Reflections on the field

Discussing common issues the public can encounter with acoustic concepts, technical terminology, the interpretation and application of British Standards, and how to recognise good service in acoustic consultancy and professional competence in the industry.


Southampton University's Institute of Sound and Vibration Research (ISVR) was established in 1963.  Southampton have various BEng and MEng Acoustical Engineering degrees available, and are one of the primary universities in the UK for acoustics.  ISVR is divided into four groups: Dynamics Group (modelling, measurement, and control of structural vibrations); The Fluid Dynamics and Acoustics Group (aero-acoustics, ultrasonics and underwater acoustics, noise source imaging and virtual acoustics); The Human Sciences Group (human response to sound and vibration); and The Signal Processing and Control Group (acoustics, dynamics, audiology).

The University of Salford's first acoustic laboratories were established in 1965.  In the early 1970s, the Department of Applied Acoustics was formed in Salford.  Salford University began teaching undergraduate acoustics and audio courses in 1975.  Salford is adjacent to Manchester and runs BEng and MEng Acoustical and Audio Engineering degrees.  Salford is another primary UK university for budding acousticians.  The BEng Electroacoustics course is no longer run.

Other courses are available - sometimes architecture with acoustics, sometimes music with acoustics - and these courses provide more or less acoustics, depending on specialism.  South Bank University/South Bank Polytechnic started teaching acoustics in the late 1980s and currently offer a MSc in Environmental and Architectural Acoustics.  Salford, Derby, Leeds Beckett, Edinburgh, and Southampton all have masters in acoustics of some sort.  Edinburgh Napier University, Huddersfield University, etc. have related masters degrees.

There is no shortage of universities offering acoustics degrees. Any acoustician you employ should have been correctly educated, ideally a degree at university.  

Blue Tree Acoustics have degrees in acoustics from Salford and consider this a fundamental requirement for competent practice.

Underneath the degree level is an IOA Diploma in Acoustics and Noise Control, offered by various bodies since 1975.  This is much shorter than a degree (usually part-time over a year).  The Diploma course was set up to provide specialist academic training to meet the educational requirements for applying for Corporate Membership of the Institute of Acoustics.  This itself is not a particularly high bar. 

The IOA say, "The normal minimum requirement for admission to the Diploma in Acoustics and Noise Control is a degree in a science, engineering or construction-related subject or an Environmental Health Officer's Diploma.  Alternative qualifications, with related professional experience, may be acceptable and will be considered on a case-by-case basis."

IOA short courses are a lower level qualification again.  We provide some IOA short course sessions, which are delivered in the Sheffield / Rotherham / Barnsley area.

Chartered status is another level above university qualification, as it requires peer review of actual acoustic engineering.  Through the IOA, suitably qualified and experienced engineers may gain this internationally-recognised award.

For CEng registration, candidates are required to present evidence of their professional development and responsible experience for consideration at a professional review interview.  The 'standard route' is for those holding an accredited engineering degree listed on the Engineering Council website.

The IOA confirm that, "Many of the successful Institute of Acoustics registrants are graduates of one of the acoustical engineering degree courses at ISVR at Southampton University or Salford University."

Chartered Engineers are able to demonstrate:

  • The theoretical knowledge to solve problems in new technologies and develop new analytical techniques
  • Successful application of the knowledge to deliver innovative products and services and/or take technical responsibility for complex engineering systems
  • Accountability for project, finance and personnel management and managing trade-offs between technical and socio-economic factors
  • Skill sets necessary to develop other technical staff
  • Effective interpersonal skills in communicating technical matters.

In addition to personal education and qualification, acoustic consultancy firms can be members of the ANC.  The Association of Noise Consultants (ANC) is a UK-based body of organisations that are engaged in the business of acoustics, noise, and vibration consulting.  It was established in 1973.  Firms are subject to interview and peer review before being able to join, and the ANC is another benchmark for ability and experience.

In short, you should ensure the acoustic consultancy firm you engage has staff with degrees in acoustics, has staff with IOA CEng status, has staff with MIOA status, and is a member of the ANC.  These are the steps to ensuring competent advice, and Blue Tree Acoustics ticks all of those boxes.

BS4142 terminology

BS4142 terms are complicated, and a simplified explanation of the terms is set out below.  These parameters, terms, and units normally include subscript notation.

Ambient sound level - this is an LAeq measurement that contains the specific sound source and residual sound.

Residual sound level - this is an LAeq measurement that contains no specific sound source.

Specific sound level - this is a logarithmic subtraction of Ambient minus Residual, i.e. the LAeq of just the specific sound source.

Background noise level - this is an LA90 made when the specific sound source is off.

Acoustic feature correction - up to 6dB can be added for tones, up to 9dB can be added for impulsivity, other features can also be penalised.

Rating level - this is the Specific sound level plus the Acoustic feature correction (arithmetic addition).

Difference - This is the difference between the Rating Level and the Background noise level arithmetically.

For example:

Plant is operating and a measurement of 60dB LAeq (Ambient) is made in a garden.

Plant is turned off and a measurement is made of 55dB LAeq (Residual) and 40dB LA90 (Background).

Specific is logarithmic subtraction of Residual from Ambient i.e. 60 - 55 = 58dB LAeq, i.e. how loud is the specific item.

If the specific sound is just tonal, a 2dB character correction can be added to the Specific to make a Rating level of 58 + 2 = 60dB

This is compared to the Background noise level to find 60 - 40 = +20dB

The +20dB would then be considered to be 'significant adverse impact' as per BS4142.

Acoustic modelling

Use of computer modelling can be helpful in acoustics, but there is a growing trend for practitioners without suitable ability, qualification, and experience to use computer modelling to 'prove' a finding.

The difficulty is that the calculation is often obscured.  The actual calculation method undertaken can be hidden, and the assumptions of the model are not stated.

Rubbish in will equal rubbish out. At best, the results of modelling can be unreliable, and at worst they can misrepresent a situation and lead to planning permission being granted or refused incorrectly.

Acoustics is complex, and the findings of a firm or individual should be weighed considering their education and qualifications, etc.


We use calibrated precision grade sound level meters for all of our work.  Most likely a Class 1 sound level meter will be used; on occasion a Type 1 sound level meter will be used.  Type 2/Class 2 are reserved for training, where final results do not matter. 

A Class 1 sound level meter is a type of sound measuring instrument with microphone and processing that meet the performance criteria set out in the BS EN 61672-1:2003 standard.  There is also a newer version  BS EN 61672-1:2013 which also defines Class 1 sound level meters.

Prior to that, standards IEC 60651 ‘Sound level meters’ and IEC 60804 ‘Integrating-averaging sound level meters’ provided specifications for four performance classes: Type 0, Type 1, Type 2, and Type 3. 

Class 1 is more accurate than Class 2.

Type 1 is more accurate than Type 2.

Classes are newer than Types.

Type 0 is not for fieldwork.  Type 3 is poor.

We have various sound level meters, and Rion Class 1 sound level meters are our primary tools of the trade. We have various items of equipment from Norsonic, Bruel and Kjaer, Cirrus, Castle, etc.

Sound insulation testing

The Building Regulations Approved Document E (ADE) sets out standards for English and Welsh residential development in terms of sound insulation between dwellings, and between dwellings and other uses of buildings.  A reasonable resistance to the passage of sound is required, which is normally met by providing a particular level of sound insulation to protect dwellings from other uses.  Either a particular Robust Detail is used, else anything can be built and then the structure should be tested to show compliance.  Testing can only be done by UKAS-accredited firms or by testers registered with the ANC scheme.  Blue Tree Acoustics is registered with the ANC scheme, and can therefore undertake ADE testing in English and Wales following the requirements of ADE, ISO140, and ISO717.

Airborne sound insulation testing involves Pink Noise played at high volume in the source room, with sound levels being measured in the source and receiver rooms. DnT,w +Ctr is the term of the final result measured in dB.  A better airborne performance has a greater value.

Impact sound insulation testing involves a tapping machine being placed in the source room, with sound levels being measured in the receiver room. 

L'nT,w is the term of the final result measured in dB.  A better impact performance has a lower value. 

ISO 140-4:1998
'Acoustics — Measurement of sound insulation in buildings and of building elements — Part 4: Field measurements of airborne sound insulation between rooms'

ISO 140-7:1998
'Acoustics — Measurement of sound insulation in buildings and of building elements — Part 7: Field measurements of impact sound insulation of floors'

BS EN ISO 717-1:1997 'Acoustics - ratings of sound insulation in buildings and of building elements. Airborne sound insulation'

BS EN ISO 717-2:1997 'Acoustics. Rating of sound insulation in buildings and of building elements. Impact sound insulation'


Frequency is the term for consideration of the tone or pitch of a sound.  It is the number of times per second that a sound wave repeats. 

The units of frequency are Hertz (Hz).

A bass guitar or bass drum has a lower frequency than a whistle or a bird chirp.

Low frequency sound sources have fewer oscillations per second, and have smaller frequencies than high frequency sounds.  A bass guitar might make sound at around a few hundred Hertz, say 50Hz to 400Hz.  A sound level meter calibration tone is 1kHz (1000 Hz).  

Young humans with perfect hearing can hear sounds between 20Hz and 20kHz. Sound content at frequencies above 20kHz is ultrasound. Sound content below 20Hz is infrasound.

Sound is not heard equally across all frequencies.  Different animals have different hearing abilities across different frequency ranges.

Sound behaves differently at different frequencies, and octave band or third octave band analysis is usually required in order to consider sound break-in or break-out, barrier performance, audibility, etc.  

Acoustics is complex, and correct measurements and calculations are necessary to establish the reality of a site or situation.


Acoustic parameters, terms, and units normally include subscript notation.

LAeq is the main parameter used in acoustics in the UK.  It is the equivalent continuous sound level - that is to say, if a sound was played at a constant sound pressure level, what would it need to be to have the same energy as the real fluctuating sound over the measurement period. The 'eq' part means 'equivalent continuous'.

LAeq is essentially a logarithmic average, and is used to describe a fluctuating sound in terms of a single simple sound pressure level.

The 'A' is for A-weighting.  This means that the sound signal has been passed through a filter to mimic normal human hearing response, so bass sounds and high pitched sounds are effectively reduced in volume, which means that the numbers are more aligned to our actual human hearing, rather than the naturally really occurring sound that the sound level meter can measure. 

LAmax is again A-weighted, but can be thought of as the highest sound pressure level occurring during a measurement.

LAmin is again A-weighted, but can be thought of as the lowest sound pressure level occurring during a measurement.

LA10 is again A-weighted, but is the level exceeded for 10% of the measurement period.  It is a statistical parameter, and is used for road traffic noise as it tends to find the sound pressure level of events.

LA90 is again A-weighted, but is the level exceeded for 90% of the measurement period.  It is a statistical parameter, and is used to establish background noise levels as it tends to ignore events and find the underlying level that occurs when no events are occuring.  

There are hundreds of acoustic parameters, including peak, C-weighting, unweighted Z-weighting, as well as NR curves, NC curves, etc.

Acoustic errors

Many acoustic reports contain errors.  A high percentage of reports written by others that we look at either have errors or are incomplete.   Errors can include misrepresenting what measurements were made and when they were made, overstating how long measurements were made for, miscalculation from raw data to summary tables, etc.  We commonly see examples where, because a window was seen closed in November it is assumed to be unopenable, or because a background noise level on the roadside of a house was 50dB LA90 it is assumed that the background noise level in the garden to the rear is also 50dB LA90, etc.

Doing acoustics well is difficult - it takes experience and care, it takes time and checking.

We provide high quality level acoustic consultancy work for reasonable fees.

Colloquial terms 

Soundproof.  There is no such thing as sound proofing.  Sound-proofing is no more possible than bomb-proofing - if you have a big enough bomb, it will break anything.  Degrees of sound insulation are what is possible.  This is a particular level of reduction, a certain dB attenuation at a certain frequency.  Claims of sound proofing are always over-promising and under-delivering.  It is an unhelpful and inaccurate turn of phrase.  
Sound test.  We are asked weekly to undertake sound tests.  This is vague - a sound test might mean an acoustic survey and report for a proposed residential site, it might mean an ADE sound insulation test for Building Regulations compliance, it might mean measuring trading noise levels of a touring band. 

Noise report.  As with sound tests, we are asked all the time to write noise reports, but the devil is in the detail.  We need to establish the purpose of the assessment, and then we design our workflow to deliver the acoustic assessment the site or project requires.

db/DB.  The unit 'Decibel' is shortened to 'dB' - lowercase d uppercase B. It is one tenth of a Bel. The bel B and the decibel dB were invented by Bell Labs and named after Alexander Graham Bell.

The misspelling of 'acoustics' with two c's is also common - acoustics, not accoustics.

Heat pump noise

'Microgeneration Installation Standard: MCS 020, MCS Planning Standards for Permitted Development Installations of Wind Turbines and Air Source Heat Pumps on Domestic Premises' is a simple method used for heat pump noise assessments.

It is simplistic and is designed to permit heat pumps and wind turbines on many properties across the country.  Its use can lead to noise problems further down the track, as it does not consider the likely impact on neighbours in the usual way. 

Non-permitted development, such as alterations to listed buildings, will most likely require a full BS4142 assessment.  This will require measurement of background noise levels as part of the assessment, and will better reflect the realities of the potential impact.

Reverberation times (RT) 

This is how quickly sound dies away in a room.  It is probably correct to say that there is no such thing as a correctly established reverberation time.  Depending on the sound source type and location, and the receiver type and location, the results will be different.  Simply, the RT is the time it takes for the sound to reduce by 60dB.  This is usually not measured directly; rather, the reduction from -5dB to -25dB is measured and then multiplied by 3 T20, else -5dB to -35dB measured then multiplied by 2 T30.  Even in these measurements, there can be a large influence depending on whether Pink Noise is used as a source from a loudspeaker, or whether an impulsive bang (gun, balloon pop, clapper) is used, but also whether reverse integration is or isn't used, and exactly how the rules are applied to the signal.  How is the source level established?  How is the background sound level established?  Is the data smoothed or not?  Are 6 measurements made, or 600? 

RT can only ever be an estimate when measured.  We have developed various calculation methods to compare results and hone measurements.  

When calculated, the correct RT is even more elusive.  Global Sabine or Erying calculations have strict assumptions that may well not be met in reality.  Ray/cone tracing software has limitations, as does all software calculation.  Some assumptions used are not obviously stated.

MIOA status

IOA sponsors - Open to firms, organisations or individuals engaged in or having an interest in acoustics and whose support can advance the aims and objects of the Institute.

A sub-set of Sponsor member is the Institutional Subscriber which is a grade of membership directed primarily towards institutional departments, such as Local Authority Environmental Health Departments and educational establishments involved in acoustics, who wish to keep informed of the Institute's activities

IOA students - Open to people who are bona-fide students who are studying in the field of acoustic or are on acoustic related course

IOA affiliate - Open to persons who satisfy Council that they have a serious interest or involvement in acoustics or a related discipline. No formal qualifications in acoustics are required.

IOA technician (TechIOA) - Open to persons who have a suitable level of general education and who satisfy Council as being suitably qualified educationally in a limited field of acoustics with a minimum of one years experience of work in acoustics. This grade of membership is intended for practitioners working in a wide variety of areas who wish to be involved in the profession or to be able to access the services provided by the Institute but who are not yet able to qualify for Associate Member or Member grades.

IOA associate member (AMIOA) - Open to persons who satisfy Council as being suitably qualified educationally in acoustics or have an appropriate period of experience in acoustics in lieu of such qualification. This class of membership is aimed primarily at persons who have obtained the appropriate academic qualifications for the grade of Member but who do not (yet) have the relevant period of experience in the profession for the grade of Member.

IOA member (MIOA) - Open to persons who satisfy Council as being suitably qualified educationally and who have a minimum of three years experience of responsible work in acoustics.

IOA fellow (FIOA) - The senior professional class, granted to those who have made significant contributions to the field in responsible acoustical work. In addition, the applicant must fulfil all the requirements for Corporate membership of the Institute as set out for the grade of Member below.

IOA Honorary fellow (HonFIOA) - By special invitation of the Council and is restricted to extremely distinguished individuals in the field of acoustics who have made an outstanding contribution to the art and science of acoustics.

Poor sound insulation of a new build home

All new build dwellings in England and Wales are subject to the Building Regulations ADE, and sound insulation performance between dwellings should meet the minimum standards.  If a resident thinks that the sound insulation through to next door is not good enough, they should complain to the developer initially.  It might be that the warranty provider needs to be contacted.

New build warranties are issued to protect buyers from the costs of fixing structural defects caused by faulty materials or poor workmanship during construction. 

Either way, eventually it should be that the issue is investigated.

This would probably then lead to an ISO140 sound insulation test being undertaken by an ANC or UKAS accredited firm.  Blue Tree Acoustics have undertaken many such investigations.  We independently and accurately undertake a sound insulation test and establish the existing levels of performance with reference to the ADE criteria.  If the test result shows a failure, then our report will state this, and the developer would then be required to improve the sound insulation performance of the building in order to meet the stated airborne sound insulation values.  In the case of separating floors, airborne and impact sound insulation values will be tested and reported.


'BS8233: 2014 Guidance on sound insulation and noise reduction for buildings' is one of the main acoustic standards in the UK.  BS8233 sets criteria for internal ambient noise levels within bedrooms, living rooms, and dining rooms.

Usually the local authority should require that the BS8233 criteria can be met and that the building(s) are designed to allow that to happen.

There might be an additional maximum noise level requirement dB LAmax in bedrooms at night to consider sleep disturbance.  

The importance of having a competent and capable acoustician can't be overstated.  If planning permission is granted with a validation testing requirement, then any miscalculation will be found and the building will not be able to be occupied until the issue is rectified.   

Many firms offer BS8233 assessment, but you should check MIOA, CEng, and ANC status to ensure competence.

Noise floors

Sound measuring equipment is imperfect.  It may be that equipment can be easily overloaded, so that stated values understate the reality.  It might be that equipment can be easily under-range, with stated values being too high.

All equipment has a self-generated noise floor, which means that the sound analyser cannot accurately measure beneath that level. It might not be possible to show that a particularly low sound level criterion has been met directly.  There are some techniques that can be used to work around this issue, and good quality consultancy is required for this.  

Sound pressure levels

Sound pressure level measurements are physical measurements of the sound energy experienced at a location.  The location is critical.

Sound pressure is measured in Pascals (Pa), but usually these are converted to sound pressure levels (dB), as these logarithmic units are more manageable and compare better to human response.
Sound power level is a theoretical measure of the total sound emissions from a particular sound source.   This does not change with location, and is a fundamental property of the sound source itself.

Sound power is measured in Watts (W), but usually these are converted to sound power levels (dB).
Sound intensity is concerned with the sound pressure level, but also the direction of travel. 

Flanking noise

Flanking noise is sound that transmits between two spaces indirectly, such as when the flanking sound passes around the main separating wall or floor element.  These routes will allow more sound to pass and can lead to the sound insulation performance of the main element not being realised.

Flanking routes can include external walls passing between demises, steel or timber beams passing through the primary structure, or even noise passing via doorways and corridors.  


A 10dB increase in sound is perceived as a doubling in sound level.
A 10dB decrease in sound is perceived as a halving in sound level.

A 3dB increase in sound is a just perceptible increase in sound level.
A 3dB decrease in sound is a just perceptible decrease in sound level.

CRTN - Calculation of road traffic noise

Calculation of Road Traffic Noise (CRTN - ISBN 0 11 550847 3) issued by the Department of Transport in 1988, considers traffic flows, speeds, percentage heavy vehicles, inclines, etc. to estimate road traffic noise.  However, CRTN is now outdated.  CRTN does not consider electric vehicles, and will require an updated or replaced method.  Clearly, tyre noise will remain, but engine noise will significantly change over the next 20 years.  The ban on petrol and diesel car sales due in 2030 or so will significantly reduce the number of petrol and diesel cars on the roads in the UK, and it will be difficult for CRTN's update/replacement to keep up with the change.

CRTN does not consider non-highways at all, but even for highways, the estimated results are likely to be +/-3dB generally, with this gap to reality increasing as time goes on. 

Essential operations

Some activities in the UK are essential.  We need airports, harbours, minerals extraction and processing, hospitals, nurseries and schools, shops, farms and food production, and all types of manufacturing, energy production and water treatment, amongst other things.  

Some activities in the UK are desirable.  We would also like to have restaurants, cinemas and theatres, music venues, pubs and bars, hotels, sporting facilities, stadia, and gyms, amongst other things.

Things have to be built and connected together, and this will often lead to conflicting needs and competing desires.    

The planning process needs to balance these needs and try to allow noisy activities to take place in the right places, whilst protecting the needs of the noise-sensitive places.  Some uses can be both noisy and noise sensitive - hospitals for example. 

It is difficult to balance needs and difficult to correctly assess the magnitude of impact that one operation might have on another use.

In come cases, there are standards and guidance documents that try to introduce benchmarks for certain levels of acceptability.

Planning permission versus nuisance 

This is one of the main issues in day to day acoustic consultancy.  The two regimes come from different viewpoints, and are generated in different ways.

Planning permission cannot grant permission to be a nuisance, but objectively finding the place where nuisance will occur is difficult.  An ideal system would set nuisance at XdB, and would permit planning permission at YdB such that there cannot be nuisance.

However, quite often planning permission is granted where the new operation will cause nuisance or where the occupants of the new noise-sensitive development will suffer nuisance.  Often, planning is determined by measurement and assessment, whereas nuisance is determined by listening and subjective impression.

Schools, colleges, and education

Building Bulletin 93 (BB93) is the main document for consideration of schools and education buildings.

BB93 considers ambient noise levels, plant noise, reverberation times, and sound insulation across separating walls and floors between room uses.  It is critical that a music lesson does not impact on a library, and that rooms control sound reflections sufficiently well so that speech is intelligible.

Road traffic noise, rail noise, aircraft noise, etc. need to be controlled sufficiently well so that the teacher can be heard.

All of this requires educated, competent acousticians, who are few and far between.    

Noise at work

Noise at work assessments are critical in ensuring that workforces are not exposed to noise that will either immediately or over time cause them to lose their hearing.  

There is a strong link between hearing loss and dementia.  According to one study, people with mild hearing loss are two times as likely to develop dementia, compared to those without.  This increases to three times for those with moderate hearing loss, compared to those without.  

The regulations use C-weighted peak sound pressure levels and a daily exposure LAeq levels taken over 8 hours.  'Upper exposure action value' and 'lower exposure action value' are the benchmarks where different levels of risk are determined.  

Noise at work can cause hearing loss that can be temporary or permanent.  Permanent hearing damage can be caused immediately by extremely loud noise, such as an explosion.

Hearing loss is usually a gradual process caused by prolonged exposure to noise. 

Workplace noise can also cause tinnitus, which can have huge impacts on the health and quality of life of those with the condition.  

We undertake control of noise at work assessments.  We also provide sessions of the IOA short course CCWNRA. 


Calibration is crucial in acoustics.  Equipment can easily be damaged or can malfunction, and without correct calibration and routine calibration checks, the data collected could be ultimately incorrect and worthless.  

Sound level meters can be calibrated to BS EN 61672-3:2006 or BS EN 61672-3:2013 for newer Class 1 sound level meters or BS 7580:Part 1:1997 for older Type 1 meters.  

BS 7580:1 only applies to sound level meters manufactured in accordance with IEC 60651:1979 and IEC 60804:1985 or IEC 60804:2000.  

Sound level meters manufactured according to IEC 61672-1, BS EN 61672-3 applies.  If made to IEC 61672-1:2002 then BS EN 61672-3:2006 applies and if made to IEC 61672-1:2013 then BS EN 61672-3:2013 applies.
Sound calibrators are also lab-calibrated.


Train movements (particularly freight train movements) and activities such as pile-driving often make noise, but they also generate vibration.

Vibration can cause damage to buildings, but usually it will annoy people well before building damage levels are met.

Ground vibration can be measured in terms of Peak Particle Velocity (PPV) in mm/s. This is the instantaneous maximum velocity reached by a vibrating element as it oscillates. The PPV is a simple indicator of perceptibility and risk of damage to structures due to vibration. 

VDV is used to measure human exposure to vibration in buildings and the effects of vibration on humans in terms of annoyance. Use of VDV is a way to quantify vibrations as an exposure dose over a daytime or nighttime period. The vibration dose value is measured in m/s1.75.

Plant noise

Plant noise, or HVAC noise, can be a big problem.  Manufacturer's data can be unclear or have crucial elements missing from it.

Different types of mechanical services are treated in different ways, which is logical when some thought is given to the issue.  Some processes are essential, and a situation where it is impossible or financially unviable to keep food cold and preserved would be bad for everyone.

A resident living in close proximity to a food storage location might experience higher levels of noise than they would like, but food storage is essential and has to happen somewhere.  Farming areas will need to be able to store food, and supermarkets, butchers, fishmongers, etc. will also need to be able to keep food fresh.  Most retail units will have HVAC equipment installed to allow the business to function.

Most likely air conditioning systems will be installed in more and more offices, warehouses, dwellings, and schools as time goes on.  As the world continues to warm, it seems that this is an inevitability.

Railway noise

Train movements on railway lines make noise.  Most people accept that trains are generally good and that well-designed and efficient rail use is preferable to reliance on cars, trucks, vans, and HGVs in terms of emissions and fuel consumption. 

Developing residential sites in close proximity to railways can be challenging, but it is possible with a good noise survey and competent calculations and reporting.  Maximum noise levels in bedrooms at night are usually the main issue - can the LAmax result in bedrooms be sufficiently controlled?

At Blue Tree Acoustics, we have more than 40 years' combined experience in assessing train noise and ensuring criteria are met.  We have a back catalogue of data from previous sites and we have in-house calculation methods to predict noise ingress into rooms.  We also have validation test data for many sites, so we have been able to review previous assessments to be sure that clients' future designs are correct.

Not all noise assessments are the same - it really is critical that the qualifications and experience of the assessor are considered when you are looking to instruct a noise consultant.

Witnessing of testing

Our sound insulation testing is witnessed and quality-checked often by professional membership bodies and  both internal and external audit cycles.

The ANC witness our testers - indeed, our two Senior Consultants are either former or current ANC examiners, who witness other testers.  In addition to this, we witness each other regularly and we also have Robust Details witness us often.  Finally, we have also had UKAS witness our testing.

As a result, Blue Tree Acoustics is one of the most witnessed and quality-checked firms working in the UK.  We do not send junior staff with little experience to your site to cut their teeth on your project.  We only work at a high quality level, and our testing comes with decades of experience.

Correct testing, fault-finding, and remedial advice are not straightforward, and it is a mistake to employ an inexperienced, underqualified practitioner.

Changing trends

Radio, recording, and post-production studios are often critical locations in terms of acoustic performance.  There are many different elements to consider when clients design such spaces, and if any one of them fails, a final polished audio recording or broadcast will either be much more difficult to achieve, or simply will not be achieved at all.

As podcasts and home-streaming increases, acoustic performance of rooms in dwellings might also need improvement.  Teams and Zoom meetings can also test the acoustics of a home setting.  There is usually less background noise at home since there is no babble from other colleagues nearby, but there can be reverberation issues as well as noise intrusion from pets, home appliances, etc.

Trends change over many years, and some problems are removed while new problems are created.  It is likely that road traffic noise reduces over the next few decades, but so does the noise masking that road traffic currently provides.  This will probably lead to commercial sound and industrial sound, noise from pubs and clubs, and noise from music venues being more audible and more annoying.  The planning and enforcement systems are currently not adapted to consider this issue.

Workers' cottages where everyone in the village worked for the mill, the pit, or the factory are now largely gone.  Not everyone living next to Old Trafford supports Manchester United.  Clay pigeon shooting areas might draw new residents seeking a slower pace of life outside the city, but who have no interest in the traditional countryside activities.  The world does change and the impacts shift.

Low frequency noise

Every so many months, we will receive an email from someone who can hear and is annoyed by low-frequency noise from an unknown sound source.  Some people are particularly tuned in to low frequency noise, and they find it difficult to escape it once they have noticed it.

The first question to us is usually whether we are able to prove it exists, as they want to know for sure that they are not imagining it or hallucinating in some way.  Low frequency noise is particularly difficult to trace and to control.

It is possible for us to undertake an assessment, but it tends to be labour-intensive and require nighttime investigation while other noise sources are quieter.

AI in acoustics

Artificial intelligence is moving forward at faster speeds every year.  AI of course might make it possible for non-acousticians or practitioners with little experience to work beyond their professional ability, but there will be big risks.

Blue Tree Acoustics has been involved in cases where associated professions, such as environmental health practitioners, have caused millions of pounds worth of damage to developments by doing poor testing, giving bad advice, and not being able to navigate the realities of competent acoustics.

Practitioners are usually members of the Institute of Acoustics, but acoustics is more than just measuring noise at a location during a noise survey.  Surveys have to be designed to ensure that the correct data is gathered, at the correct times, and in the correct conditions.  Once the data is collected, it must then be correctly processed and used to inform design via quality calculations and estimations.

AI will be better at this than those businesses/companies/workers, but unproven methods and untested processing will probably lead to unworkable solutions.  As with noise modelling, it is possible to present a scenario with a computer image that has little relationship to the actual realities of a site.

Drone noise

There are no standards to deal with drones, but it seems very likely that in the near future takeaways, Amazon,  and possibly chemists and corner shops will use drones to deliver small packages to carparks, driveways, or gardens.

There will be little buzzing drone motors and blades emitting noise while landing or hovering for a few minutes, near most houses.  The houses or flats near the warehouse or shop sending all the drones out will have a large increase in noise, and this will no doubt lead to complaints and nuisance issues.

This is slightly different to road traffic noise or rail noise, and is probably more like commercial noise since it is made by a business, but Government and Local Authorities/Councils will have to consider placing limits on the numbers of drones, sound power levels of each drone, and hours of operation, together with acceptable flying routes.

Other safety issues might prevent drones from crossing motorways or railway lines, but drones might become the usual method for delivering small packages, akin to couriers and delivery bikes in larger cities now.  Localised restrictions may also be brought in, i.e. it might be that drones in Lincoln and Derby are permitted, but are banned in Doncaster and York.  

Driverless vehicles will also be introduced eventually, with deliveries being made over a wider range of times since driverless vans, like drones, won't need drivers to want to or be able to work at 5am - they will just need a recipient to want a 5am delivery.

Desirable sound

A lot of acoustic consulting work involves 'noise' - stopping noise getting from A to B, determining if a level of noise in location C is acceptable or not, assessing noise exposure for staff in a workplace and trying to protect them from hearing loss, etc.

However, sometimes our work deals with the more desirable 'sound'.  This might be music in a concert hall, or speech in an office space or school classroom.

'Soundscape' is a buzzword in acoustics currently, where playing sound into a space or manipulating sound sources to provide more pleasing sound in a location is being attempted.  This can improve the situation since listening to birdsong is usually nicer than listening to traffic noise, but who should decide what everyone in a  given space will now listen to?  Piping in soundscapes creates another area where cultural preferences, personal phobias, and the needs of different neurotypes can clash within an organisation, and conflict can arise around how 'desirable' is defined and what amount of additional sensory input is tolerable or necessary.  No-one wants to listen to traffic noise, but this is not a chosen state - it is a side-effect of modern life.

Blood donation premises have music playing at fairly high sound levels to mask the private conversations that take place there, but for most people this is experienced as endless blaring from a pop radio station, which interferes with communicating important information and understanding speech within that space.

UK regions

We work on lots of projects in Sheffield, Leeds, Wakefield, Barnsley, Manchester, and the surrounding Yorkshire, Derbyshire, and Lancashire areas.  We also travel further afield, particularly where a client has an issue that needs to be dealt with professionally by competent, experienced experts.

One project we were recently involved in was in Oxfordshire, where an unqualified, unregistered tester appears to have undertaken sound insulation testing leading to legal action.

Another situation was in Lincolnshire where we undertook assessment and review to comment on and correct an incomplete noise assessment done by others.  We've assisted with luxury hotels in Cumbria and residential development in Rutland, dog kennel noise in Boston and shooting clubs in the east riding of Yorkshire.

We work across the UK and we do what we say we'll do - if we have confirmed that we will measure at a wedding from 0700 hours until 0000 hours, then we'll be there.

Each site location is different from the next, and will have different specific needs from other, even similar, projects.  Planning and care to suit the situation at hand are at the centre of everything we do.

Acoustics library

We have an in-house library of over 100 acoustics books and related subject volumes.  Plenty say "noise control" in the title, but we've also collected audio and architectural tomes over the years.

Analogue and digital signal processing, environmental noise measurement, control of noise at work, Beranek, Lord, Wood, Rayleigh, Ghering, Magrab, Kuttruff, and Crocker all feature, along with original Bruel and Kjaer texts.

Information and knowledge are important, and we are always looking for more ways to expand our understanding.


SEL is now LAE.  Unweighted data is now Z-weighted data.  B-weighting is gone, D-weighting is gone.  Sampling and A-D converters have replaced paper printouts.

Acoustics moves forward.  There might be time when A-weighting has less of a hold on criteria, and C or Z (or indeed something else) starts to take over.  Digital data is usually better than analogue, but incorrect settings and poor understanding will lead to incorrect conclusions and assertions.


Decibels are complicated and do not behave as you might expect.  

20dB(A) is pretty quiet - a quiet bedroom at night might be this quiet or might be 30dB(A).  A living room is more like 40dB(A) or 45dB(A).  Restaurants might be 70dB(A), a pub might be 80dB(A), and a nightclub might be over 100dB(A).  These are just rough ideas, and of course a particular pub could be more like a living room or more like a nightclub.

How large is a living room?  How large is a nightclub?  These are similar questions, where there are no firm rules.  At around 194dB, the rules stop working and shock waves are being generated.  0dB(A) is the line in the sand taken to represent the threshold of human hearing.  Most of our lives, we are exposed to sound levels between 20dB(A) and 80dB(A), with periods of high sound levels over 80dB(A), but probably under 120dB(A).

Two signals, both generating an experienced 50dB(A), say, would not create 100dB(A).  They might create 56dB(A), they probably create 53dB(A) - they might create 0dB(A), depending on coherency and phase.

Competent person

'Competent person' is a phrase used in a few places related to acoustics.  It is interpreted differently by different people with differing views and positions.

Noise at work assessments require a competent person to undertake the assessment, and there is of course a significant risk that an invalid or incomplete assessment can lead to a workforce being permanently damaged by high noise levels.

Of all the work Blue Tree Acoustics is involved in, workplace noise assessments are the most important since an error can lead to life-changing medical conditions.  It is important that a competent person is indeed competent.  BEng/MIOA/ANC/CEng status must be deemed competent by any consideration, and this is the gold standard.

BREEAM is another area where a competent person is required.  Again, university degrees in acoustics, chartered engineer status, and corporate memberships of the Institute of Acoustics and the Association of Noise Consultants are the gold standard for BREEAM assessment.

Kitchen extracts

Noise coming from kitchen extracts is a common issue in town centres and city centres.  Most food-serving venues will have a kitchen extract, and most likely the system was installed with air extraction as the primary (and possibly only) area of interest.

Kitchen HVAC equipment is often installed in tight and restricted areas and commonly terminates in the vicinity of a residential window.  Sooner or later, a resident will complain to the local council, and then a noise impact assessment and mitigation advice report will be required.  The council criteria might be open to interpretation or might not be stated, and the notice might simply require that the noise be abated.  Alternatively, the notice might set out the criteria to be met, else suggest a standard such as BS4142 to be used as a basis for the sound  survey and sound assessment.

Non-trading sound levels will probably need to be established, and the difference between the plant noise and the other noise will need to be established.  It might be that a Council criterion requires an increase in total sound level of not more than XdB.

Whatever the situation, it is usually possible for us to provide assistance by assessing the scenario and advising appropriate mitigation measures to resolve the issue.

45 dB LAmax

Most likely this is a criterion applied to bedrooms at night for rail noise ingress, or possibly any noise, including traffic noise ingress.

The 45dB LAmax level is not in a standard; it is in a WHO guidance document and loosely reflects sleep disturbance research noise levels.

If a local authority writes a planning permission condition that requires 45dB LAmax to be met in a bedroom at night, they might not actually mean that precisely.  A number of exceedances are commonly permitted - perhaps ten - although interestingly enough the world is never the same from one day to the next, so the night or nights of the original noise survey cannot reflect the events that will occur throughout the lifetime of the building, or even  through the night of the validation test (if there is one stipulated).

Various companies have methods for calculating noise ingress to meet the 45dB LAmax criterion - many of them are wrong.  Logical errors and processing missteps are often made, which can lead to massive problems once a site is built.  The errors can't be listed out here, but they are real and their consequences can be nuanced beyond the understanding of inexperienced assessors.

PA systems 

Sound systems and music noise are part of the many areas where you need a competent acoustician.  We have seen many venues over the years that have incapable sound control in place or an ineffective PA limiting system set up.

Qualified acousticians will understand PA systems, and will be able to ask the right questions and notice the right details.  Premises licences are at risk if music noise from recorded music playback or live music is not controlled correctly and/or limiter devices are not configured relevantly to the venue's typical trading noise levels.


Controlling noise and designing areas for patients, staff, and visitors in hospitals and other healthcare settings is something you probably only appreciate when you are ill or in need of treatment and an overly noisy or reverberant space increases the suffering while also muddling speech intelligibility and reducing the ability to rest.

As with all acoustics, the issues can be dealt with and controlled so long as an experienced acoustic engineer is involved in the client's design or redesign of the space.  HTM08-01 sets standards and requirements that we can help you meet.

Noisy neighbours 

Most activities in the UK are not restricted in terms of sound level.  The issue of restriction starts to arise where the activity impacts on others.  Depending on the activity, the local authority might be able to take action to reduce the sound level or activity time, or to restrict the times of day that the activity is undertaken.

Some activities cannot be limited - a baby crying, for example, is natural and normal and is permitted regardless of the impact on others.  There is little that could be done in any case for this sound source.

Generally, the local authority should be the first port of call.  It might be that then advice is needed to take private action against a noisemaker, or perhaps to improve the sound insulation and separating between the noise source and the noise-sensitive area.


Continual professional development (CPD) is where experts keep up to date with new developments in their field and ensure that they know and have not forgotten how to do the existing important things in accordance with the existing standard and guidance.

Our senior consultants' examiner involvement with the ANC is CPD, as is our previous role with BSi.  Our preparation for and teaching of IOA short courses is also CPD.  Our staff attendance at IOA lectures and presentations is also CPD, although the content of such sessions varies in that some contain matters of fact and others are clearly a vehicle for the opinions of the author/presenter.

Some CPD sessions are about very precise topics, such as the attenuation provided by different types of trees and shrubs over distance, or a manufacturer explaining why they have made a new product and what they hope it will be able to do.  CPD sessions are sometimes slow and repetitive, as a particular standard may not have changed for 20 years, but still some practitioners have not yet read or understood it.

Understanding standards usually requires a level of mathematical and logical ability.  Acoustics is a branch of physics, and engineering requires mathematics.  Standards are sometimes written to assume a fundamental level of ability, without which the steps will not be understood.

However, there is more than maths to be understood.  An important element of CPD is working closely with experienced, competent acousticians.  This is especially important in the early years of a career when one develops the critical skillset to practice competently.  There are many individuals who claim 5 or 10 or 15 or 20 years of experience - but experience of what quality, gained working under what calibre of senior acoustician?  Working alone or under someone who is doing the work badly will clearly be detrimental to professional development, and will lead to corner-cutting or even not realising what is required in an assessment in the first place.

Our Senior Consultants have worked for ANC-registered firms from the late 90s onwards.  Blue Tree Acoustics became a member of the ANC in its first year of trading.  We gained our university qualifications, then cut our teeth working for existing firms before creating our partnership and implementing best practice into our working methods - keeping up to date with new developments in the field all the while.

Planning conditions 

The National Planning Policy Framework reinforces the view that planning conditions should be kept to a minimum, and only used where they satisfy the following tests:

  1. necessary;
  2. relevant to planning;
  3. relevant to the development to be permitted;
  4. enforceable;
  5. precise; and
  6. reasonable in all other respects.

In practice, many planning permission conditions are not precise, and are open to debate.  Another group of conditions are unenforceable and would fail this planning test.

If conditions were challenged more, it would probably be concluded that a decent percentage of conditions were not phrased well enough for correct implementation to be assured.


Rw is a measure of the sound insulation performance of a material in controlled conditions in a laboratory test.

It is a little like car manufacturer's claims of a typical MPG, in that a given Rw value is highly unlikely to be achieved in reality on site.  It is also unlikely that the material will be installed perfectly onsite.  It is unlikely that only the sound passing through the material is consequential, as probably there is one or more flanking routes for sound to pass around the material in addition to passing through it.

It is critical that architects and others do not assume a laboratory value will be achieved onsite - acoustic calculations, advice, and experience are necessary for a project to be optimised and to perform as required.

Variable conditions

Noise surveys are undertaken as quickly and efficiently as possible.  The weather always plays a part in planning and undertaking sound surveys, as the wind usually needs to be no more than 5m/s and ideally there also needs to be no rain.  Wind direction is a more difficult issue as 'ideal conditions' might be vary or be impossible for a given site and surroundings.  Prior consideration of the specific site location and its quirks is also essential to time-effective and cost-effective survey logistics planning.

Weather conditions might shift results significantly.  One solution is to undertake many surveys over many days.  Research from Salford has considered this issue for a specific dataset, but ideal data collection is usually not the same thing as viable data collection.

Equally, some companies/firms survey over periods that are demonstrably too short and have no hope of delivering quality data.  For example, the night-time situation for a site cannot be known unless the night time situation for that site is sampled, but some practitioners simply guess at what might happen at night since this is easier than physically attending site.  This is poor practice and usually leads to large errors between the assumption and the reality, for which the client ends up having to handle the fallout financially and also sometimes legally.

Building Regulations sound testing

ADE sound insulation testing can result in a range of outcomes.

If the party walls and/or floors are not designed or installed correctly, it is possible that one or more of the structures sampled will fail the test.

Each failure is reported and submitted into the ANC database.  Building Control then decides if remedial work is necessary and if retesting is required.  Remedial work would normally be required after a shortfall in sound insulation performance has been confirmed, and testing of that improved structure (probably with additional testing of previously untested areas) would be required.

It is much better practice to get good advice pre-build rather than chasing the mistake near completion or after completion.  There is no reason to avoid getting good advice from experts, and it is more cost effective in the long run as it avoids the spiral of test failure, remedial works, and retesting costs.  It also saves time delays and  the contractual issue of plots not being able to be handed over on contracted dates as a result of poor design and the ensuing remediation and retests required.

Acoustic mats

Acoustic mats and acoustic underlays have a limited benefit, and there is a very short menu of situations where they provide real benefit.  The same firms offering 'sound proofing' will no doubt offer acoustic mats and acoustic glue, etc.

Why is there a problem on the site?  What is it that is heard and that you would like to have reduced?  How does the building currently perform?  What are the primary routes of noise transmission?

There are many questions, and acoustic matting is rarely the answer.

Impulse sources

Sometimes we undertake reverberation time measurements with loudspeaker sources.  The room is filled with sound, which travels and bounces around the room until the moment that the loudspeaker source is cut off from power.  Then the reduction - more importantly, the rate of reduction - is established in each frequency band, and this dictates the reverberation time.

There are other instances when an impulsive source is used.  This could be a blank firing pistol, or similar explosion or impact.  Blank-firing guns can cause issues, particularly when testing in schools or healthcare settings, and measurement of reverberation time in the USA must be all the more risky.  Some testers prefer blanks, as they can be quick and generate high sound pressure levels while not being heavy to carry around the site like a cabinet loudspeaker source is.

Gym noise

Gyms can make noise, in particular with weights impacting on the floor.  If gyms are building into a multi-use development including retail, offices, or even accommodation, there can be an issue with structure-borne noise transmitting from one space to another via structural elements.

There are methods to calculate likely transmission and there are ways of measuring existing transmission conditions.  The most common noise generation route is the free-weight lift releasing onto the concrete face floor below, with this noise passing in all directions into adjacent sensitive spaces like residential flats.  There can also be other sources such as running machines, weights 'machines', and rope pull exercises banging on the floor of the gym, which might be only a few metres from the nearest apartment.

Gym noise can likely be assessed at the design stage, and the resulting noise levels can also be measured in validation tests within accommodation or other spaces.

Peak signals

Sound level meters usually sample every 20.8 micro seconds (i.e at a 48kHz sampling frequency), and peak sound levels are essentially the highest of those individual measurements.

But what if the actual physical peak occurred between these samples?  What if the peak value stated underestimates the actual real peak?

It is possible to estimate this if the adjacent peak values are known.  Blue Tree Acoustics has a proprietary calculation method to estimate this and determine the likely magnitude of the error.  The error is usually insignificant, but understanding the processes that the signal goes through is important for good understanding of data as well as proper data processing and manipulation.

Piling noise

Construction noise, and in particular piling noise, can be very loud.  It usually goes on for only a few weeks, but the percussive piling method can involve 3, 4, or even 6 tonnes of hammer banging onto a pile every few seconds.  This can elevate average LAeq noise levels and maximum LAmax noise levels, and might impact on neighbouring properties.

A local planning authority might have placed noise conditions on the building site operations, or perhaps will respond to complaints from occupants being annoyed by the piling.  Piling should be done in daytime only, but it can cause issues for locals even then, particularly with the increase in telework and remote working from home recently.   

Blue Tree Acoustics Ltd

We are a partnership, not a company - we are not, and have never been, "Blue Tree Acoustics Ltd".  A few years ago, a company of that name was started up, potentially in a passing-off attempt.  That company is now dissolved, but this type of naming is not welcome and appears to be an attempt to win custom whilst trading on our reputation.

Nursery noise

Children can be loud and can make noise as they learn and play, or even when they've fallen over and are upset and crying.  Creche and nursery locations can be on the same site as a primary school, or can be in a purpose-built other location, or can be run from a private home.

There are thousands of nurseries operating in dwellings in the UK, running along happily adjacent to other dwellings.  It is quite common for elderly citizens to report actively enjoying being able to hear the sound of children playing.  However, there can also be conflicts, and shift workers such as fire-fighters of police officers are more likely to dislike daytime nursery noise reaching their dwelling.

Acoustic assessments can be made to estimate the potential impact at any given site, and the existing noise environment will most likely change the view of the likely impact.  Nursery noise assessments can be made across the UK, and Blue Tree Acoustics have recently worked on this type of assessment in the Sheffield area and in Peterborough.

Absorbing noise barriers

Noise barriers might be used to protect dwellings for BS8233 or BS4142 type assessments.  They might control road traffic noise, or rail noise, or even industrial and commercial noise radiating off towards noise sensitive uses.

Noise barriers might also be used for external live music operations such as festivals, or for noisy construction activities, particularly in city centre locations such as Manchester, Birmingham, or London.

Barriers with a noise-absorbing face are desirable, and can be an essential part of the noise mitigation strategy depending on the site.  If the surface of the barrier is reflective, then the noise will rebound elsewhere and potentially impact on a different adjacent area.  A reflection will also possibly allow noise to reflect a couple of times and bounce back towards the area that was trying to be protected.  There's no need to guess when calculations can estimate all of these issues and propose the most targeted solutions.


'Anechoic' means 'without echo'.

There are two ways of making a space anechoic.  Firstly, it could be placed away from reflecting surfaces - so, up in the sky would be anechoic.  Alternatively, it could be simulated by ensuring that all of the nearby surfaces absorb sound, rather than reflect sound.  If the sound is absorbed, then acoustically it is as though the surface were not there.

Measurements of sound emissions of plant equipment and machines or loudspeakers might require an anechoic location, depending on the aim of the assessment.

There are a few universities and other laboratory sites in the UK that have anechoic chambers.  Salford University, Liverpool University, Bristol University, Imperial College London, London Southbank University, University of Southampton, and Loughborough University all have such chambers, and there are additionally some company-owned sites that have them.

Anechoic chambers are expensive to build, but necessary for some tests.


Acoustic consultants are often working at night in strange, remote locations.  Sometimes on a undeveloped site near a railway line.  Sometimes on overgrown land near a derelict building.  Sometimes on a unoccupied building site.  Noise happens at night, and without measuring and assessing at night, the noise that occurs cannot be known.

Commonly, a noise consultant will be working alone in silence and there is a risk of injury or attack, a risk of a failing mobile phone or car, and a risk of medical emergencies occurring whilst being uncontactable.

Blue Tree Acoustics has various methods to quantify and address these risks of lone working, whereas many organisations do not.

Same-day reporting

There is a trend for sound tests (sound insulation testing) to be presented as 'same day sound tests' or similar.  Same day reporting equals unchecked reporting.  It is highly likely that same-day reporting will generate errors or incomplete reporting.

We are well-educated and experienced.  We do not report on the same day, as we check the data, check the calculation, carefully report the issues, internally peer-discuss the results, register the results with the correct national database, check the report before finalising it - and only then do we issue it to the client.

We could issue reports on the same day as a test was done, if we halved or quartered our professional duties by not checking anything.  The latter approach is unfortunately very common in the industry - but Blue Tree Acoustics does not participate in the race to the bottom of quality, ability, and experience that is ongoing at the moment.


Some European standards are concerned with LDEN, which is a calculated value trying to weight and combine daytime LAeq, evening LAeq, and night-time LAeq levels.  Evening levels have a 5dB penalty, and night levels have a 10dB penalty.  In the UK, we usually do not combine the levels and we assess different periods separately.

Pink noise

'Pink noise' is a strange term.  Sound like a detuned radio is white noise.  This static/hiss is random and effectively is all sounds, and if it were light, it would look white.  Pink noise has a particular spectral content that has more energy at low frequency.  If it were on the colour spectrum, it would look pink - as it is all sounds (white) with more low frequency content (red) added into the white.

Pink noise is used for various testing, including ISO140 and ISO16283 airborne work.


The Construction Skills Certification Scheme (CSCS) is a UK-based scheme trying to protect workers on building sites by making sure workers have some health and safety knowledge and therefore do not get injured at work.

This scheme changes often.  The CSCS Visitor card has recently been retired as an option, so currently acousticians must instead pass the same intensive CITB health & safety test as building site managers in order to gain either a 'Professionally Qualified Person' (PQP) CSCS card or an 'Academically Qualified Person' (AQP) CSCS card.

The logic of this is difficult to follow given that acousticians are not responsible for, and do not undertake, any construction activities on any site, and are only ever attending building sites in a data-collecting, visiting capacity.  Nonetheless, Blue Tree Acoustics consultants have each met either the PQP standard or the AQP standard, and hold current CSCS cards for such visitor access onto active sites across the country.


Acoustic design often conflicts with other requirements.  Soft sound absorbing ducts would be better for noise, but worse for hygiene.  Heavy self-closing doors would be better for noise control in schools, but would risk trapping children's fingers.

It is acoustically desirable to enclose noisy machines in concrete boxes, but this would stop airflow and the machine would typically overheat and break.

Competing needs must be balanced and a solution found for optimum design to suit all needs.

Noise cancelling

Noise cancelling headphones are now commonplace.  These work by measuring the sound waves coming towards the ear and attempting to add the opposite signal to the original signal to cancel out the energy.

This can be very effective for certain situations, particularly when the relationships between the microphone, loudspeaker, and ear are fixed.  If the relationships are not fixed, it is possible for the new energy to be in phase with the original, in which case the noise level heard will actually be increased.  Active noise cancelling is difficult to implement successfully in rooms and spaces, as the distance and angle relationships are ever-changing as a person moves around a room.

As technology improves, it will be possible to make it work for a person, but it won't work for multiple people.  It's possible to imagine a situation where a CEO has a workplace set up to be perfectly quiet to them as they move through the space, but for everyone else the random phase relationships of noise and anti-noise combine to make it at least as loud as it was in the first place.

Precision medical equipment

Some scanners, imaging equipment, testing equipment, and microscopes are very sensitive to movement.  Vibration in the structure from traffic, footfall, or other equipment can stop precision equipment working altogether, or can reduce the quality of the final results.

It is possible to install 'solutions' that have no effect, or even worse, that can amplify the issue and cause a greater magnitude error.  Autoclave equipment can generate high vibration levels and can transmit vibration through to hospital labs, blocking the correct operation of the measuring kit.

Vibration isolation is complex, and is another area where it is a mistake to employ a firm that is not sufficiently educated and experienced.

I was offered a job as a noise pollution officer...

...but I had to turn it down.

There are a lot of Environmental Health Officers in the UK who have to deal with various noise issues daily. Barking dogs, shouting, arguing, sites starting up too early or running too late. Propping open lobby doors during trading or failing to attenuate mechanical services kit. Noise complaints are common, and dealing with them can be difficult. At what point does a noise become a nuisance or a statutory nuisance?

Over the years, we have worked with the EHOs in Sheffield a great deal, as well as with EHOs in other areas such as Leeds, Hull, York, Rotherham, Barnsley, Wakefield, and Manchester.

Usually there is a reasonable solution to be found to resolve an issue. Sometimes a particular case goes to court for the matter to be resolved.

Forging and furnaces 

From the many high noise activity areas we have visited in order to measure and assess noise, forges can be amongst the loudest.

The operations are very hot, cooling of the air is difficult, and noise is intense and relentless.  Working conditions for a visitor are difficult, but for longterm workers the PPE and conditions can be especially tough.  Peak noise levels can be high and uncomfortable, even with hearing protection in place.

Newer technologies involve compression via hydraulic presses rather than via hammering.  This can lead to a large reduction in overall noise level and lower overall risk to staff and visitors on the shopfloor.

Building Regs of the past

Building regulations are not normally retrospective/retroactive.  Buildings are usually designed to meet the regulations in place at the start of the project.

Old developments probably do not meet current standards; certainly, they do not have to.  An old house converted into flats can have very little sound insulation between dwellings.  The 1992 ADE improved matters somewhat, and that version of the noise part of the building regs was in place until 2003.  The current version came into force in 2003 and has run for over 20 years.

We can remember testing under the 1992 Building Regulations, which had its own complications and quirks, but sound insulation testing became much more commonplace after 2003 and thousands of sites are now tested annually. 

There are two sound insulation testing schemes currently in England and Wales: the ANC and UKAS.  UKAS involves a firm being accredited and testers following the scripted approach that is written down and pre-approved.  ANC involves firms being competent, then individuals being competent and individually assessed and reviewed.  They do not have to follow a set of rules, but they can apply the standards and regulations to a site as they have a deeper understanding of acoustics compared to a non-acoustician UKAS tester.

To be fair, there is a small number of firms that have some acousticians and that choose the UKAS route.  However, many are not acousticians and choose the UKAS route.  There are none that are not acousticians and are able to follow the ANC route.

Greatest impact 

Various studies have been done, but a reliable method for determining impact of noise on humans is illusive.

Would you prefer one loud train that passes for 1 minute at night, or three quieter trains passing for 2 minutes each?  Would you prefer a loud bang every hour, or a constant noise all day every day that is fairly quiet but always there in the background?  Of course, most people can agree that they would like neither, but would you prefer no noise and a 10-mile walk to the shop to buy milk?

There have to be tradeoffs and give and take, but the line as to where 'reasonable' is is not simple.  There is a guidance document that sets out how to find whether low frequency sound exists, which in fact does not always find that low frequency sound exists even when it can be measured, heard, recorded, and complained of.

How much bass is too much bass?  The answer will be different depending on the location and the activity.  Usually a sufferer is frustrated where the sound is audible and annoying but the noise-maker does not take suitable steps to reduce the sound.

White van men

Noise surveys often involve standing near a road measuring sound levels, waiting for a white van to drive past and beep its horn.

It is almost certain that a measurement location clearly visible from the road will result in at least one beep and possibly a few shouts from an open window.

The basic protocol of noise surveys is that any data that is occurring because of the survey should be dumped from the dataset.  If someone shouts generally (not directed at the acoustician), that data stays in.  If someone shouts at the acoustician, that data does not stay in.

Acousticians having to stay silent throughout sound surveys so as not to contaminate the dataset with extraneous noise from our own speech is also why we can't take phone calls for most of any workweek.

Hydrogen noise 

Global warming solutions will likely tend towards wind turbines and solar panels extracting hydrogen from sea water, which is then piped into homes with hydrogen boilers replacing gas boilers.  Vehicles will probably also eventually use hydrogen engines.

We have not yet measured the noise emissions of a hydrogen boiler or hydrogen engine.   It is possible that the type of sound generated is unfamiliar and that people or animals do not respond well to the new normal.      

Multi-use spaces

A living room is usually just that.  A village hall, or a school, college, or university multi-use hall is probably many things.  One moment it might be a 5-a-side football pitch or tennis court, the next minute an examination hall, the next a venue for a classical music recital, or finally a rock concert venue.

The changing needs of the space can be very different and the acoustics of the space will probably not perform well for the whole range of likely activities.  Flexible solutions and innovation in design might be required for peak performance in acoustics.

Blue Tree Acoustics are able to deal with unusual situations and offer innovative ideas to solve complex issues.

Bin lorries and bins 

Sound power level statements are usually printed on the side of noisy machines.  89dB LWA is the statement usually placed on the site of a bin.

It is unusual to have this type of sticker on something like a bin rather than a machine.  The bin itself can make noise as the lid is opened and closed, or as it is wheeled across the footpath, but the sound power level sticker is unlikely to relate to reality as the bin can be used in a whole manner of different ways.

We have yet to undertake a noise at work assessment for a bin collection worker - but it would be interesting to know how the bin loading noise compares to the bin lorry engine and grinding noise, and whether the 89dB LWA statement bears any relationship to reality.

Institute of Acoustics Royal Charter application

The IOA is seeking chartered status.  What this means to acousticians is currently unknown.  Chartered acousticians may be around the corner.  The standard may be high or it may be low....

Pop Code 

The Noise Council's 'Code of Practice on Environmental Noise Control at Concerts' is a familiar tool for considering festivals and music noise from concerts.

A music noise level of 75dB LAeq is the usual starting place for the acceptable noise level at dwellings, assuming up to 3 days per calendar year for an urban stadium or arena.  There can be a lot of debate about the limits, and 65dB LAeq might be proposed or a noise level relative to background could be stipulated.

We have seen a wide range of different limits applied at different sites as Councils try to balance the desire to allow trading against the protection of local residents' amenity.  Some venues are unworkable to the Pop Code, but still put on events.

We have monitored in realtime at various festivals and concerts, and can assist with the noise management plan as well as measuring and monitoring noise levels during events.

Cattery noise

Noise from dog boarding, dog kennelling, and dog breeding is a familiar area of assessment.  Dogs in kennels can bark and whine, which can be audible.  Neighbouring residents might hear the noise.  What is the planning and nuisance balance for barking, etc.?  This area does require better guidance, but at least there is some guidance.

Conversely, cattery and cat boarding noise impact does not have guidance.  Cats are usually quieter, with mewing, purring, and hissing generally not being loud.  Cats can growl and vocalise while scrapping, while in pain, or while caterwauling, which can all make noise, but usually this noise comes from domestic cats interacting outside as they cross each other's paths.  This type of noise from interaction is what can wake people up at night.


Calculation of Road Traffic Noise (CRTN) does not take account of unmade roads or potholes.  Potholes seem to be on the increase, and may be partially linked to heavier electric cars due to battery weight.  Vehicles driving into potholes, speedbumps, etc. generate a different type of noise with higher maximum noise levels and a potentially jolting noise, rather than a more continual noise from free-flowing traffic.


Global warming is concerning.  Noise impacting on existing or future residents is unwelcome.

Energy and ventilation calculations tend to assume open windows to resolve potential overheating.  Noise assessments tend to assume closed windows for noise control.

These two approaches do not easily coexist.  HVAC, air conditioning, and cooling systems may be required to provide cooling whilst allowing closed windows for noise attenuation, but ironically these systems are powered and will use even more electricity.

Ideally, sufficient solar production will provide the power for the cooling during warm periods.  The solutions for the problems and possible innovation in the area are still developing.

Estate agents

Some areas of conflict in residential development are caused by sales teams not understanding sound issues.

Newly built attached dwellings will have to meet a minimum level of performance, but this is no guarantee that noise from next door will always be inaudible.  An unreasonable expectation of inaudibility can be set by uninformed salespeople, which often later results in occupants feeling dissatisfied and lodging complaints with the developer.

The planning permission for adjacent commercial uses should also be researched, understood, and explained to potential buyers/renters so that future occupants realise on what basis the nearby noise sources may operate before they sign a contract.

The planning permission conditions for the residential development should also be kept with the deeds and contracts so that the basis on which the development was granted planning permission is understood and made clear to the potential occupants.   

Audio recognition 

Recorded sound or real-time sound signals are able to be passed through a detection process to attempt to identify the type of sound based on some signature profiling - perhaps speech, perhaps a motorbike.

Some sounds have similar frequency profile content.  There are times when the context of the microphone location informs a human - more likely a train near a train track, or more likely a plane near an airport.

The detection systems currently make mistakes, and might always make mistakes.  It might be that AI will listen to a million trains as identified by humans, then be very good at identifying trains.  But what if it has never come across a particular birdcall before, or an emergency vehicle siren in a particular country.

Time of measurement

If the time setting on the sound level meter in use is wrong, then the data collected has problems.  Issues with time include a sound level meter timecode slipping compared to reality through a long set of logging measurements, a time being set wrongly initially, British Summer Time (BST) and Greenwich Mean Time (GMT) 1-hour movements presenting as an hour missing in the data or two 0100 hours measurements.

Care with time and with data is essential.  Curfews for concerts rely on the time being right - as Lana Del Ray discovered at Glastonbury 2023.

Bonfire Night

It is a strange time in terms of noise around Bonfire Night.

Suddenly anyone can buy explosives and make a lot of noise in residential areas (shouldn't be in a public place unless organised).  The noise might start days before the 5th of November, and it might last for days after.  Some people also seem to buy fireworks in the window of availability and then use them for a birthday celebration or similar later in the year.

Fireworks can be set off between 0700-2300 hours any day of the year usually.  This extends to midnight on Bonfire Night, and 0100 hours on New Year's Eve, Diwali, and Chinese New Year.  Given that it is possible any day, it is surprising that fireworks aren't heard every week.

Noise surveys around the named days above will be difficult to conduct, as this additional extraneous noise might dominate measurements and would not reflect a normal day with typical environmental conditions.

Processing improvements

Sound signal processing is always advancing.  Processing channels and multiple parameter analysis is one area of advancement.

Multiple frequency weighting channels can be run - A, C, and Z would be three.  Octave and one-third octave analysis might be additional channels, or might be filtered from the main signals.  Different time weightings would add more channels - fast, slow, and impulsive would be three.  Usually fast A-weighting is most important, but other measurements are needed for some calculations.  The risk of more capable sound level meters being developed is that the practitioner doesn't understand the complexity of the tool and doesn't capture the relevant data for the particular survey.

We have seen this happen in other companies' reports.  A lack of ability or care will lead to problems, and those problems will feed through into poor development.

Conversely, a wide dynamic range is beneficial to everyone, and a shift from 30dB or so to over 100dB over the past few decades is welcome.  Soon 200dB and no need for any selection will be common.


BS8233 gives some criteria relating to desirable noise levels in bedrooms, living rooms, and dining rooms. There are many other pages of content in the document.

This British Standard deals with noise control in and around buildings, specifically on an objective and quantifiable basis.  General criteria for conditions for sleeping/resting are stated, but it does not automatically follow that meeting those criteria guarantees that noise levels in a room will allow a person to sleep.

The standard says that it is necessary to remember that people vary widely in their sensitivity to noise.  That is to say that there is a Gaussian distribution, with some people sleeping on a noisy train and others failing to sleep in near-silence.  The guide states that the criteria might need to be adjusted to suit local circumstances, but does not explain how one should go about doing so.

The standard also states that, as important as psychological factors may be in response to noise, it is not practicable to consider them in that guide.  The issue is left unattended to.

Construction noise 

Sound Section 60 and Section 61 of the Control of Pollution Act 1974, as amended provide the legislative basis for the control of construction noise.

BS5228-1 gives some guidance on construction noise limits.  BS5228-1 does contain some logical issues and some hidden inaccuracies in the calculation methods, which must be understood and navigated.

As with many standards, the acoustician needs to decide which path to take and which calculation type is most representative for the likely activity.

Many construction sites have complicated and largely undefined activity.  Certainly, it would be unusual for the acoustician to know months ahead of the activity precisely which vehicle or machine will be exactly where, when, and making which noise.


Scattering of sound is diffusion that comes from diffusors.  Diffusors can be purpose-made or can be a feature provided by an element such as a chair or table.

Diffusion is helpful in many areas, and is assumed in some.  Omni-directional sound sources are not diffusors, and it is debatable as to whether they improve diffusion in test conditions.

Wind turbines 

Most will agree that energy prices are too high and that energy security is essential.  Some pundits are very negative about wind turbines, but these more sustainable energy sources must have a place in the future of energy production in the UK.

Currently very little new wind farm development comes onshore, with more being offshore, but most likely onshore development will need to be ramped up in order to meet targets.  On-shore means closer to dwellings and  then noise becomes a potential problem.  If the policy became that every dwelling within 1km of the turbine receives free energy, then that might offset any potential noise issues - certainly, if a turbine is proposed at close proximity with no benefit to locals, support for the proposal will be reduced.

ETSU is the standard for wind noise, with daytime and nighttime assessment.  It has issues, including wind speed gradient, but government and planning policy needs to find a way to allow for wind turbines to come into operation, preferably with local support and benefit.

Empty room RTs

Some standards either state or imply that tests should be done in empty rooms.  The question follows as to whether it matters what is happening in a room if no-one is in the room to be bothered by / enjoy / experience the sound.  There are few situations where it can matter if a reverberation time for example is x seconds when no-one is there.  It is reasonable to assume one person as a minimum.

Years ago, we dealt with a site in Leeds where the impossible situation was considered important.  Sometimes the reality of a situation must be considered rather than the absolute perfect case.

Acoustic awards 

We don't enter awards competitions.  We don't particularly value awards.  We do good work subjectively and objectively, and we don't need an award to know it.

There are awards available to those that apply for them, in the following categories:

Acoustics for inclusion
Building acoustics
Environmental acoustics: Infrastructure
Environmental acoustics: Non-infrastructure
Vibration prediction assessment and control
Small Consultancy

We do work in all of these areas, from schools and education facilities in London to multi-use developments in Leeds, from agricultural processing in Lincolnshire to data-processing and innovating coding across every site we visit.  

Monitoring of concert noise

Monitoring music noise levels from concerts can take a lot of energy and time.  Usually there are a number of locations to assess noise at, and these require walking from place to place over many hours of an event.  We have monitored at a range of concerts and festivals, including a 2023 Sir Tom Jones open-air concert.

Inaudible hearing damage

More research needs to be done regarding noise induced hearing loss caused by sound that is not well represented by the LAeq or LCpeak parameters.

It is possible for a loud noise at say 30Hz (which would have a -40dB or so A-weighting) to damage the hearing mechanism, but to be acceptable to the Regulations.

It is possible that A-weighting assessments are insufficient for noise-induced hearing loss dose assessments. 

It is possible that noise at 19Hz, thus typically 'inaudible', could also cause damage.

Reasonable resistance to the passage of sound

A 'reasonable resistance to the passage of sound' is the Approved Document E and Building Regulations requirement.  Sound insulation testing and demonstration of compliance with numerical values are the usual method of showing that this has been met.  Robust Details gives another route.

Light switches, toilets flushing, bath/shower noise, kitchen appliances and chopping noise, footfall across separating walls, are all not listed in the regulations and have no Building Regulations standard to meet.

An airborne sound insulation test is the only requirement for attached dwellings, and this only considers noise made into the air.  Television noise, speech, telephones, music, vacuum cleaners - these are all airborne noise sources.  These sources are not required to be inaudible; it is only required that the separating wall reduce the level of noise by the required amount when tested in accordance with ISO 140 and ISO 717.

It is advisable to test and/or experience the sound insulation performance of the structure prior to building the dwelling or signing a lease document.  Of all of the potential issues with dwellings, noise is amongst the top complaint. 

We are registered and peer-reviewed for sound insulation testing.  Our testing is automatically accepted by NHBC, all Building Control departments and approved inspectors, and warranty providers LABC, Premier, Zurich, etc.

Little Red Book for other territories

We have been asked at least twice, maybe more, if we can write a Little Red Book of Acoustics for other countries.  The short answer is that we could.  The longer answer is that it is difficult and takes a lot of time to put any book together, and there is not an immediate need for us to act now.

Copyright infringement is a growing issue, and book users need to have a physical copy of the book.  There is no digital format publication of The Little Red Book of Acoustics - so if you possess a digital copy, you are infringing our intellectual property rights.  Infringing our IP rights will probably lead to us not writing additional versions in the future.


Sound can convey information.  Speech is an obvious example, but there can also be bleeps signifying that a process has started or stopped, and information as to whether or not a machine is functioning or whether it is running correctly.

Speech is the most important, as it conveys the most information.  Speech being audible in the wrong places, or by the wrong people, can lead to problems.  The police, politicians, and the military need to be able to make decisions without information being leaked, either intentionally or accidentally.  Sound insulation of walls, doors, windows, and services, as well as microphone and intercom/address systems, are important.  

As ever, technology gets smaller and better, and privacy can be breached with phones, recorders, and transmitters.  Privacy in courts, tribunals, interview rooms, and boardrooms is vital, and must be considered and tested correctly.  


Inexperienced practitioners often fail to use equipment properly.  Anyone can buy a sound level meter or a dose meter/dosimeter or noise meter, but using it correctly is unlikely without training.

Council workers, start-up consultancies, and industry-adjacent consultancies seeking to expand into acoustics, often get it wrong.  There are processes and steps.  There are protocols and procedures.  The role is not for everyone, and it is probably fair to say that more than 50% of people using a sound level meter are underqualified and underexperienced.

Calibration of equipment is vital.  This must be done either side of measurements, not at some point earlier in the day after which the kit was turned off and moved.  The sequence should be: turn on, calibrate, measure, calibrate, turn off.

Concert noise measurement

One-off events can be difficult to assess and measure.  If the main act takes to the stage when the heavens open, then measurements of the noise levels reaching dwellings will be difficult, and will contain residual noise caused by the weather.

Correctly gathering data over a 12-hour concert is not easy.  Equipment, parameters, locations, notes, the public, weather, safety, security, and feedback to front of house are all being considered, plus the physical effort/endurance and the probable lack of facilities.

A 75dB LAeq Music Noise Level (MNL) cannot be measured directly unless there is no residual noise - which is rare in urban or festival locations.

Generator noise 

Building sites, temporary operations, concerts, and festivals all need electricity in order to function.  Some power might be provided by solar sources, but usually there is a generator or two running to give sufficient wattage for the consumption of the build or the show.

Generators make noise, and the siting of the generator is important.  Locating it up against a neighbouring dwelling will most likely cause issues.  Generators also make fumes, so that is another reason to keep generators away from dwellings.

Wind noise

The wind makes noise in a few ways.  It rustles trees and bushes, it blows litter around, it makes fences, bins, flagpoles, and streetlights bang.  It also makes noise on microphones, such that the measurement made does not reflect the true noise level experienced.  This can be experienced with mobile phone microphones used outside.   

NIHL sound testing 

Hearing damage from sound insulation testing is possible.  Sound pressure levels over 100dB(A) are experienced for more than half an hour, which is potentially damaging.  Hearing protection is worn in the source room to protect the tester.  Noise levels in adjacent rooms are high, but are unlikely to be damaging as the exposure would be 85dB(A) for an hour or so, which is <80dB(A) over 8 hours.

85dB(A) for 8 hours  = 88dB(A) for 4 hours = 91dB(A) for 2 hours = 94dB(A) for 1 hour  = 97dB(A) for 30mins = 100dB(A) for 15mins.

Also 85dB(A) for 8 hours  = 82dB(A) for 16 hours

Internal walls and floors

Newly built dwellings or newly converted dwellings have to meet Building Regulations requirements.

One part of ADE is a requirement for internal walls and internal floors.  The level of sound insulation performance is not tested onsite, but the construction build-up used onsite must have a laboratory-tested sound insulation value of 40dB Rw.

The design must comply, and then the installation of the design must be well implemented.  This requirement does not apply to every wall within the dwelling.  The 40dB Rw requirement attempts to protect bedrooms and to protect other rooms from toilet areas.  The protection against sound within a dwelling-house requirement is ADE E2.   

What is true online 

All of this content has been created by us - by humans.  There is a lot of incorrect information online, and the more you search the more you find.

There is a website stating that you can hear a 100dB sound at a maximum of 16 metres.  Well, assuming the 100dB occurs at 1m, from the source, 2m would be 94dB, 4m would be 88dB, 8m would be 82dB, 16m would be 76dB, 32m would be 70dB, 64m would be 64dB, 128m would be 58dB, 256m would be 52dB, 512m would be 46dB, 1024m would be 40dB.

This would usually be audible, and only wouldn't be audible if there was a decent amount of other noise at that location.  So over a kilometre is a bit more than 16m.  The Silvertown explosion was heard 100miles away.

Whispering walls

There are a few locations around the world where the curve of a wall is just right and a sound can bounce along the curve so that a signal, music, or speech can reach and be heard much further away than it usually would be.

St Paul's Cathedral in London has one, Gloucester Cathedral has one, Birmingham University has one.  20log distance attenuation does not apply.  There must be one in Yorkshire, but we can't think of one in Sheffield, Leeds, or York.  There are some disused railway tunnels around Sheffield where sound will reflect over large distances.    

Sounds of Sheffield  

The Human League, Joe Cocker, Def Leppard, Pulp, and the Arctic Monkeys are some of the big names to start and play in Sheffield.  West Street, Cambridge Street and even Snig Hill all have or had venues.  The Arena, Don Valley, Sheffield Hallam University, University of Sheffield, and of course The Leadmill have all hosted large shows over the years.  Tramlines tends to be in Hillsborough, recently off Middlewood Road.

We have worked with some of these venues and dealt with development near all of them.  Music noise impact is becoming a larger issue as technology increases and high sound power levels become cheaper to achieve.

2.5 inches of mercury

Krakatoa is supposed to have made the loudest noise ever experienced on earth in 1883.  Measuring 100 miles away, it is reported that a barometer measured a pressure of 2.5inches of mercury, which is 8465Pascals.  This is 173dB at 100 miles or so, so around 160,000 metres, that calculates to 277dB at 1 metre.  That is beyond the point where it is acting as a sound wave - it is a shock wave.

The Who are reported to have played at 126dB in London.  Leftfield reportedly played at 137dB at the Brixton Academy.  Probably others have played louder.  Typically 100dB(A) at the front of house desk is sufficient.  NIHL risk increases as the levels increase above around 85dB(A).  

The RNID Don't Lose the Music campaign attempts to inform musicians and music lovers that hearing loss caused by loud music is largely avoidable and that hearing loss is not insignificant if it happens to you.        

Airport noise  

Aeroplanes can generate significant levels of sound.  The issue is usually that residents near airports get a significant noise contribution as aircraft land and take off.  Living near Gatwick, Heathrow, Birmingham or Manchester airports will mean increased noise levels compared to not living near airports.

This noise was much reduced during lockdowns, and now leads to additional annoyance as flights are back to normal levels.  Eventually technology will reduce aircraft noise significantly, else the number of flights will have to reduce to attempt to reduce global warming related fuel burning emissions.

6dB rule

The ISO140-4 6dB rule relates to sound pressure levels in the source room measured in adjacent 1/3 octave bands.  The reason for the rule is to try to prevent signal bleed from one octave band into another.  Bands do not perfectly reject sounds outside their range, and so sound in the adjacent 1/3 octave can influence the sound levels in the band in question.  If the source sound levels are flat from 100Hz to 3150Hz, then the bleed does not matter.

Tapping machine  

ISO140-7 requires 5no 500g weights, each falling the equivalent of a 40mm free-fall distance every 100ms.  That is 10 bangs per second.  The 40mm free-fall requirement means that the actual drop height will most likely have to be 41mm or 42mm, as the system will have friction.  There are more requirements to standardise the test so that every tapping machine gives the same test signal within permitted tolerances.                

Line Source

A line source radiates cylindrically and is a 10log distance attenuation which is 3dB per doubling of distance.  So standing adjacent to the M1 in Sheffield or Nottingham, or the M62 in Leeds or Manchester, as you move from 50m to 100m away the noise only reduces by 3dB, everything else being equal.  From 100m to 200m the noise would reduce another 3dB, so 50m to 200m would be a total of 6dB reduction in noise level.  Line source attenuation might be factored into BS4142 or B8233 calculations for NPPF or ProPG for example.

Printing presses  

Printing presses are traditionally very loud but laser printing has reduced noise levels considerably.  In our experience printing press workers have had noise exposure reduced by 6dB or so in the last few decades compared to the previous work environment.    There remain some inherently noisy activities and machines in factories and press locations, but workers tend to be more remote, controlling from afar.  Digital systems on computerised control is usually quieter than mechanical systems.  Emails and digital publications also reduces the amount of printing needed.  Hopefully the industry does not die altogether and can find a way to operate without damaging hearing.             

Sound insulation testing following complaints

Once dwellings are built and occupied, the residents might complain that they can hear more noise from next door than they think they should.  We have undertaken many tests for large housebuilders to establish the objective sound insulation performance of the separating wall or floor.

The Building Regulations do not consider if the neighbour is a DJ, or a librarian, or perhaps both.  The Regulations do not consider what the lifestyles of either household are.  Children, shifts, type of people - are all not considered.  The sound insulation performance of the structure is the only area tested.  It can be that the wall passes, but that the neighbour is loud.  It can be that the wall passes, but that the type of sound causing the issue is not tested.  It can be that the passmark is considered too low by the residents.

It can be that the structure does not pass the minimum standards and that remedial work is required.     

Thunder and lightning   

Sound travels at around 344m/s.  Light travels at around 300,000,000m/s.  So if an event was 344m away, it would take  | 0.000001seconds for the light to arrive, so more or less instant, whereas it would take a second for the sound to arrive.   1km away would be more or less instant for the light, and about 3 seconds for the sound.  5km away would be again more or less instant for the light and about 15 seconds for the sound.  So roughly divide the time between the flash and the bang by 3 to get km.

Miles is roughly the time between flash and bang divided by 5.        

That's how you can estimate how far away lightning is using the speed of sound.

AD plant biogas

One way to produce fuels without the need to take fossil fuels is to use anaerobic digestion plant to turn food waste and other energy-rich material into biogas and fertiliser.  Biogas is a renewable fuel, and its production reduces carbon emissions.  The process itself is quiet, but heat and power units can generate noise that might impact on local residents.  BS4142 assessments might be required to establish that the impact is suitably low when Rating Sound Levels are compared to Background Noise Levels.


c = fλ
Which is:
speed of sound = frequency * wavelength

c /f= λ

c is around 344m/s, so 20Hz has a wavelength of 17.2m; 100Hz has a wavelength of 3.4m; 20kHz has a wavelength of 17.2mm.

CIBSE Guide B5

'Noise and Vibration Control for HVAC' by The Chartered Institute of Building Services Engineers (London, 2002) is a useful document that sets out some rules of thumb relating to mechanical services noise.

The introduction states, "This publication is primarily intended to provide guidance to those responsible for the design, installation, commissioning, operation and maintenance of building services. It is not intended to be exhaustive or definitive and it will be necessary for users of the guidance given to exercise their own professional judgement when deciding whether to abide by or depart from it." 

Advice regarding fans, pumps, generators, boilers, lifts, escalators, and a wide range of other noise-generating equipment are all included in the document.

ISO140 BG correction

ISO140 says 'background' when it should really say 'residual'.  'Background' usually equates to LA90, whereas in ISO140 it is an LAeq in the absence of the specific sound, and really this is 'residual'.

There are three equations for correcting the receiver level depending on the difference between the receiver level and the background level.  This is commonly confused, and we have troubleshooted various spreadsheets and calculation models used by others that do not implement the calculation correctly.  We have seen examination questions that implement the corrections wrongly as well.

For a difference of 10.0dB or more, there is no correction.
For a difference of less than 10.0dB, but more than 6.0dB, there is a log subtraction correction.
For a difference of 6.0dB or less, there is a fixed 1.3dB correction.

The standard is slightly unhelpful, as its author fails to present the decimal places correctly, but the above is the correct implementation.

>=10.0dB no correction
<=6.0dB 1.3dB correction 
Anything else is log subtraction correction, i.e. <10.0 to >6.0

If these are coded wrongly, then the wrong equation will be triggered and the wrong value found.  We have presented at conferences about this and the surrounding issues multiple times.

Floating point errors are common and remain misunderstood, again despite this being the subject of various presentations.  Implementation of ISO140 is often poorly done.

Japanese impact ball

Some countries use a 2.5kg ball that is usually dropped from 1m height onto a walking surface to generate an impact force, and thus low frequency noise, in order to test separating floors.  The impact signal is usulaly <=500Hz, with most of the energy occurring at lower frequencies.  There are a few obvious disadvantages to this compared to tapping machines.  There needs to be a method to release the ball at the time the measurement is ready, two testers are usually needed, and there is only a moment when the energy is released rather than a number of bangs each second - so there is no averaging over time, which does occur for tapping machine measurements.

Spring CCENM

The Spring 2023 IOA CCENM results are out for our Sheffield South Yorkshire course.  We retain our 100% pass rate and are looking ahead to the Autumn 2023 IOA CCWNRA course, which will also be in Sheffield.


Data has limitations.  Every item of equipment has a tolerance.  Calibration equipment is correct +/- an amount; sound level meter equipment is also correct +/- an amount.  Therefore all data is correct +/- an amount, and is not precise.

The imprecision is probably on the order of tenths of decibels, but might be a whole decibel for Type 1 or Class 1 sound level monitoring equipment.  This is for a given location, at a given time, in given conditions.  When the range of locations, times, operations, and weather conditions are considered, the +/- tolerance increases further in terms of what is 'typical' or 'normal' or 'reasonable', but the reality of the data captured remains within 1dB tolerance.

The question is what range of conditions are possible from day to day.  This might be unknowable.  Should the absolute worst-case event on the worst-case day be considered?  Should only the average activity on an average day be considered?

Acousticians are not usually statisticians, and can misunderstand statistical mathematics.  Many attempting to be acousticians make fundamental errors in data processing and do not take sufficient care in reporting.

Street sweeping

We have undertaken hundreds, if not thousands, of sound surveys on the streets of Sheffield, Rotherham, Barnsley, Leeds, Manchester, London, etc.  One common source of high sound pressure levels is street sweepers.  They can be very loud and they can rework the same area multiple times in an hour.  Dustbin lorries can also generate high levels of noise, as can modified cars with large exhausts.  Other sources include high noise flights, such as Concorde (although Concorde itself no longer operates).  One survey with elevated noise levels was in Nottingham, where a break-in took place during the survey period.  Other odd examples of increased noise levels include streakers at sporting events, when crowds react differently.  In most sport, spectators make more noise when events occur, i.e. goals, points, etc.    


Flights have restrictions in terms of times of day and routes of flightpaths, to try to keep noise away from dwellings and residents as much as possible.  Rivers and main roads might be followed, rather than a direct flight path that might otherwise route directly over dwellings.

When cars are all driverless and computers control the routes, it is possible that systems will be developed to only permit some routes and some flows at some times, so as to ensure a noise limit is met at a certain location.  It is possible that a limit of, say, 1000 vehicles per night on a particular A-road will automatically force the 1001st vehicle to route via another road in an attempt to reduce noise levels.

There will be a tradeoff with fuel use, so the algorithm must balance the different elements when directing traffic.  Logically, a more polluting vehicle will be allowed to drive more directly to minimise emissions.

Quietest places

Without first-hand information, we don't know where the quietest place on earth is.  It will most likely be an anechoic chamber in a developed country such as the UK, USA, Germany, France, Japan, etc.

These spaces are installed in reasonably quiet places, are kept away from traffic sources, and are designed to have thick walls inside a building with thick walls, vibration isolation, and acoustic absorption.

Anechoic chambers can achieve less than 0dB(A) .  Microsoft has a chamber that is reportedly rated at -20dB(A).   This is much quieter than any natural location on earth.

Quiet locations on earth need to be remote from animals, and remote from transport and human activity, and remote from rivers and seas.  The Antarctic on a still day will be one such place.  Similarly, mountain tops will  typically be quiet when it is calm.  Dormant volcanos might also be quiet, as might large craters.      

28 days permitted development 

The permitted development right of Class B allows a temporary use of land for any purpose for up to 28 days in any one calendar year without the need to make a planning application.  

There are two main exceptions to the 28-day rule:

  1. If the land is located within a Site of Special Scientific Interest (SSSI), then it cannot be used for motorsports including motorcycling or ‘war games’ such as clay pigeon shooting.
  2. Of the total 28 days' permitted development, only a maximum of 14 days can be used for the purposes of holding a market, such as car boot sales or for motorsports. 

That said, permitted development cannot permit nuisance, so it might be that the activity can take place for 28 days so long as there is not significant noise impacting on the neighbours.  

Clearly, the type of activity probably matters, and the relationship between the activity land and the surrounding land matters.  Seeking planning advice is sensible, as planning matters can be nuanced, and if noise assessment are needed then we are here to assist.

Dawn chorus

Depending on the location, the dawn chorus of birdsong can be very loud, and can be the dominant sound source in the area.  Certainly, remote areas of Yorkshire, Lincolnshire, Cumbria, and Derbyshire will most likely have a significant increase in sound levels half an hour or so before sunrise.  Farming areas can also have increased levels of noise at times that are different from city/developed areas.    

The change in these noise sources and timing of these sources might effect the data collected on a given day in a given location.

Transmitting a pink noise signal 

Some testers send a pink noise signal via bluetooth / wifi / radio from the device in their hands to the amplifier and loudspeaker.  This introduces complexity into the signal chain.  There is potential for the signal to degrade or be corrupted, and for the test to be non-compliant.  There is no need for a test signal to be broadcast through the building.  It is unnecessary and likely to lead to the signal cutting out.

Suitably Qualified Acoustician (SQA)

Various standards and guidance documents are starting to require suitably qualified acousticians.  This is an attempt to require actual qualified experienced acousticians, rather than people with lower or no qualifications.  BREEAM, ProPG Gym noise guidance, etc. are starting to move in this direction as acousticians try to improve standards.

MUGA sports  

Multi Use Games Areas (MUGAs) are growing in popularity.  Basketball, football, cricket, etc. can all be played from the same one space.  MUGAs usually have hard metal wire sides, and do not make too much noise when the ball hits.  Hockey can generate moderate levels of noise as hard hockey balls strike the sides, particularly when players miss goals.  We successfully dealt with hockey noise for a site in North Yorkshire recently. 

Football and rugby are usually quieter, but fans and supporters can make noise.  Players can shout and can swear on occasion when the game is not going their way.  School sites can have this type of noise source, and we recently dealt with an extension to a games area for a school in West London.

Skate parks can be indoors or outside, and there will be some noise at source as skateboards bang on and scrape across surfaces.  We have assisted with various indoor and outdoor skate parks and similar venues across Yorkshire and beyond.


Overall sound insulation values are given in Rw terms.  This is a single figure that generally describes how much sound the glass will block.  This is not enough information to know what will actually happen inside a room as the sound strikes the window inside.

The frequency content of the sound source is all-important.  If there is a lot of low frequency noise, then the glass will not attenuate as much as the the Rw figure suggests.  If there is a lot of high frequency noise, then the glass will stop more than the Rw figure stated.  Glazing sound insulation values must be known and stated in terms of octave band performance.  These values should be used in any high quality calculation.  We always undertake octave band calculations in accordance with BS8233 unless a planning permission condition or Local Authority require or imply a different method should be used.

Decent calculations are more complex and require more skill, but they are more accurate and should be done as standard.

BS4142 shredders

Machines that can churn up and shred materials like trees and pallets can be very powerful, and can generate high levels of noise.  Having this type of specific sound source in close proximity to dwellings can cause issues.  Shredders can be used to feed boilers and generators, and to provide heat and power, which can be a good use of waste material.  This type of noise would usually fall within the scope of BS4142, and the Rating Sound Level should be compared to the typical Background Noise Level.  A difference of around +10dB or more is an indication of significant adverse impact.  A difference of around +5dB is an indication of adverse impact.  A difference of around +0dB would be seen as low impact in most cases.


Speedway and other motor sports can generate high noise levels.  Some sport types have handbooks and methods for assessment.  There is a 2 metre max method for some activities, that is 100dB(A), 96dB(A), or 94dB(A) depending on the bike.  Some post race requirements are 112dB(A) or 115dB(A).  There can also be a 81dB(A) at 100m criterion.  There might well be planning permission conditions relating to the activity, and these might be written in terms of noise level at 2m, 10m, 100m at the boundary or at the nearest dwelling.  

Radio waves

Radio waves are electromagnetic waves.  They have frequencies around the low GHz.  Radio waves travel at the speed of light 299,792,458 m/s.

Sound has a frequency around the hundreds and thousands of Hz.  Sound waves travel at the speed of sound, which is around 344 m/s in air more or less, depending on temperature, etc.

A concert being recorded/transmitted by the BBC or similar can have the sound picked up on microphones on the stage, then transmitted via radio.  The signal could be received and replaced through loudspeakers before the natural signal arrives at the rear of the venue, as it would take, say, 0.2seconds for the natural acoustic sound to arrive, whereas it could take less time for the signal to be turned into an electrical signal, transmitted, received, then turned back into an acoustic signal.   

If a listener was positioned 1km from a sound source, it would technically be possible to send a radio signal and bounce it off the moon and back to the listener before the sound signal arrived.

This slow speed of sound is also used at concerts where repeating loudspeakers can be placed further into the audience and the signal can be played to align with the natural sound passing that point.  The signal would be delayed to make it align as the electrical signal is moving much faster than the soundwave is.

Affordable housing 

NPPF says that, where major development includes the creation of housing, at least 10% of the housing should be affordable housing.  'Affordable homes' can mean a few different things, but starter homes for first time buyers and homes discounted 20% below market value are the usual types falling under the affordable homes banner.  

'Affordable' is a difficult term to deal with the cost of houses compared to income, i.e. the house price to earnings ratio has increased from around 5 in 2000 to 9 in 2020.  The graph is interesting and shows around 12 in 1845, dropping to 2 around the First World War.  From there, it creeps up to 9, now.

Acoustics is one of the industries that no doubt increases house prices.  The cost of building a house increases if acoustics are to be considered.  Equally, it increases if thermal efficiency, or fire ratings, or any other Building Regulations issue, is properly considered and applied.  This is not a bad thing, as most would rather pay a little more for a house, but be able to sleep, and be able to escape if there were a fire.  Other less desirable factors also influence house prices, and these should be dealt with before standards are lowered in order to reduce costs.

As global warming increases, we will probably need additional regulations to protect residents from heat, and perhaps planning regulations will require particular types of development in particular locations to reduce emissions and protect dwellers.


'Mission Impossible: Dead Reckoning Part One' has a plot where AI collects information and is in effect always listening in to conversations and data streams.

Listening in to signals is not new.  There is benefit to having information and knowledge, and governments, companies, and individuals have been spying on each other since cave man times.  Technology develops and  its complexity increases, but listening in is not new.

For example, The Thing was a listening bug that was covertly transmitting audio from inside United States government buildings out to spies outside.  Similar to the Trojan Horse tactic, it was concealed inside a wall mounted crest gifted by the Soviet Union to the United States Ambassador to the Soviet Union in 1945.  It was a passive transmitting device, so it didn't need batteries or a power source and could run for a long time.   It was small and didn't contain any active electronic components, and was difficult to detect.

Sound waves from conversations in the office passed through the casing and made the membrane vibrate.  This movement altered the capacitance of the system, and this variation was transferred into radio signals, which were radiated out from The Thing. The radio waves could be picked up outside the room, although probably not at great distance as the signal strength would be low.  The received signal would then be decoded and turned back into sound waves and listened to.

We don't advise on devices listening in to conversations, but we do advise on separating structures that allow sound to pass directly out of a sensitive room.  There are experts who deal with the radio / digital transmission issue, and it is possible to create a room that does not leak signals in any way.  Military sites clearly have a need for this, but other sensitive uses will also increasingly need to improve their defences against lost / stolen information as smart devices become more and more ubiquitous.

Historic buildings

Approved Document E has a development type where the normal standards might not be required.  The logic is essentially that if a building is being converted (i.e. a material change of use is taking place) to create one or more dwellings, but the building is special or the location is special such that the measures needed to meet the normal sound insulation standards would not be possible as they would interfere with the protected features, then the standard does not have to be met.

Historic buildings can be Listed, or in conservation areas, or buildings of architectural and historical interest that are referred to as a material consideration in the Local Authority development plan, or buildings of architectural and historical interest within national parks, areas of outstanding natural beauty, and world heritage sites, or vernacular buildings of traditional form and construction.

If it is not practical to improve the sound insulation performance to meet the stated values because of a need to conserve the special characteristics of the building, then the work should aim to improve sound insulation where possible, and by as much as possible.  Local planning officers and historical buildings experts would be able to advise on what is and is not possible, given the features of the building.

To be clear, the Regulations do not say that old buildings do not have to meet the Regulations.  They say that if, for example, you have a Listed floor and a Listed ceiling and cannot alter them, then it might be reasonable  to not meet the normal standards as doing so would not be possible without damaging the structure.

Whatever the performance of the building actually is should be correctly tested and recorded and stated in a conspicuous place inside the building - presumably so that estate agents, buyers, renters, mortgage valuation firms, surveyors, etc. will be able to see that the separating wall or floor does not meet the normal requirements and they are then in a position to decide to buy / rent / loan, etc. accordingly.

'Historic building' status is often misunderstood and misused, and it does not automatically apply to an old building.  Indeed, it does not apply to most conversions, and cannot apply to new build development.

Stolen equipment

Sometimes acoustic equipment is stolen.  The industry is poor at setting out what has been stolen and allowing owners and prospective buyers to check a central database.

We have asked the IOA to list stolen equipment on their website, but they have not set this up.

In order to improve the situation, we are offering to list stolen sound level meters, Class 1 microphones, Class 1 calibrators, tapping machines, omni-directional sound sources, pink noise generators, etc. here to show the serial numbers, the dates stolen, etc. and we will put the potential buyer or finder in touch with the owner.

We have a track record of contacting a few owners who had equipment stolen when we have come across the kit for sale online.

The acoustics industry needs to do more, and we make the offer on this page to list and try to connect.

Lossless formats

Audio files can be lossless or lossy.  Lossless formats do not throw data away, and they preserve the original data.  This does not mean that they are perfect recordings, but they do not additionally lose information.  Lossy formats throw some data away; they try to intelligently dump any data that is not overly important, such as sound that can not easily be heard given other sound that is being made.  This is compression and is irreversible - once the file is saved without the data, that data is irretrievable.

There are various lossy formats and various levels of data quality.  As lossy formats are impressions of the original, there are choices to be made.  It is usually easier to consider images when thinking about lossy audio files.  For example, a perfect photo of Vincent van Gogh's 'The Starry Night' looks almost like the original does.  Compression and lossy files will lose some of the information that is not too obvious.  Greater and greater compression will make the image less and less like the original, gradually losing definition, reducing the number of colours and the number of pixels until the final ultimate maximum compressed image would be one block of blue.

WAV is an uncompressed, lossless, linear pulse-code modulation format developed by IBM and Microsoft.

AIFF (audio interchange file format) is an uncompressed, lossless, linear pulse-code modulation format developed by Apple Macintosh.

These are both high quality, but they generate large file sizes.

MP3 (MPEG-2) is a lossy, compressed format developed by the Fraunhofer Society/Karlheinz Brandenburg.  It has around 80% compression. 

AAC (advanced audio coding) is a lossy, compressed format developed by Apple Macintosh to rival MP3.  It is usually said to sound better than MP3, but that is debatable.

ALAC (Apple Lossless Audio Codec) is a higher quality format developed by Apple further to AAC.  The entire Apple Music catalogue is also encoded using ALAC in resolutions from 16-bit and 44.1 kHz (CD quality) to 24-bit and 192 kHz.

44.1 kHz 16-bit stereo format is CD quality.  WAV and AIFF can be 48kHz and 32-bit stereo, which is better than CD quality.  MP3 can be 48kHz and 320kbps, but even then CD quality would be better.  The Audio CD bitrate is always 1,411 kbps 16*2*441000.

The 'bits' in lossless formats are to do with how many possible values can be assigned to the value of the wave at the given moment.  If the system could select only two values - 1 or 0 - it would be 1 bit, and would not describe the sound very well.  With 16 bits, there are 65,536 possible values of the wave, which allows for 96dB, which is 20log(65536).  24 bits would have 16,777,216 possible values, which allows for 144dB, which is 20log(16777216).  The sampling rate for lossless formats is how many times per second the waveform is measured.  44.1kHz is needed to be able to encode up to 20kHz in audible sound.  48kHz is the other usual default that gives a little more detail, and 96kHz is also used.

32-bit float is something slightly different.  It can capture 1,528dB, which is more than is possible for a soundwave to be, so there is plenty of scope.  Compared to fixed-point 16-bit or 24-bit, which work on 2^16 or 2^24, 32-bit float files store numbers in a floating-point format.  This a fundamentally different method than the fixed point.  32-bit float stores data points with 'scientific notation', i.e. 1.2312*10^7 rather than 12312000, which opens up many more numbers for the same number of characters.  The first bit indicates a positive or negative value, the next 8 bits indicate the exponent, and the last 23 bits indicate the mantissa.  IEEE-754 sets out the format.  In the example, 1.2312 would be the mantissa and 10^7 is the exponent.  The mantissa holds the main digits and the exponent effectively defines where the decimal point should be placed.  1,528dB is massive and gives all the headroom that could possibly be needed.  32-bit float is actually 24-bit quality but with range benefits.
96kHz 24-bit lossless audio is very high quality.

There is a place for audio compression, but it should be understood and used with care.  Sound insulation testing does not need to use compressed lossy files; there is no need for it and it can only reduce the quality and introduce potential errors.  Likewise, transmitting sound signals via radio  / bluetooth / wifi for sound insulation testing is unnecessary, and is not done by careful experienced testers.     


We do not hear equally across all sound frequencies.  We also lose the top end of the sound spectrum as we age.  The Mosquito device was made to take advantage of this by playing a loud noise at high frequency that is typically inaudible to adults, but audible to children - the idea being that it would be audible and would annoy children, and therefore if played in or near a shop, it would discourage them from hanging around and being a nuisance.

One problem with this is obviously that this noise is also audible to children who aren't hanging around being a nuisance, such as toddlers and babies.  It is a grey area as to whether or not this is strictly legal.  The devices are not currently banned, and are usually not playing noise that could damage hearing.  It must be possible for the Environmental Protection Act to be used to get a particular device removed from a location, but we are not sure if this has ever actually happened.    

Languages for NIHL

Presbycusis is age-related hearing loss, where high frequencies are lost from the high frequencies downwards.  Noise-induced hearing loss is usually experienced most in the 4kHz region of hearing.  This means that the detail of speech in English is lost.  Consonants are difficult to distinguish, and words such as 'shop' and 'shot' are confusable.

As different information is encoded in different frequency registers in different languages, the effects of presbycusis and NIHL will also be different in different languages.  It must be that some languages have a massive issue if certain frequencies are lost, and it must be that other languages are less affected.  As we write, we do not know how to order this list.  English is unlikely to be the most affected.  European languages are likely to be similar, but it would be an area of interest to research across the globe.  If all the information is contained in the low frequencies, then hearing loss would have less of an effect.  However, it would seem that there would not be enough different sounds available to a speaker to say enough words and phrases.

Railway noise 

Trains make noise, and this has been true since the first steam engines started moving materials from mines and around industrial works.  Stephenson's Rocket is probably the most famous of the original trains that won the trials on the Liverpool to Manchester railway in 1829.  Steam trains eventually gave way to diesel trains, which are slowly giving way to electric trains.

Electric trains, like electric cars, generate less noise compared to their petrol and diesel equivalents.  Trains have the metal on metal interaction with wheels on tracks, which will probably remain until / unless a maglev (magnetic levitation) system replaces the UK railways.  This is not coming soon.  UK railways have a 125mph speed limit.  Trains in France can run at 186mph, and trains in Japan can run at 199mph.  The Channel tunnel lines in the UK can run at 186mph too, but that is not the UK-wide network.

The fastest trains currently being developed can run at 374mph in Japan.  It is hard to think about what this looks and feels like.  125mph feels very fast when you are standing in a field next to a line.  374mph will feel almost impossible.  We have not measured train noise above 125mph, as we have not measured train noise adjacent to the Eurostar line in Kent, and the UK generally has slow trains.

It will be interesting to measure noise and vibration of faster trains, and more interesting again to measure maglev trains.  We have read reports suggesting that maglev train noise is around 5dB quieter than non-maglev trains.  Superfast trains will have the ability to startle people, such as residents and office workers, patients or surgeons.  We have evolved to expect a certain pace of life, and to anticipate objects travelling at animal movement speeds.  374mph might be shocking as the train is not anticipated, then it is suddenly adjacent to you in a burst of energy, then it is gone again into the distance.

Maximum noise levels will be the determining parameter and will attempt to describe the human reaction to the noise, but it might be that a new parameter considering the rapid change (i.e. the onset of energy) will be needed to properly correlate to human response.  It might be that vibration levels are no higher than they are currently, and exposure times will reduce as the time a train is at close proximity will reduce.

It might be that trains reduce in popularity as self driving cars take over.  It might be that trains begin to dominate as the world warms and individual transport becomes unsustainable.                

Cinema sound levels

Sound systems improve over time.  Dynamic ranges of equipment, whether in recording, in storage, and in playback, all increase over time.  Cinema internal sound levels are increasing.

Noise at work regulations protect staff, but most cinemas will not have measured the sound levels they generate, and will not be able to consider the variation over a month or a year as the films and film genres change.  There will be a time when cinemas are able to create peak noise levels that can instantly damage hearing, but this feels unlikely to actually occur.  After all, concerts can generate very high sound levels and instant peak-based hearing damage is not common.  Exposure to noise over time could occur, but unless a patron is going to the cinema for hours day after day, damage is unlikely to occur.  Damage for staff is much more likely, depending on staff locations and times spent at loud locations.  

Security staff at concerts are on the front line of high music noise levels.  Standing for 8 or 10 hours in front of the stage with probably over 110dB(A) being generated behind you puts you at large risk.  Staff will be given hearing protection, but not wearing the protection properly or all of the time will risk the hearing of the security staff.  Others at big risk are camera operators, first aiders, sign language interpreters, technicians, etc.

Sound Pressure Levels 

Sound Pressure Level Lp = 20 log (p/p0) where p0 is the reference sound pressure of 20 micro pascals or 2*10^-5 Pa.  This is a standard equation to convert from sound pressure in Pascals to Sound Pressure Level in dB or decibels.  The equation is hardly ever used in reality as sound level meters are not presenting sound pressures but have already done this maths to present sound pressure level data.  The meter will display dB values for various parameters and the user does not need to convert to or from pascals.  Various Institute of Acoustics courses require candidates to be able to manipulate from one to the other, usually only from sound pressure to sound pressure level and show that 1Pa =94dB (93.98dB) and 10Pa =114dB (113.98dB).  100Pa would be 134dB (133.98dB) and so on.  

Sound Power Level Lw = 10 log (W/W0) there W0 is the reference sound power of 1*10-12W.  Again a standard equation and again used on various Institute of Acoustics courses. 1W=120dB, 10W=130dB and so on.  Again this is not used very often as usually a Sound Power Level is calculated from sound pressure level measurements.    

Equal loudness curves

As we don't hear equally at all frequencies there is natural interest in knowing how much louder a sound at a particular frequency has to be for it to be heard as being equally loud as another sound at a different frequency.  Adding to this, the different across frequencies changes when the sounds get louder.  There is a measurable and plottable pattern that has been understood since about 1933 when Fletcher and Munson wrote "Loudness, its definition, measurement and calculation."  ISO 226 picked up the thread and set out the values.  Different values can be obtained with headphones or loudspeaker testing.  Also there are differences depending on the direction of the sound relative to the head.  These curves, specifically the 40 Phon curve informed the A-weighting curves.         
There are no perfect curves and as ever the curves are approximations and averages across populations.           

Binaural beats

Binaural beats are an interesting effect where two tones come closer together in pitch and the sum beats that is it gets louder and quieter as and louder again as time goes on.  This can be worked out mathematically as sine waves combine and can be heard when tuning one guitar string to another playing the same note but slightly off pitch.  With a pure tone in each ear the beating can still happen - this beating is made in the brain rather than in space which is strange and not caused by the waves directly.  

Pitch circularity is another effect where the sound forever sounds as if the pitch is reduction where really it is a clever mix of tones octaves apart with low frequencies faded out and high frequencies faded in, so the sound goes on forever.        

Otoacoustic emission is where an ear emits a quiet sound, which is another unusual effect - but logically the ear is a functioning moving organ which is partially exposed to the air so it makes sense that on occasion noise might be emitted into the air from the ear.    

What is acoustics?

Acoustics is the study of sound.  That is the shortest definition possible.  A more full definition is that acoustics is a branch of physics and as sound is a physical pressure change which travels in waves in a given medium, it can be measured, experienced, predicted and expected.  Calculations and modelling can be done to determine what will happen if variables change.  Reductions in sound levels can be predicted, likely impacts on humans or animals can be considered.  The quality of sound can be assessed where the ability of a space can be considered for its ability to allow speech or music to be heard, comprehended, enjoyed etc.  So there is more than just physics, it is engineering, it is applied to the world and to human interaction and enjoyment and to human risk.

As it is engineering, the correct qualification for a practitioner to have to have is an engineering degree in acoustics and for them to be peer reviewed and qualified as a chartered engineer in acoustics.               

Human evolution

Depending on definitions humans have been around a couple of hundred thousand years.  During most of that time food was scarce.   Our bodies have relatively large brains that set us apart from other animals.  Our bodies evolved to store energy when there was excess to allow us to survive when there was not enough.  

We have not evolved for the modern world and there are areas where we are ill-equipped.  There are areas where the evolution actively works against us as we move through the modern world.

It would be nice to be able to turn off the fat storing system and pre-set an ideal amount of storage and anything over that is not stored.  Similarly, it would be nice to be able to have control over our hearing to that we could be fully protected from very high noise levels.  To be able to switch hearing off, or significantly down when sleeping might assist us.  Having more than two ears would benefit us.  Re-growing ears would benefit us.  Evolution is blind to our desires and is unlikely to accommodate the modern world any time soon.  It is difficult to see a situation where steerable hearing (like a rabbit), or a wider frequency audible range, or an ability to switch off your hearing would allow genes to outbreed compared to those without.    

Noise induced hearing loss in the future

NIHL studies and data have been done historically where exposure to xdB for y hours a day for z years has been established.  The activities the workers are exposed to are obviously noisy workplaces from 30 or 40 years ago.  They tend to be manual type activities in noisy workshops and printing presses.  The NIHL has a obvious pattern with a reduction in hearing ability at round 4kHz being most obvious.  There is a dip in the plot.  This is around the most sensitive frequency of human hearing.  There are questions relating to the likely NIHL of the future.  What if the noise the individual was exposed to had no 4kHz content, there was only say 200Hz content - what would the NIHL look like/sound like in that scenario?  Studies are needed where the frequency content of the noise over the 40 years is known rather than just the overall level.  This data will come as more and more workplaces have sound measuring equipment installed and operating daily.  The types of work we do will alter as time moves on, so it would not be unexpected if the NIHL caused by it also alters.  It is not the type of activity as such that matters, but the spectral shape as well as the overall sound levels that might change.                 

Multi-use developments

Multi-use developments can be well defined where all of the end user are known and their likely noise emissions and noise sensitive areas can be established before the foundations have been laid.

More commonly, a site will be developed with 1 or perhaps none of the end users known.  The requirements of Tesco will be different to Sainsbury's; a cafe might be different to a pub, but one type of pub will be different to the next.  Pubs have a large range of trading styles from no-music and people playing chess to foreground live music until the early hours.  The variation is also true for business and commercial uses.  The plant and delivery requirements will also vary sometimes significantly between different end users.  

Sometimes this will inevitably lead to buildings being overdesigned where a site is developed to accommodate a loud joinery operation but is actually used by a plumbing parts store.  It is also possible for a site to be designed to a relatively low specification and for a significantly louder end user to move in.

Further complications can come where the previous owner living on the site changes to be a third party living in close proximity to a noisy business type.    

Sound Reduction Index R

ISO140-3 1995 states that the Sound Reduction Index R is ten times the common logarithm of the ratio of the sound power W1 which is incident on a partition under test to the sound power W2 transmitted through the specimen.

R = 10log(W1/W2)  Where W1 is sound power incident and W2 is sound power transmission.  So if 1.0W strikes the panel and 0.1W transmits through, the Sound Reduction Index is R=10dB.

Also Sound Reduction Index is defined in ISO140-3 as
R = L1 - L2 +10 log (S/A)
Where L1 is the average sound pressure level in the source room in dB
L2 is the average sound pressure level in the receiver room in dB
S is the area of the test specimen in square metres
A is the equivalent sound absorption area in the receiving room
A= 0.161V/T
Where V is the receiver room volume in cubic metres and T is the Reverberation Time in the receiving room in seconds

This equation assumes diffuse sound fields in both rooms and that the only route from source to receiver is the test specimen

Individual R values can be found for any octave band centre frequency or 1/3 octave band centre frequency. 

R is used in laboratory tests.           

Apparent sound reduction index R'

ISO140-3 1995 states that the Sound Reduction Index R' is ten times the common logarithm of the ratio of the sound power W1 which is incident on a partition under test to the total sound power transmitted into the receiving room if, in addition to the sound power W2 transmitted through the specimen, the sound power W3, transmitted by flanking elements or by other components, is significant.
R = 10log(W1/(W2 + W3) )  Where W1 is sound power incident and W2 is sound power transmitted through the specimen, W3 is the sound power transmitted by flanking elements.  

So if 1.0W strikes the panel and 0.1W transmits through, the panel, and 0.1W transmits through the flanking routes then the Sound Reduction Index is R'=7.0dB. 

Also Sound Reduction Index is defined in ISO140-3 as
R' = L1 - L2 +10 log (S/A)    

Where L1 is the average sound pressure level in the source room in dB
L2 is the average sound pressure level in the receiver room in dB
S is the area of the test specimen in square metres
A is the equivalent sound absorption area in the receiving room
A= 0.161V/T
Where V is the receiver room volume in cubic metres and T is the Reverberation Time in the receiving room in seconds

This equation assumes diffuse sound fields in both rooms.  The route for the transmission from source room to receiver room does not matter.

Individual R' values can be found for any octave band centre frequency or 1/3 octave band centre frequency.             

Level Difference D

ISO140-4 1998 states that the Level Difference D is the difference, in decibels, in the space and time average sound pressure levels produced in two rooms by one or more sound sources in one of them:

D = L1- L2 
L1 is the average sound pressure level in the source room;
L2 is the average sound pressure level in the receiving room.

So If the average sound pressure level in the source room is 100dB and the average sound pressure level in the receiver room is 40dB the Level Difference D = 60dB.

Individual D values can be found for any octave band centre frequency or 1/3 octave band centre frequency. 

D is used in site testing.

Normalised Level difference Dn

ISO140-4 1998 states that the Normalised Level Difference Dn is the level difference, in decibels, corresponding to the reference absorption area in the receiving room: 

Dn = D - 10log(A/A0)

Once a Level Difference D is known, it can be corrected for the absorption area of the receiving room, correcting to the reference absorption area of A0 where A0 = 10 square metres.

If the D value was 40dB and the measured A was 5 square metres then:
Dn = 40 -10log(5/10)
Dn = 40 - (-3) = 43
If the receiving room is less absorbent than 10 square metres, when corrected the Dn value will increase.

Individual Dn values can be found for any octave band centre frequency or 1/3 octave band centre frequency. 

Standardised Level Difference DnT

ISO140-4 1998 states that the Standardised Level Difference DnT is the level difference, in decibels, corresponding to a reference value of the reverberation time in the receiving room:

DnT = D + 10log (T/To)
Once a Level Difference D is known, it can be corrected for the reverberation time of the receiving room, correcting to the reference reverberation time of T0 where T0 = 0.5 seconds for dwelling rooms.

If the D value was 40dB and the measured T was 1.0 seconds then:
DnT = 40 +10log(1.0/0.5)
DnT = 40 + 3 = 43
If the receiving room is more reverberant than 0.5 seconds, when corrected the DnT value will increase.

Individual DnT values can be found for any octave band centre frequency or 1/3 octave band centre frequency. 

DnT is  used for on site testing.

Weighted sound reduction index Rw

ISO717-1 1997 sets out the method for calculating a single number quantity Rw given a number of individual Sound Reduction Index R values.

R values are found at each frequency either 1/3 octaves: 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150Hz.
octave bands: 125, 250, 500, 1000, 2000Hz.

Oddly 3150Hz data is included in the 1/3 octave calculation but not the octave band calculation.

The found R values are compared to a set of reference values and the reference values are shifted as a group up or down in 1dB steps until the reference values fit the data.  The method for fitting the curve is to find the (adverse) unfavourable deviation i.e. the amount that the measured R values exceed the reference values in that frequency. then find the sum of all unfavourable deviations, then shift the curve until the sum is as high as possible but not exceeding 32.0dB for the sum of the 16 1/3 octave data, or 10.0dB for the sum of the 5 octave bands.

The Rw values is the 500Hz value of the shifted reference curve.    

Rw is used for laboratory testing and gives a single figure to show the overall sound insulation properties of an element such a s a particular window system, a particular stud wall system or an acoustic door which has been tested.  The Rw value does not give performance in terms of frequency, i.e. how well a door will attenuate bass, or how well a window will attenuate speech.  The frequency information is needed to generate the Rw figure, but the Rw figure does not show the frequency data, the R values used to find the Rw do set out the performance at each frequency band. 

Weighted apparent sound reduction index R'w

ISO717-1 1997 sets out the method for calculating a single number quantity R'w given a number of individual Sound Reduction Index R' values.

R' values are found at each frequency either 1/3 octaves: 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150Hz.
octave bands: 125, 250, 500, 1000, 2000Hz.

Oddly 3150Hz data is included in the 1/3 octave calculation but not the octave band calculation.

The found R' values are compared to a set of reference values and the reference values are shifted as a group up or down in 1dB steps until the reference values fit the data.  The method for fitting the curve is to find the (adverse) unfavourable deviation i.e. the amount that the measured R' values exceed the reference values in that frequency. then find the sum of all unfavourable deviations, then shift the curve until the sum is as high as possible but not exceeding 32.0dB for the sum of the 16 1/3 octave data, or 10.0dB for the sum of the 5 octave bands.

The R'w values is the 500Hz value of the shifted reference curve.    

Weighted normalised level difference, Dn,w

ISO717-1 1997 sets out the method for calculating a single number quantity Dn,w given a number of individual Sound Reduction Index Dn values.

Dn values are found at each frequency either 1/3 octaves: 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150Hz.
octave bands: 125, 250, 500, 1000, 2000Hz.

Oddly 3150Hz data is included in the 1/3 octave calculation but not the octave band calculation.

The found Dn values are compared to a set of reference values and the reference values are shifted as a group up or down in 1dB steps until the reference values fit the data.  The method for fitting the curve is to find the (adverse) unfavourable deviation i.e. the amount that the measured Dn values exceed the reference values in that frequency. then find the sum of all unfavourable deviations, then shift the curve until the sum is as high as possible but not exceeding 32.0dB for the sum of the 16 1/3 octave data, or 10.0dB for the sum of the 5 octave bands.

The Dn,w values is the 500Hz value of the shifted reference curve.    


Weighted standardised level difference, DnT,w

ISO717-1 1997 sets out the method for calculating a single number quantity DnT,w given a number of individual Sound Reduction Index DnT values.

DnT values are found at each frequency either 1/3 octaves: 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150Hz.
octave bands: 125, 250, 500, 1000, 2000Hz.

Oddly 3150Hz data is included in the 1/3 octave calculation but not the octave band calculation.

The found DnT values are compared to a set of reference values and the reference values are shifted as a group up or down in 1dB steps until the reference values fit the data.  The method for fitting the curve is to find the (adverse) unfavourable deviation i.e. the amount that the measured DnT values exceed the reference values in that frequency. then find the sum of all unfavourable deviations, then shift the curve until the sum is as high as possible but not exceeding 32.0dB for the sum of the 16 1/3 octave data, or 10.0dB for the sum of the 5 octave bands.

The DnT,w values is the 500Hz value of the shifted reference curve.    

DnT,w is used in sound insulation testing for on-site tests.  It includes sound passing via all flanking routes and allows separating walls to be tested against building regulation standards.  DnT,w is standardised for reverberation time in the receiver room so does not punish a site for being reverberant at the time of testing.  Reverberant spaces are preferred for testing as the diffuse status of the rooms is assumed in the test method. 

Understanding regulations

Regulations are usually complicated.  Understanding the basic gist of the words is one thing, but fully understanding what is required and what does and doesn't matter is more complicated and takes more care and effort.

The devil is almost always in the detail and the wording of each phrase matters.  Standards and regulations can also contain errors, as can examination papers and preferred answers.

10dB and 10.0dB mean different things.  10.4dB is still 10dB but clearly 10.4dB is bigger than 10.0dB.  Some standards are written poorly and intend to say 10.0dB when they actually say 10dB, this adds another level of confusion and debate.

Another difficulty comes when bands of results contain the same value.  For example if Level 1 is 70dB to 60sB and Level 2 is 60dB to 50dB. What level is 60dB?   Probably either 70dB to >60dB was meant, or <70dB to 60dB etc was meant.  The writer was not careful enough, and this type of issue occurs often in standards.
Another issue is the interpretation of words such as X should be done, Y may be done, Z can be done etc.  The English language is complex too and words can have two meanings.  Mathematics ought to be more precise but it depends how the maths is used as to the clarity it brings.

Some standards state in their scope they deal with a particular area of work and then do something different within the main text - this is also unhelpful.

Acoustics in court

Speech intelligibility matters in a courtroom.  All parties need to be able to hear what is said.  Often this includes a defendant being able to hear an advocate who has their back to them, or from behind a screen.  Courtrooms vary wildly across the country and their ability to provide high levels of speech intelligibility also vary.  Increasingly courts rely on multi-media systems either playing back CCTV and body worn footage into the court, or linking to a solicitor's office, police station or more often prison for a virtual interaction in a court hearing.  'Court hearing' pointing again to the fact that it is speech and verbal questioning that drives the system, not written essays and written questions.   

Positive noise/sound

High levels of noise impacting on a community used to be more acceptable when everyone in the community worked for the local industrial operation.  Glasgow, Newcastle and Belfast made ships for months making the community ring with the sound.  If you lived there, you wouldn't complain, you might not like the sound, but the sound meant that your business was progressing and that you had employment.  Some might have even liked the noise and might have taken comfort in it.  Textile industries in Manchester, Lancashire and Yorkshire will have has the same effect.  Noise in the area equals jobs.  Coal mining in Yorkshire and Derbyshire will also have had a community spirit and mining towns would be unhappy if the noise stopped.

In modern times there are few large employers employing the locals of a town or village where the community support the operation and welcome the operation.  Increasing there is conflict between uses and a reduced community element.   

Air Compressors 

Energy storage is becoming critical and the ability for people, firms, governments to store electricity into another medium for later release is vital.  New methods or lesser used methods might come into play to provide a method for storing green energy at times of excess.     

Fly wheels could be used, but probably won't be.  Hydrogen separating will be used a great deal and there will be some noise in this process.  Air compressors are another method that might increase in popularity i.e. compress air in a tank at times of excess, then allow the air to expand and drive a turbine to create electricity at times of need.  Gravity storage might become a massive new system with stacks made during excess and the potential energy recovered when in need.  Pumping and releasing water in hydroelectric systems might be unwise as water becomes increasing valuable.  

Whatever the new proposed methods - new noise standards are needed to tolerate higher levels in noise in an attempt to prevent the global warming crisis.  It is probably a mistake to block the systems that might save the word because some people will hear devices.  There should be limits, but the limits for global warming solving systems must be higher than something neural or negative.      

Origin of noise

We have dealt with a few sites where others claim that a given noise event originated at a given site.  There tends to be assumptions that are not certain but are built into the assessments of other organisations.  In order to claim that noise has originated from a site it is not enough to say that noise which is of the type which might emit from the site was heard, therefore the noise came from the site.

A nightclub for example might generate high levels of music sound inside the space, and moderate levels of music noise outside the space.  If music is heard at a house 1000m away, does that mean that the music came from the nightclub?  Obviously it does not automatically follow that any music noise heard at the receiver location must have come from the nightclub.  There might be other nightclubs, pubs, restaurants, neighbours, car sound systems, music practice rooms, etc and the music might be coming from there.  Determining the source is vital.     

Other types of noise where assumptions are made tend to include construction noise, clay pigeon shooting noise, motorsport noise, industrial noise and speech noise.   


Occasionally, a main road will be repaired which forces traffic to follow unusual routes, either by planned diversions or by guesswork on behalf of the driver, or following a sat nav which attempts to reroute.

The alternative route will probably have a massive increase in traffic flow.  The flow is always potentially there, it is possible for a large flow to drive down any road, but suddenly the potential is realised and a quiet route is at maximum capacity with the associated increased noise levels.

Festivals and stadium events also change the flow of traffic in the area and can bring tens of thousands of extra trips to relatively quiet areas.        

Reversing bleeper paradox

Reversing bleepers are an unusual paradox type noise source.  The reason they are fitted to large vehicles is to warn pedestrians and workers that reversing is about to happen/is happening and try to to reduce the risk of people being knocked over.  The sound is intended to be heard and intended to be loud.  Reversing bleepers are sometimes tone types and sometimes white or pink noise types.  

Residents in the area might complain about hearing the intentionally audible reversing bleepers.  It would be desirable for the sound to be audible at close proximity to the rear of the vehicle but not audible elsewhere.  It might be possible to crease a more directional sound source, it would certainly be possible to ensure that the sound source was located close to the ground and right at the back of the vehicle or trailer this would tend to reduce the level of noise transmitted elsewhere.  

It is possible to turn off or disable reversing bleepers so long as a banksman will stand and guide each vehicle.  The future of transport will no doubt have driverless machines and no people walking in the areas the vehicles move in, so reversing bleepers will probably naturally go in the next 50 years.       

Independent acoustic experts 

We are one of the few genuinely independent acoustic consultancy firms in the UK that have only highly qualified and experienced staff.  We also have CEng status and teach Institute of Acoustics short courses in and around Sheffield, Barnsley and Rotherham.         

Airport Ground Crew 

Airport ground crew is probably one of the most exposed job types.  Multiple hours in close proximity to high noise level environments adjacent to aircraft has the potential to impact on hearing very quickly.  Other high noise level environments are probably offset with the reduces times of exposure.  Military staff such as jet pilots for example will not typically be exposed for hours day after day.  Some highly exposed workers will be entirely coincidental.  For example a railway or underground worker who happens to be positioned near the rails on a bend where noise levels are increased compared to straight sections or where a loudspeaker broadcasting announcements in centimetres from the staff location.

There are industrial processes where staff must stand in the middle of out machinery for 8 hours per shift.  These workers are not seen by the public so are not normally in lists of risky professions, but there are thousands of staff exposed to noise, hidden away from society. 

Globalism tends to see manual work subcontracted away from the UK, but the reality is that staff in countries in Asia doing the outsourced work will be at risk and welfare in other countries might have different and perhaps lower standards than in the UK.

Generally websites writing about hearing loss are incorrect.  They tend to talk about dB and decibels, but they don't understand A-weighting, equivalent continuous sound levels,  exposure, peak sound pressure level etc.  100dB is not the same as 100dB(A) and 100dB(A) is not the same as 100dB LAeq.  100dB LAeq is not the same as a daily personal noise exposure of 100dB(A).

Focusing sound 

Soundwaves are created and they radiate away from the sound source.  Usually they spread into the space they travel into, waves spreading out like ripples from a stone dropping in a lake.  Soundwaves can be reflected, be absorbed, experience diffraction, diffusion, etc.  Reflections can be off flat planes or off concave or convex surfaces.  It is possible for sound to be focused off a concave surface in a similar way to satellite dishes focusing radio signals onto a receiver at the focal point.  There are various places where the effects of this can be experienced and they usually sound surprising and unusual as it is not a normal experience to hear sound very clearly from a source as the sound is focused on your ear.  

Kircher understood elliptical focusing in 1670s with his Phonurgia Nova.                

Phonurgia Nova 

'New science of sound production' by Kircher in 1673 was the first book dedicated to acoustics.  He wrote about echoes and how sound moved in waves and that sound bounced off surfaces like light bounces off mirrors.  He also wrote about how music can influence the mind.  We don't know who wrote about echoes first, but Giambattista della Porta wrote about concave reflection of sound in Magia Naturalis (Natural Magic) in 1558, but then Aristotle wrote about echoes in 300BC, Vitruvius did in 15BC being keen on theatre design. 

Lord Rayleigh (John William Strutt) wrote 'The Theory of Sound' in 1877 and explored vibrations and resonance in solids and gases.  The second volume in 1878 considered sound propagation.  Rayleigh is probably as well known for his discovery of Argon in 1894.

"As a general rule we shall confine ourselves to those classes of vibrations for which our ears afford a ready made and wonderfully sensitive instrument of investigation.  Without ears we should hardly care much more about vibrations than without eyes we should care about light."   


Evaluation and measurement for vibration in buildings guide to damage levels from groundborne vibration uses peak particle velocity PPV to assess vibrations from activities like construction and mining in the UK and consider potential damage to buildings.  BS7385-1 is withdrawn and superseded by ISO4866.  ISO4866 is Mechanical vibration and shock - Vibration of fixed structures - Guidelines for the measurements of vibrations and evaluation of their effects on structures.

BS7385-2 considers possible cosmetic damage for transient vibration from 50mm/s PPV for reinforced or framed structures Industrial and heavy commercial buildings and 15mm/s at 4Hz up to 20mm/s at 15Hz up to 50mm/s at 40Hz PPV for unreinforced or light framed structures, residential or light commercial type buildings.              

XLR cables 

XLR cables are External Line Return and are usually 3 pin in audio cables.  James Cannon designed XLR cables in the 1950s.  They have a shield cable usually at pin 1 and  hot/in-phase cable at pin 2, pin 3 is a cold/out of phase cable.   XLR cables are usually balanced and this allows them to better reject interference.  The hot and cold wires which carry the audio signal, but with reversed polarity, i.e. if the wave goes up on the +ve it goes down on the -ve and vice versa.  Any interference would be picked up on both hot and cold lines.  The two signals can then be used to get rid of the noise, as the noise cancels when the hot is added to the inversed cold signal.  The cancellation should leave only the original signal.  Unbalanced cables do not cancel out noise in this way.  

Often audio is digitised close to the source, then this digital signal is sent and can be received  without interference over large distances, the undigitized XLR signal is also sent, rejecting noise, and this is used as a backup to the digital signal.  Some systems automatically switch if the digital feed is lost.  XLR cables can be successfully run over a hundred meters or so.   



We are asked most months if we work with a lot of egg boxes.  The answer is no.  Eggboxes have a 3D shape which is similar to that of some high performing acoustic absorption wedges used in anechoic chambers - that that is probably where the idea comes from.  Sites do not use egg boxes to solve any problems, and neither to chambers.  The public also generally confuse absorption with insulation.  Adding acoustic absorption into a room will not stop sound passing through a wall or floor into another space.  It will change the portion of sound which would have reflected back into the room and stop some of that reflection, but it will not change what next door hear.              


The Institute of Electrical and Electronics Engineers is largest technical professional organization in the world.  The IEEE has a wide ranging membership  composed of engineers and scientists of various types and interests including computer scientists, software and hardware developers, physicists, electrical and electronics engineers and of course acousticians.  

IEEE is dedicated to advancing innovation and technological excellence for the benefit of humanity.  

IEEE's originals start back in 1884 when electricity began to become significant in society and the path forward where electricity would become dominant was clear.  At that time there was one major established electrical based industry which was the telegraph.  As early as 1840 telegrams could be sent around the world and messages could be sent faster than via other means and certainly faster than physically travelling with a physical message.  By the end of the 1800s telephonic communications, the electricity power source and electric light industries has begun and an organisation to knit the industries together and develop standards and share knowledge was desirable. 

The IEEE developed from the American Institute of Electrical Engineers and also the Institute of Radio Engineers.  The AIEE was larger but the IRE has more students and more younger members.  The AIEE and IRE merged in 1963.

There are many technical societies within the IEEE including: 

  • IEEE Aerospace and Electronic Systems Society
  • IEEE Antennas & Propagation Society
  • IEEE Broadcast Technology Society
  • IEEE Circuits and Systems Society
  • IEEE Communications Society
  • IEEE Electronics Packaging Society
  • IEEE Computational Intelligence Society
  • IEEE Computer Society
  • IEEE Consumer Technology Society
  • IEEE Control Systems Society
  • IEEE Dielectrics & Electrical Insulation Society
  • IEEE Education Society
  • IEEE Electromagnetic Compatibility Society
  • IEEE Electron Devices Society
  • IEEE Engineering in Medicine and Biology Society
  • IEEE Geoscience and Remote Sensing Society
  • IEEE Industrial Electronics Society
  • IEEE Industry Applications Society
  • IEEE Information Theory Society
  • IEEE Instrumentation & Measurement Society
  • IEEE Intelligent Transportation Systems Society
  • IEEE Magnetics Society
  • IEEE Microwave Theory and Techniques Society
  • IEEE Nuclear and Plasma Sciences Society
  • IEEE Oceanic Engineering Society
  • IEEE Photonics Society
  • IEEE Power Electronics Society
  • IEEE Power & Energy Society
  • IEEE Product Safety Engineering Society
  • IEEE Professional Communication Society
  • IEEE Reliability Society
  • IEEE Robotics and Automation Society
  • IEEE Signal Processing Society
  • IEEE Society on Social Implications of Technology
  • IEEE Solid-State Circuits Society
  • IEEE Systems, Man, and Cybernetics Society
  • IEEE Technology and Engineering Management Society
  • IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society
  • IEEE Vehicular Technology Society

We have IEEE membership.  IEEE membership is not essential in acoustic consultancy, but is a measure of qualification as membership is unlikely to be granted without sufficiently robust formal education and experience.


The Audio Engineering Society is dedicated to audio engineering and is the only organisation/institution to be so.   The idea for the society first arose in the 1940s post war.  Legend has it that audio engineer Norman Pickering arranged the original meeting at the RCA Victor Studios in New York City in February 1948. 

The aim was to create a professional, non-commercial and independent organisation fostering an exchange of knowledge between experts in the audio engineering world.

The Audio Engineering Society remains a professional body for engineers and scientists with an interest or involvement in the professional audio industry.  The membership comprises a range of professionals engineers developing devices, methods, systems or products for audio, or working with audio production or manipulation.  It includes acousticians audiologists and a few other disciplines related to audio.

We have AES membership.  AES membership is not essential in acoustic consultancy, but as work can include audio recording and manipulation, it is desirable.    

Cocktail party effect 

The cocktail party effect is the phenomenon of the brain's ability to focus on a particular sound while filtering out other sounds.  The name comes from the idea of a listener trying to listen to one voice in a room full of voices.  Listeners usually have the ability to segregate different sound sources into different streams of data then decide which stream to focus on and which streams to exclude as noise.

Different people have different abilities to pick out information and being under the influence or being tired will reduce the potential.  Accents, familiarity, location of the source and sound pressure level of the signal compared to the noise will also play a part.  


Doppler effect 

The Doppler effect is the apparent change in frequency(pitch) of a wave as an object moves towards or away from a listener.  Christian Doppler, who described the effect in 1842.

As a police/fire/ambulance vehicles drives towards you with it's sirens sounding, you hear a pitch or a few pitches, then as the vehicle drives off into the distance having passed you, the pitch is lower. 

The reason for the Doppler effect is that when the source of the soundwaves is moving towards the listener each successive wave crest is emitted from a position which is closer to the listener than the last. Each wave takes slightly less time to reach the listener than the last. The time gap  between the arrivals of successive wave crests at the listener is reduced and this caused the frequency or pitch to be increased.   The waves are bunched together and squeezed in a the reducing space.  As the vehicle moves away, the opposite happens and the waves are more spread out as the distance increases.

The effect is about relative velocities and can happen with a moving listener or source or both.   

The idea was written in Dopplers paper  'Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels'  - 'On the coloured light of the binary stars and some other stars of the heavens' for light waves.

The hypothesis was tested for sound waves by Buys Ballot in 1845.  Perhaps it would be called the Doppler-Ballot effect.  


John Napier published findings and discovery of logarithms in 1614.  Napier was a Scottish mathematician from Edinburgh. 

Henry Briggs from Sowerby Bridge, near Halifax then worked with Napier to improve the form of the mathematics.  Napier's logarithm compared L points moving on a graduated straight line moving uniformly from minus infinity to plus infinity to the X point moving from zero to infinity at a speed proportional to its distance from zero.  L=0 when X=1 and their speed is equal at this point. Napier discovered that this constitutes a general relationship between the arithmetic and geometric series.  It became possible to use addition to do multiplication.  Up until this point in history multiplication was difficult to do and lack of easy multiplication was holding back all of science.  This breakthrough allowed science to move on, but Napier's idea was difficult to understand initially.      

Briggs published his table of logarithms calculated to 14 decimal places for numbers from 1 to 20,000 and from 90,000 to 100,000. In 1628 Adriaan Vlacq produced a 10-place table for values from 1 to 100,000, adding the missing values between 20,000 and 90,000.  The tables allowed science to finally move on.  A complicated multiplication could be turned into a simple addition.

'Napier's bones' is a manually-operated calculating device used for the calculation of products and quotients of numbers.  The method was based on lattice multiplication.

Logarithms are used for a few main reasons:

They allow transformation from multiplication into addition as log(xy) =log(x) + log(y)

They allow us to solve exponential equations easily.  Exponential equations become linear equations which are easier to solve.

They allow us to measure in orders of magnitude.  It is easier to express how much bigger one value is than another.

They are useful for visualising data particularly when data is over large ranges.

Logarithms simplify mathematics in some cases, and in the case of acoustics they also align with the hearing mechanism, we hear logarithmically not in a linear way. 

As log(xy) =log(x) + log(y) multiplication can be done as addition.

Given these Log base 10 tables:
 Value     Log 
1        0.0000
2        0.3010
3        0.4771
4        0.6021
5        0.6990
6        0.7782
7        0.8451
8        0.9031
9        0.9542
10      1.0000
11      1.0414
12      1.0792
13      1.1139
14      1.1461
15      1.1761
16      1.2041
17      1.2304
18      1.2553
19      1.2788
20      1.3010
21      1.3222
22      1.3424
23      1.3617
24      1.3802
25      1.3979
26      1.4150
27      1.4314
28      1.4472
29      1.4624
30      1.4771
31      1.4914
32      1.5051
33      1.5185
34      1.5315
35      1.5441
36      1.5563
37      1.5682
38      1.5798
39      1.5911
40      1.6021
41      1.6128
42      1.6232
43      1.6335
44      1.6435
45      1.6532
46      1.6628
47      1.6721
48      1.6812
49      1.6902
50      1.6990
51      1.7076
52      1.7160
53      1.7243
54      1.7324
55      1.7404
56      1.7482
57      1.7559
58      1.7634
59      1.7709
60      1.7782
61      1.7853
62      1.7924
63      1.7993
64      1.8062
65      1.8129
66      1.8195
67      1.8261
68      1.8325
69      1.8388
70      1.8451
71      1.8513
72      1.8573
73      1.8633
74      1.8692
75      1.8751
76      1.8808
77      1.8865
78      1.8921
79      1.8976
80      1.9031
81      1.9085
82      1.9138
83      1.9191
84      1.9243
85      1.9294
86      1.9345
87      1.9395
88      1.9445
89      1.9494
90      1.9542
91      1.9590
92      1.9638
93      1.9685
94      1.9731
95      1.9777
96      1.9823
97      1.9868
98      1.9912
99      1.9956
100    2.0000

Multiplying 2*5 is simple enough, but for the example:
2*5 = 0.3010+0.6990=1.000 which converts back to 10.

This isn't needed when the numbers are small, but as the difficulty increases the need becomes obvious:
13*2*3 = 1.1139+0.3010+0.4771=1.8920 which converts back to around 78.

Clearly the more place values given the better the accuracy, and non-integer numbers can also be given allowing calculations which would have taken hours in the 1600s to be done in minutes. 


Computers use a binary number system of 0 and 1 rather than our decimal system of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.  Conversion from decimal to binary involved dividing by 2 and noting the remainders.   

If 103 is the starting decimal number. Take 103 and divide by 2, is there a remainder? If so count the 1.  51 divided by 2, is there a remainder, count the 1, etc. as set out below.  Perhaps an easier question is, is this value odd - if it is count a 1.

Number    Is there a remainder?
103              1  (least significant number i.e. last)
 51               1
 25               1     
 12               0
   6               0
   3               1
   1               1 (most significant number i.e. first)

so 103 decimal is 1100111 binary (starting from the most significant number to the least, so writing the numbers down from bottom to top)

Converting back is about powers of 2.  2^0=1, 2^6=64 and so on.

2^6  2^5  2^4  2^3  2^2  2^1  2^0   (values to check)

64    32   16     8       4      2      1     (above converted to decimal)

 1       1     0      0       1      1      1      (our binary number)
yes   yes  no   no    yes  yes yes     (values are contained)
64 + 32 + 0  +  0  +  4 +   2 +   1 =  103 in decimal

So 103 = 1100111
1100111 = 103

Taking 42
Number    Is there a remainder?
42               0
21               1
10               0
5                 1
2                 0  
1                 1

42 = 101010

Checking back to Binary

2^5  2^4  2^3  2^2  2^1  2^0   (values to check)

32   16     8       4      2      1     (above converted to decimal)

 1      0      1       0      1      0      (our binary number)
yes   no  yes   no   yes    no     (values are contained)
32 + 0  +  8  +  0 +   2 +   0 =  42 in decimal

So 42 = 101010
101010 = 42

Addition in decimal is simple for us.  3+5=8

In binary, the numbers are lines up like this.  Then 1+0 in a column =1.  
0 + 1 in a column = 1.  0 + 0 in a column = 0. But 1 + 1 in a column =0 but also an extra 1 in the column to the left 

Taking 3 which is 11 and 5 which is 101

It looks like this:
                   1   1
          +  1   0   1
=      1   0   0    0

1000 in binary is 8 as shown below

8    0
4    0
2    0
1    1 

1000 in binary

Now adding the 103 to the 42:
103+42 = 145 in decimal, this is simple.

        1  1  0  0   1  1  1 

      +    1  0  1   0  1  0 

=   1 0  0  1  0   0  0  1

So 103 + 42 =  10010001

Transforming back to decimal:

 2^7    2^6   2^5   2^4   2^3   2^2   2^1  2^0
128     64     32     16      8      4        2       1       
  1        0       0       1        0       0       0       1
128 +   0   +  0 +   16 +  0  +   0  +  0  +  1

128 + 16 + 1 = 145

Binary multiplication, as you would expect 0*0=0, 1*0=0, 0*1=0 and 1*1=1.  Multiplication is simple, but the place holders marked - need to be used to keep the columns straight.

3 is 11 in binary
5 is 101 in binary
multiplying these gives:

         x   11
=          101 (this is 101* the rhs 1)
+        101- (this is the 101* the lhs 1)
=        1111  which has 8+4+2+1=15 which is correct.

10 is 1010 in binary
5 is 101 in binary
multiplying these give:

        x  101
=        1010 (1010* the rhs 1)
+         000- (1010* the middle 0)
+     1010--  (1010* the lhs 1)  
=   110010        which has  32+16+0+0+2+0= 50 which is correct.

ISO 140-7 

ISO140-7 is the standard which sets out the requirements for impact testing of separating floors in most areas of the UK.  It is used in Approved Document E testing.   'Acoustics — Measurement of sound
insulation in buildings and of building elements — Part 7: Field measurements of impact sound insulation of floors' is the full name of the standard.

The standard sets out various requirements including distance requirements of microphone positions including:
— 0.7 m between microphone positions;
— 0.5 m between any microphone position and room boundaries or diffusers;
— 1.0 m between any microphone position and the upper floor being excited by the tapping machine.

The standard also states that the tapping machine should be places in at least four positions randomly distributed on the floor under test and that the tapping machine should be at least 0.5m from the edges of the floor (i.,e. the walls)  In the case of anisotropic floor constructions (with ribs,
beams, etc.), more positions may be necessary.  It also requires the tapping machine to be positions at 45° to the direction of the beams or ribs.  Usually this is taken to be 45° to the walls even if there are not any beams or ribs.

Fixed measurements must be a minimum of 6 seconds * 6 positions.
It gives an unhelpful example of two microphone positions for each of two tapping machine positions (giving 4 measurements) plus two microphone and two tapping machine positions, make measurements on a one-to-one basis (i.e one measurement of tapping position 3 and one measurement of tapping position 4).  It is more straightforward to measure with two locations for each of 4 tapping machine positions for a total of 8.  Each measurement being 6 seconds minimum.  

A sweep method can also be used with a 30 second sweep per each of 4 tapping machine positions.  

Animal hearing ranges 

Animals hear over different ranges to humans.  Humans typically hear over 20Hz to 20kHz but this is really just for young healthy humans and the 20kHz value reduces with age. 

Animals have different hearing mechanisms and different abilities.

Dogs can hear up to around 45kHz so can hear very high frequencies.  Cats have similar ranges.  Chickens reportedly can only hear up to 2kHz.  Bats famously use hearing for location also and can hear over 100kHz well into ultrasound.   The soundwaves bats make naturally bounce off objects in the environment and some of these reflected waves return to their ears.  Bats are able to hear their own signal and do not tend to confuse other bats in the area.  This processing is vital otherwise the bats would not be able to create an image of the space and would fly into objects.  

Pigs can hear similar ranges to dogs.  

No ears 

Some animals have no ears, others have no outer ear, but still have hearing ability through inner ears.

Worms can sense vibrations in the ground and can detect sound in that way, but have no ears.  Other animals such as turtles do not have outer ears but do have an inner ear and can process sound.  Pigs have ears.

IOA short courses CCWPNRA/CCWNRA and CCENM  

These Institute of Acoustics courses each have two exams a year.  We tend to run one of these courses in the Spring and the other in the Autumn.  Our pass rates are high and we have the experience to assist and help you through the course.  We are an unusual education centre as we actually undertake the various assessments as we are working consultants/engineers whilst teaching the courses alongside normal working activities.  We are degree educated and full members of the Institute of Acoustics and the Association of Noise Consultants.     

CCWNRA is the correct shortening, sometimes it is written CCWPNRA but it is the certificate of competence in workplace noise risk assessment.

CCENM is the certificate of competence in environmental noise measurement.


ISO16283 has issues.  One of the issues is that omnidirectional loudspeakers are required "The directivity of loudspeakers shall have approximately uniform, omnidirectional radiation".  Why is this a problem? It limits the type of loudspeaker that can be used in a sound insulation test.  A normal sized room in a dwelling testing across a normal performing wall is probably fine.  Testing from a large concert hall, through a high performing wall to another space is not.  Under ISO140 large and loud cabinet loudspeakers could be used and so testing in a nightclub could be undertaken with the type of loudspeakers that usually make it loud in a nightclub.  There are no sufficiently loud omnidirectional loudspeakers on the market.  The authors of ISO16283 did not consider the range of rooms that require testing.

The contortions required for the overly simplistic room sweep diagrams are also poorly thought out.  The first sweep is impossible for anyone without a dislocated shoulder.  The second is is possible but not for small rooms.  The third is parallel to room boundaries and will emphasise room modes.  The forth again requires dislocated shoulders but also has two points where oversampling occurs.  It is also likely to fail the 0.5m distance requirements.        

There are better ways and it might be best for practicing acousticians to write to the next ISO.

Mobile phones 

Most months someone will mention that they can measure everything on their mobile phone.  This is loosely true but it is not accurate.  It probably won't be following standards to do fast time weighting or 'A' frequency weighting, it certainly won't be using precision microphones and pre-amps.  There might be bias.  It might always be 5dB down, or it might be 5dB high sometimes and 5dB low other times.  It might not measure at 100Hz or 4kHz accurately.  It will probably have directional tendencies.  It will not comply with BS5228 or Approved Document E or BS4142 or BS8233.  It can't be used for BB93 measurements in Leeds or concert noise assessments in Wakefield.  It gives some information +/- a large margin and is usually insufficiently accurate to be of any use.

Cannon Noise

There isn't a standard to consider cannon noise potentially impacting on residents.  Recently four different groups of residents approached us to tell us that a particular concert was not as loud at the cannon firing that had happened a few weeks earlier.  Cannon firing might be restricted to 10 or 20 individual bangs, but how loud should those bangs be permitted to be?  How many weekends should be permitted before the residents should be entitled to quiet.  This is an example of the type of consideration that needs to be decided across the country on any given weekend and each local authority might take a different view.

Matters become additionally complicated if the event takes place in one county but the residents are in another.  This could even occur across country boundaries.    


This standard sets out attenuation calculations for sound propagation outdoors.  Barrier attenuation, point source distance attenuation, air absorption, ground reflections, reflections from buildings etc are all induced and effects can be calculated.  ISO9613 is one of the main methods which are are implemented into sound propagation / noise mapping  software.  So a detailed calculation to estimate the noise level from a heat pump or factory at a given location can be calculated following the ISO9613 steps. 

Meniere's disease

This is an ear condition that can cause tinnitus and hearing loss but also vertigo.  Vertigo is a sense of spinning and dizziness (not a fear of heights).  It can be caused by poor fluid drainage inside the ear.  It can be genetic.  Viral infections can also cause the disease.  Sudden unprovoked attacks of the vertigo symptom might result in serious accidents and most likely a driving licence will be lost as driving can be affected.

Room modes

Room modes are where a soundwave resonates in a room as a function of the wavelength and the dimensions of the room.  Some pitches will boom in particular locations as the resonance dominates.   There are axial, tangential and oblique room modes.  Axial room modes are only in one direction of the room, so only involve one room dimension.  Tangential room modes are across two dimensions, oblique are across all three and move from corner to corner.  Axial are 1D, tangential are 2D, oblique are 3D.  

If the length of the room is L, the width is W and the height is H then
Axial is calculated from speed of sound c and one room dimensions

f = k*c/(2L) for length where k is an integer and 1 is the fundamental 

f = k*c/(2W) for width where k is an integer and 1 is the fundamental 


f = k*c/(2H) for height where k is an integer and 1 is the fundamental 

The equation for all room modes is:

f = (c/2)* [   (p/L)^2  + (q/W)^2 +  (r/H)^2  ] ^ (1/2)
f is frequency, c is speed of sound, L, W, H are length, width, height of room, p, q and r  are the wave numbers in each case. 
p=1 q=0 r=0 is the first axial mode in the length direction.   Solving with two of p, q and r with values gives tangential.  With values for all three it's solving oblique room modes. 

So if L=5 W=4 and H=2.4 for example the results are:

p  q  r  Frequency
1 0 0  34.4Hz Axial
0 1 0  43.1Hz Axial
1 1 0  55.1Hz Tangential
2 0 0  68.9Hz Axial
0 0 1  71.8Hz Axial
1 0 1  79.6Hz Tangential
2 1 0  81.2Hz Tangential
0 1 1  83.7Hz Tangential
0 2 0  86.1Hz Axial
1 1 1  90.5Hz Oblique
1 2 0  92.7Hz Tangential
2 0 1  99.5Hz Tangential



Dn,e is a measure of sound insulation of items like vent in terms of frequency.  It is a measure of airborne sound insulation of small elements and is detailed in BS EN ISO 10140.  It is the element normalised level difference.   Dn,e = L1 - L2 + 10log (A0/A)
L1 is the energy average source room sound pressure level 
L2 is the energy average receiver room sound pressure level
A0 is the reference absorption area in square metres, A0=10
A is the equivalent absorption area in the receiver room in square metres 

Dn,e,w takes the individual frequency values and rates them to provide an overall performance and is commonly used for trickle vents and other acoustic ventilation products.  The values are dB decibels.   

Dn,e values can be used in calculations such as BS8233 noise ingress calculations for planning noise assessments for housing schemes near road or rail noise sources.

Temporary acoustic barrier

Various products are available that provide a temporary acoustic barrier or fence.   Often they are fixed to Heras fencing and are around 2m tall and 3.5m wide.  They usually weigh between 10and 15Kg.  Typically they are water resistant with an outer PVC layer designed to be breathable so that air and sound can penetrate but water cannot.  They have a sound absorbing and insulating inner construction.  Of course the amount of absorption and insulation provided varies with frequency and usually very little insulation is provided at low frequency.  Products tend to claim 30dB or 35dB performance.  The devil is in the detail.    

Acoustic plasterboard

'Acoustic plasterboard' products are usually designed to be a little thicker and a little denser than standard 'plasterboard'.  Normal plasterboard or products that are not advertised as being 'acoustic plasterboard'  also have acoustic properties as do all products.  Everything reflects sound to a degree, absorbs sound to a degree and attenuates the magnitude of the wave transmitting across it.  'Acoustic' products are usually just give a little more performance or perhaps similar performance but measured and stated performance.  

Why use 'acoustic plasterboard' etc?  Well it might be the only way to solve the problem and meet the criteria at hand.  It might also be possible to achieve the same criteria with standard products.  Pricing and physical size and weight come into play but it is possible that three layers of standard product do the same as two layers of acoustic product.  It might be that the standard product has identical properties to the acoustic product.  It might be that the acoustic product is needed as it achieves the same performance but in a smaller space. 

Typical acoustic plasterboard density is around 840kg/m3 with standard wallboard around 650kg/m3.  Usually 12.5mm and 15mm plasterboard options are available for both, and a 19mm plank type version might also be available.  SoundBloc and Soundshield are the British Gypsum and Knauf versions of this type of dense plasterboard.   Acoustic plasterboard is commonly used in suspended ceilings and both separating and internal wall types.  

Sound Insulation Advice

Largely speaking Approved Document E allows anything to be built as long as it can be justified and in the case of separating structures tested.  The testing should be done by firms like ours who are experienced and approved to undertake the work.  Advice for separating structure and for internal walls and floors does not need to be sought, but really should be sought.  Advice is relatively cheap and we have been involved in hundreds if not thousands of sites across England and Wales.  Sailing too close to the wind and failing a sound insulation test is expensive  as often kitchens and bathrooms have to be removed so that structures can be upgraded.  We have Salford degrees and decades of experience, it is not a good idea to have a white van 'tester' give you advice when you could have engineers.  We have given sound insulation and building regulations advice for projects across Yorkshire, Leeds, Sheffield, Wakefield, York etc as well as Manchester, Nottingham, London and beyond.      

Wave equation

This is a way to describe waves radiating through media in space and in time. x is a spatial direction as it normally it, t is time as it normally is.  In one dimension we only need x.  c is the speed of the wave, so in air c=340 or so.  u is the displacement of the wave, rather the displacement of the given point in space we are considering.  d represents differentiation which allows us to see displacement but also the slope of the graph, the speed of change.

If x is location then dx/dt is speed and d^2 x/dt^2 is acceleration.  
x is the axis to the right and horizontal, u is the axis to the top and vertical
The basic wave equation is :
d^2 u/dt^2 =c^2 * d^2 u/dx^2

Fast Fourier Transform

Fast Fourier Transform (FFT) can be used to convert a waveform signal into its individual frequency components.  A complex wave is made up of individual tones at a number of specific frequencies of a given amplitude.  Each tone also has a phase, which is how the wave is shifted in time when added to the other waves.  

FFt is used in acoustics and audio manipulation all the time.

The sampling frequency fs must be known (the number of samples per second taken of the signal) this might be 48kHz

The block size  B must be known this is how many data points are being used in the calculation.  It might be  2^10=1024 or 2^11=2048

The Nyquist Fn frequency is half the sampling frequency so here it would be 24kHz this is the highest frequency that can be determined.

Measurement duration or Frame Size T is the block size divided by the sampling frequency  So let's say 2048/48k =0.0427s  

Frequency resolution is df is fs/B so 48000/2048 = 23.44Hz.  This is how the results will be spaced out. In this case not every frequency is calculated only every 23.44Hz.

There is a block size tradeoff between accuracy and refinement of df and speed of calculation. 

This is the first element in understanding FFT and Nyquist limits.

ETSU Wind Turbine noise

ETSU R 97 is a standard considering wind turbine noise.  “The Assessment and Rating of Noise from Wind Farms” is the full title.  The document was published in 1996.  ETSU-R-97 is the UK government’s preferred method of assessing wind turbine noise for planning purposes.

ETSU-R-97 generally compares the turbine noise with a level 5dB above background noise. The standard sets a lower limit which in the day can be 35dB LA90 to 40dB LA90.  The night time lower limit is 43dB LA90. 

Turbines are permitted to make more noise at night based on the logic that residents will be inside a dwelling at this time.

New onshore turbine sites have recently been largely halted but government policy is changing and new measures including broadening the ways that suitable locations can be identified and requirements to include communities might allow more development in the coming years.

Ideally those most affected would receive free energy for the life of the turbines.  

Amplitude modulation (AM) of wind turbine noise is the change in amplitude of the sound wave (loudness) which can occur over time as the blades move around.  If it occurs for a given unit, usually amplitude modulation occurs as each blade passes the main structural support of the turbine.  For a three bladed turbine the whooshing sound might occur three times per rotation of the turbine, as each blade passes the base.  Essentially, how loud are the moments when the noise is occuring compared to the moments when the noise is not occuring - this is the question to be considered.  There is no method for measuring or assessing currently in place.  It might be that comparison of the LA1 to the LA90 is best, as this would most likely include the sound in the LA1 measurement and would certainly exclude it in the LA90 measurement.  Over a 15 minute measurement for a 3 bladed turbine, 9 seconds of noise would be needed.  Turbines might turn at at least 10rpm, so 150 revolutions in 15 minutes.  9/150 = 0.06 seconds so as long as each whoomp sound lasts at least 0.06 seconds it would affect the LA1.  Of course LA5 might be a better describer, but many different ways of assessing the noise is possible.   Over a 10 minute measurement the 0.06 seconds would also be found per noise.  Depending on the site the LA1 or LA5 might also be influenced by other noise sources and clean evaluation of the turbine noise alone will be difficult with varying wind speed and direction.   

BS4142 Rating Level

In BS 4142:2014+A1:2019 the following definition is made:

rating level, LAr,Tr 
This is the specific sound level plus any adjustment for the characteristic features of the sound.

Tr is the reference time interval which is 1 hour in the day and 15 minutes at night.  

BS4142 Specific Sound Level

In BS 4142:2014+A1:2019 the following definition is made:

specific sound level, Ls = LAeq,Tr

This is the equivalent continuous A‑weighted sound pressure level produced by the specific sound source at the assessment location over a given reference time interval, Tr which is 1 hour in the day and 15 minutes at night.  


Maximum noise level at night LAmax

Published guidance on implementing limiting LAmax criteria in bedrooms (e.g. ProPG: Planning and Noise, and WHO Community Noise Guidelines) advises that individual noise events should not normally exceed 45dB LAmax more than 10no times per night, as this represents a threshold below which the effects of individual noise events on sleep can be regarded as negligible.  

Usually it is accepted that 1 minute measurements of LAmax is best practice.  The night time period is 8 hours which is 480 minutes.  

This means that 470 minutes should be under or meeting the 
45dB LAmax criterion and 10 events can be above the criterion. 

Some researchers erroneously take this to be the 10th highest external noise level.  This is not the case.

Noise ingress for all of the 480 spectral data sets must be undertaken, then the design should be altered in order to find a specification whereby the internal sound level within a bedroom exceeds 45dB LAmax no more than 10 times per night.

There are other issues to debate.  Should sirens or alarms be included in the dataset? Is there a limit that should be placed on the exceeding levels?  Is it reasonable to allow the worst maximum to be 70dB LAmax so long as the 470 are <=45dB LAmax?

What it there are 11 exceedances on night 1 and 9 on night 2?  What if there are 5 exceedances on each of 29 nights and 100 exceedances on the 30th night?  The method could and should be standardised.

The method for the actual noise ingress calculations should be standardised, but BS8233 is silent on the method.  CRN and CRTN are also silent on methods for estimating likely maxima and statistical spread of maxima.

Direct measurement is a relatively robust method for data collection and a modified BS8233 ingress calculation is possible.  We have successfully undertaken these calculations and correctly found the statistical data and results relating to LAmax maxima.     


Some councils have started to use LA1 as an alternative to LAmax.

Taking data where we have 15 minute logging data for a week and considering the correlation between LAmax and LA1, it appears that the correlation between the two is not always strong.  Taking the periods containing noise events at night, the relationship is around 0.75.  0.50 being a weak link.  The correlation between LA1 and LAeq is stronger at around 0.90.  1.0 being a perfect correlation.  The correlation between LA1 and LA10 tends to also be around 0.75.

LA1 is the A-weighted, fast weighted sound pressure level exceeded for 1% of the measurement time.  A 15 minute measurement would have 9 seconds as the critical time.  The LAmax value will be influenced by a split second event, whereas the LA1 will be generated by the level occurring over 9 seconds within the 900 seconds of the measurement, this might be 10 events or 2 events or 100 events depending on the time over which the event occurs.

Different sound source types will have different onset and offset periods and different profiles.  It a train passes a quiet countryside location site in less than 9 seconds, then the increase in sound pressure level caused by the train will not change the LA1 value, but will change the LAmax value.  It will affect the LAeq.  

Depending on the site and sound sources it might be that the LA1 best correlates to the LAmax it might be that the LAeq best correlates to the LAmax. 

This selection of data shows that a 15 minute period of 78.9dB LA1 happened along with a 92.0dB LAmax, but that another time generated 78.9dB LA1 along with a 97.6dB LAmax.  The parameters measure different things and perhaps it is not surprising that the values do not correlate perfectly.

All values dB(A) 15 minute measurements.
 | Time   | LAeq  | LAmax  | LA1  | LA10  | LA90
 | 23:00  | 68.0   | 82.2       | 75.2 | 71.7    | 56.7
 | 23:15  | 70.4   | 97.0       | 77.3 | 71.2    | 56.5
 | 23:30  | 68.6   | 91.2       | 75.3 | 70.8    | 56.2
 | 23:45  | 70.1   | 92.0       | 78.9 | 70.9    | 56.3
 | 00:00  | 67.2   | 85.9       | 74.4 | 70.5    | 56.3
 | 00:15  | 70.9   | 97.6       | 78.9 | 70.6    | 56.3
 | 00:30  | 67.4   | 88.4       | 75.6 | 70.0    | 55.4

More research is needed relating to LAmax and LA1 use for night-time noise ingress predictions and calculations.

Some New South Wales Australian guidance states "the LA1, (1 minute) descriptor is meant to represent a maximum noise level measured under 'fast' time response. The EPA will accept analysis based on either LA1, (1 minute) or LA, (Max)."  which implies the two are interchangable, when clearly they are not. 

First chart of data for LA1

This data shows two hypothetical trains passing a quiet site

Second chart of data for LA1

More trains might look like this

Third chart of data for LA1

A moderately busy road might look like this

Fourth chart of data for LA1

One train would control the LAmax but the LA1 and LA10 would not shift.

Array Gain

There are times in acoustics when additional information can be gained by using more than one measuring device.  An array of sensors is more than one, and usually they would be in a set pattern.  The signal to noise ratio (SNR) can be improved by taking the signal from more than one sound sensor/microphone.  This is the case if the signal is coherent across the sensors and the relationship can be established reliably.

Ideally the noise has no coherence between the array sensors and an array can improve the SNR and array gain by 10log(N) dB where N is the number of sensors.     


Vibration Dose Value (VDV) is used to estimate the probability of adverse comment from people in buildings subject to vibration.

VDV attempts to measure all sources of vibration and value the impact in a comparative way.  The assessment period is 16 hours in the day and 8 hours at night.

VDV is similar to root mean squared, but is actually to the fourth power, so root mean quad.

BS6841 weightings of Wb for vertical and Wd for horizontal motion are used.

The VDV in the dominant direction is to be identified so the VDV need only be identified in one direction.  However, x/y/z orthogonal simultaneous measurement is commonplace and logically this can be used to find a single 3D Pythagoras vector value.

If VDV values in m/s1.75 were  x = 0.1, y=0.1 and z=0.1 then the overall value would be 0.2 m/s1.75
Single value = (x^2+y^2+z^2)^(1/2)

Establishing Priorities

One method for considering how to prioritise works to noisy workplaces is proposed by Howard Pelton.  Modified for UK use and paraphrased it would be:

PN = ( n * L * t ) / ( C * F^2 )

Where PN is the priority number for the issue the equation generates
n is the number of employees affected
L is the sound pressure level LAeq
t is the exposure time in hours
C is the cost /£10,000
F is the feasibility factor and 1 is off the shelf, 2 is available, 3 is major work, 4 is unproven and R&D required for new method

So four different situations could be:

Boiler 1
F = 1

Boiler PN = ( 5 * 80 * 8)/(2*1)  =  1600

Furnace PN = ( 8 * 92 * 7)/(3*9)  =  191

Pump PN = ( 1 * 90 * 8)/(1*1)  =  720

Compressor = ( 30 * 75 * 5)/(2*16)  =  352

So this method would say:
Boiler = 1600
Pump =  720
Compressor =  352
Furnace =  191

The UK logic would be what is the impact on the staff?  Assuming each staff member is only affected by one item.
Boiler = 80dB LEP,d  at lower exposure action value
Pump =  90dB LEP,d over upper exposure action value
Compressor =  73dB LEP,d below lower exposure action value
Furnace =  91dB LEP,d over upper exposure action value

So the Furnace and the Pump are the priorities.  The Boiler and the Compressor require no action.  

One problem being that there are no limits.  A sound pressure level of 60dB LAeq for 8 hours a day, affecting 1000 people is not damaging.  If this was cheap and simple to fix the maths might be:
   ( 1000 * 60 * 8)/(1*1)  =  480,000
Of course this would not even be rated in reality, but it highlights the point.


Loudspeakers are usually grouped into categories by the following criteria:

1) Direct
2) Horn loaded

Driving Element
1) Magnetic
    i) Dynamic
    ii) Moving armature
   iii) Magnetostrictive
2) Condenser
3) Piezoelectric 
4) Ionophone
5) Air jet

Reproduction range
1) Low frequency (woofer or subwoofer)
2) Mid frequency (midrange)
3) High frequency (tweeter)
4) Full range

Diaphragm shape
1) Cone
2) Dome
3) Flat

Acoustic Beats

Chart of beats 1

 The above chart shows two sine waves of equal amplitude but of slightly different frequencies, and the resulting combined wave.  The two waves can combine to increase the amplitude but can also cancel out when one is a positive value and the other is negative.  The first display is 100Hz and 111Hz.

Chart of beats 2

The above chart shows 800Hz and 850Hz with the resultant wave.  The beating can be seen where the amplitude comes in and out and the mistuning can be heard.

Chart of beats 3

Here is another example, this time 500Hz and 506Hz

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Sound Power Level Measurement Standards

There are many different acoustic standards for the measurement and determination of sound power levels from various types of sound source.  Some require reverberation rooms, some require a free-field over a reflecting plane.  Some are engineering methods which are less precise, some offer precision methods.  Some offer a survey method.  The ISO 3740 range provides many standards.

BS EN ISO 3741 Acoustics – Determination of sound power levels of noise sources using sound pressure – Precision methods for reverberation test rooms
BS EN ISO 3743-1 Acoustics – Determination of sound power levels of noise sources using sound pressure – Engineering methods for small movable sources in reverberant fields – Part 1: Comparison method for a hard-walled test room 

BS EN ISO 3744 Acoustics – Determination of sound power levels of noise sources using sound pressure – Engineering method in an essentially free field over a reflecting plane.

BS EN ISO 3745 Acoustics – Determination of sound power levels of noise sources using sound pressure – Precision method for anechoic and hemi-anechoic rooms.

BS EN ISO 3746 Acoustics – Determination of sound power levels and sound energy levels of noise sources using sound pressure – Survey method using an enveloping measurement surface over a reflecting plane

There is also a range of ISO 9614 standards:

BS EN ISO 9614-1 Acoustics — Determination of sound power levels of noise sources using sound intensity — Part 1: Measurement at discrete points

BS EN ISO 9614-2 Acoustics  – Determination of sound power levels of noise sources using sound intensity  –  Part 1: Measurement by scanning

BS EN ISO 9614-3 Acoustics — Determination of sound power levels of noise sources using sound intensity — Part 3: Precision method for measurement by scanning

Rules of Thumb

We recently came across a document which set out likely improvements in terms of improvements in STC ratings for separating walls.  STC is  Sound Transmission Class and is a single number rating for wall and floor partitions used in America which is similar to Rw in the UK, but it is not the same.  It is calculated from 1/3 octave band STL (Sound Transmission Loss) data as set out in American Society for Testing Materials standard ASTM E316.  

Rw considers the frequency range 100-3150Hz whereas STC considers the frequency range 125-4000Hz
STC does not consider low frequency content and focuses on speech frequencies.

The rules of thumb for STC in america are:
Doubling plasterboard on 1 side +3dB
Doubling plasterboard on both sides +5dB 

Resilient bar on one side +6dB
Resilient bar on both sides +10dB

Staggered studs +10dB

Absorption in the cavity +5dB

Care should be taken with rules of thumb and these most likely do not apply to Rw values.

Noise at work course / Workplace noise course

Our Institute of Acoustics workplace noise course will run this autumn in Sheffield starting 25/09/2023.  Places are still available.

Public Buildings

Concrete RAAC buildings are becoming an issue in the UK with the safety of the building coming into question as buildings are seen to decay.  Schools, Hospitals and Court buildings are amongst those at risk and the future policy to repair or replace the buildings will need to be implemented quickly. The acoustic performance of buildings is rarely tested post occupation.  It is likely that many schools built post 2003 do not meet BB93 it is certain that some older schools will not meet the standards  - but they are not required to.

The acoustics of prisons and courtrooms are also rarely considered and a new policy of assessing the existing stock should be taken place in order to bring the existing buildings up to standard.  

Hospital and healthcare buildings are also certain to under perform if tested with modern methods.  Many wards have curtains to divide between beds which is obviously poorly performing in all regards.     

Swimming pools and sport centres are often overly reverberant and in some cases staff are exposed to high levels of noise for prolonged periods.  

Curtain walling

The idea of a fully glazed wall is old.  It is probably 500 years or more ago that the idea was formed, but technology took hundreds of years to allow it to be possible to create such a structure.  There are gothic buildings and 17th century estates which have a lot of glazing but are not quiet curtain walls as we consider them in the modern day.  Sealed double glazed units with thermal panels (spandrel panels or spandrel glass) usually replacing some of the glazed areas to reduce light and increase thermal performance, with transoms horizontally and mullions vertically connecting the elements together.

Acoustically lots of glass will give lower overall building envelope performance compared to a masonry wall with a section of glazing.  The framing mullions and transoms will also allow noise to flank around the separating walls and floors and there might be building regulations sound insulation issues between dwellings or spaces.   

Triple glazing with an outer pane then inner double is a way to improve performance acoustically but also thermally.  

Noise Cameras

The UK is trialing noise cameras to detect and then fine drivers of noisy vehicles, or perhaps vehicles being driven noisily.  Great Yarmouth  Norfolk, Birmingham west midlands, Keighley near Bradford in west yorkshire and north Bristol / south gloucestershire have been selected as the trial locations for the government trial of newly created cameras.

The cameras are number plate recognition devices, video recorders and sound level meters.  They are installed road-side and attempt to trigger when the sound pressure level is high, then to establish if the passing vehicle caused the noise.  If the noise exceeds a threshold, there will ultimately be a fine posted out to the owner in a similar way to speeding offenses and speed cameras.

The government has said that road noise pollution contributed to health problems, such as heart attacks, strokes and mental issues such as dementia.

Until these devices are rolled out nationally, boy racers will drive quietly until they are out of sight and or 'earshot' of the devices.  

Speed cameras tend to catch people who are not local and it is uncertain if boy racers etc will leave their patch and be caught.  There will probably be an app to tell them where the cameras are so that they can avoid them. 

Noise exposure of audiences

There is no specific legislation setting noise limits for audience exposure to noise at a concert, show or festival. 

HSE say that they "strongly recommend" that the average A-weighted sound pressure level of the event in all parts of the audience should not exceed 107dB LAeq.  There is no time consideration in the statement.  IF an event lasts 1 hours the LEP,d would be 98dB, if the event lasts 8 hours then the LEP,d would be 107dB these are orders of magnitude different.

The HSE also say that "where practical, the audience should not be allowed within 3m of any loudspeaker" and that  "under no circumstances should the audience and loudspeaker separation distance be less than 1m".  

The advice regarding informing the audience is that "where the Event LAeq is likely to exceed 96 dB, advise the audience of the risk to their hearing in advance, eg either on tickets, advertising or notices at entry points."

The advise also states that the C-weighted peak sound pressure level should not exceed 140 dB in audience areas.

Typically the 3m loudspeaker distance is more or less achieved for concerts and operators.  Typically catchall wording is applied to tickets saying something like "Warning – Prolonged exposure to loud noise may cause damage to your hearing".  Terms tend to be unread and are there to protect from legal action rather than to protect the individual. 

Hearing can be damaged in non-work activities and we have been involved in research into this area since 2000.

There was a Greater London Council GLC Code of Practice for Pop Concerts in 1985, this thought about the audience and the wider public.

In terms of hearing damage, the code set 93dB LAeq at 50m from the speakers over 8 hours.  So a 4 hour concert could be 96dB LAeq etc.

The code wanted no more than three events per year for a site.  The code wanted a residual sound level to be found over fifteen minutes then the ambient sound level including music noise to be no more than 10dB higher for rehearsals and performances between 0700 and 2000 hours.  This was 6dB between 2000 and 2300 hours.  the limit was inaudibility from 2300 hours to 0700 hours.

The increase was limited to 1dB(A) if there were more than 3 events per year.

All measurements to be made outside affected dwellings. 

Some data to support the 10dB(A) value showed that the number of complaints significantly increases when the increase is more than 13dB(A).  A 20dB(A) increase leads to many complaints.  

A similar GLC document 'Disco Rules ok?' set 90dB LAeq for 8 hours.  0700-2300 required the same approach as the concerts guidance but a 0dB increase at dwellings.  2300-0700 required inaudibility at dwellings.  The code requires a maximum permissible exposure MPEL of 100dB(A)

An Evaluation of Hearing Damage Risk to Attenders at Discotheques by J Bickerdike and A Gregory for the Department of the Environment in 1979 found a mean level of 96dB LAeq and 97dB LAeq in the 1970s.  They found MPEL values of 102dB(A) and 100dB(A) depending on whether the venue was licenced or not.  They have not reduced since.  

The average attendee age was 21.

They found 50% of people attended for 4.5 hours a week, and 5% attended for 15 hours per week.

This work should be repeated in 2029 to see if anything has changed.

Voice Recognition

The Crown Prosecution Service (CPS) prosecutes criminal cases that have been investigated by the police and other investigative organisations in England and Wales.  The CPS of the police and of government.

Expert evidence as to the identity of a voice is admissible where required, but an a Northern Irish case called  R v O'Doherty 2003, the Court of Appeal in Northern Ireland stated:

"Voice identification should come from a suitably qualified expert in acoustic analysis: the examination of the differences in the acoustic properties of speech which took into account the individual's physical characteristics;

Auditory analysis of a person's dialect or accent was insufficient, except where the purpose of the evidence was to identify who from a known group was speaking at a particular time; where there were rare characteristics to identify the speaker or where the issue in the case related to accent or dialect; and

The jury should be provided with tape recordings to allow them to evaluate the expert evidence but they should be warned about the dangers of relying upon their untrained ears.

For voice recognition both by experts and by lay people, and the importance of the need for caution"

Doctored Audio

As technology develops it will be more difficult to know what is true and what is real.  A recent UK child custody hearing was reported as having a doctored audio file brought into evidence in an attempt to discredit another party.

Apparently audio was modified to incriminate another party by manipulating the sound file to include words not actually used, allowing an accusation of their dangerous and unsuitable status for custody.

Manipulated video or audio recordings increasingly referred to as 'deep fakes', risk corrupting criminal and civil hearings in the future.  

Hydrogen vehicles 

A recent study into the operation of a commercial internal combustion engine vehicle with engine modified to run on petrol and hydrogen found that sound levels with hydrogen were around 1dB to 2dB higher depending on the actual use.

Most hydrogen cars will be as quiet at electric cars as the hydrogen fuel cell electricity production should not generate significant levels of sound, the electric motor will generate some noise particularly then accelerating. 

Tyre and turbulence type noises will remain. 

Those supporting hydrogen buses say "Customer experience is even better on a hydrogen bus. They’re smooth to ride, and make very little noise – creating calmer, quieter streets for the local community to enjoy when compared with cars or diesel-powered vehicles."

Room Criteria RC

Room Criteria (RC) is used in America for mechanical services noise measurements.  Noise Criteria (NC) is also used.   

NC and RC are both used to evaluate mechanical system noise (HVAC noise) in a room.  Noise Criteria curves were defined in the 1950s by Beranek. Room Criteria curves were first proposed by Blazier for ASHRAE in the 1980s.

Noise Rating NR curves are typically used in the UK.

All of these plot measured spectral sound levels against a defined set of curves.

Noise Criteria NC curves plots octave band sound pressure levels in 8 bands finding the highest curve crossed.  

Room Criteria RC curves plots the same 8 octave band sound pressure levels.  The RC curve is not so much a curve but a straight line with a downward slope of 5dB/octave.  RC assessments require additional information about the potential Neutral, Rumble of Hiss content in the sound.  Essentially the perceived balance between low and high frequency sounds is required to be factored into the RC value.


Sound in space 

Soundwaves require a medium like air or water to travel in.  Space is a vacuum so sound can not travel...usually.

The Earth is in space, but there is sound, as it isn't a vacuum, so any area of space that isn't a vacuum might have sound. 

Recently Nasa recorded some sound in the gas of a plasma cluster around 250 million light year from Earth.

The acoustic waves generated by a black hole were first identified in 2003 but have not been recorded or heard until now.  The natural wave is well below human hearing so the sound was pitch shifted to make it audible to humans.

Underwater sound

Microphones are transducers that convert pressure changes in air into electric signals.  Hydrophones do the same but in water.  The electrical signals can then be amplified, recorded, measured, reproduced, transmitted etc. The electrical signals can also be analysed to determine the amplitude, the frequency and all the usual acoustical parameters.  Once the electric signal is made the processing and manipulation of the signal can be done in the same way as for a microphone.

Hydrophone technology was developed, progressed and used in WWII to locate and detect enemy vessels.

Hydrophones can be omnidirectional and sensitive to sound coming from all directions equally, or they can be directional and 'listening' from one direction more than another.  Arrays can be used to create a directional effect from omnidirectional sensors.

Noise in oceans and seas can impact on marine wildlife, but usually the military are most interested in measuring and using sounding water.  There are some Sonar uses to detect fish and obstructions in the sea under the vessel.

Sonar (Sound Navigation and Ranging) is useful for mapping and sensing in the oceans and seas because soundwaves travel further and deeper into the water compared to light waves.

Fork lift truck noise 

Fork lift tucks have a large range of possible noise level emissions.  Are they moving on flat surfaces or bumpy and uneven surfaces? Are they carrying maximum loads or are they empty?

In addition fork lifts have many types in terms of shape and length of reach.  They also have diesel, LPG and electric versions.  Large diesel forklifts can be 10 or 15dB louder than LPG which can be perhaps up to 5dB louder than electric forklifts.

The received sound level will depend on the activity and the surface, and the type and the distance from the source, barriers and so on.  Sound power levels can be in the high 90s dB(A) or even 100dB(A) or higher.  Some fork lifts declare sound power levels much lower.  


Vocal effort means how loud someone is talking or shouting.  Vocal effort varies a lot from a quiet whisper through normal speech to raised voices and ultimately a shout.  Various levels of singing also occur.  Speech sound pressure levels vary from person to person, but can be grouped into categories such as males/females/children and by vocal effort.  

As with all sound sources, the distance from the sound source to the receiver changes the sound pressure level received.  There are also directional elements so that for a given distance the relative directions will change the resulting sound level.  The environment will also change the level, inside or outside, absorbent or reflective etc.    

Assuming all things are equal, the normal distance people stand near each other to talk is around 1m, maybe slightly less, and facing each other.  So this is the normal to compare other things to.      

This might give a sound pressure level of around 60dB LAeq for normal speech but of course it will vary.  Rough figures would have a raised voice 6dB louder, a loud voice another 6dB louder and shouting another 6dB louder, so around 78dB LAeq at 1m.

Speech is a point source so 10m away and the 20*log(10/1) reduction would be 20dB(A) lower, so 40dB LAeq for normal speech and 58dB LAeq for shouting.... more or less.

The LAmax result would of course be louder than the LAeq figure.

The frequency content of speech is complex and the pitch or fundamental frequency is usually around 90Hz - 150 Hz for men. For women it is around an octave higher at around 170Hz  - 250Hz is found approximately one octave higher. For children it is around 300Hz.  The sound is not just at this frequency range, and the bulk of the energy content ranges from 200Hz to 3500Hz.   

Vowel sounds are usually <1kHz whereas consonant sounds are >500Hz.  Vowel sounds are the volume and the power of the speech and the consonants are the delicate parts made by movement of the tongue and lips.

It is easy to increase the sound level of the vowels but difficult to increase the sound level of the consonants.  

500Hz, 1kHz and 2kHz octave bands are critical for speech and speech intelligibility.  

A 15dB S/N ratio is desirable for good intelligibility.   

Noise Mapping 

Since the 1990s noise mapping in computer software has become possible.  Noise mapping allows complicated calculations to be undertaken in a way which used to have to be done in spreadsheets and before that on paper.

Noise maps calculate and display predicted sound levels at all the points on a map and apply colours or contours to allow the data to be easily read.  Contours show sound levels just as height contours on a map show height above sea level.  A sound source or series of sound sources are created in a the 3D model with x, y and z locations being set.  The map inputs topography and buildings and barriers are input into the model then calculations can be done following standard methods.  Noise sources might be roads and motorways, trains and railway lines, aircraft and flight paths, industrial sound sources and factories etc.  Once the key elements are inputted and the 3D model is in created it is possible to calculate the sound level at any point within the virtual environment by splitting the area into a series of points on a grid.  The results at each point are calculated.  The definition of the grid can be varied and might be 1m or 2m or 5m point grid spacings.

Noise mapping methods allow acoustic engineers and consultants to assess and report on complex environments and design attenuation solutions where required for projects.  Noise mapping software also allows anyone else to attempt to create a noise map and unfortunately it is very easy to create inaccurate and incorrect maps.

Many noise maps display road or rail noise sources and do not consider all other noise sources.

Noise maps are created via calculation given a set of input conditions.  They are not usually made via measured data in the field.  There can be a large difference between predicted levels and actual levels which should be considered.

There are various software products available CadnaA, NoiseMap, Soundplan etc.

Mapping or modelling can usually accommodate BS4142 CRTN CRN ISO9613 BS5228 etc.

Gun silencers

There is a high pressure and high temperature of gas created when the gun is fired.  Then this gas comes into contact with the atmospheric air, sound is generated.  The sound pressure level will vary depending on the atmospheric conditions and base on the energy in the gas, but also the way the gas comes into contact with the air.  Does the contact happen suddenly or does the contact come spread out over time.  A silencer tries to increase the time of the contact and uses a series of baffles to do this.

The baffles reducing the pressure and velocity of the gas associated with the bullet.  The gas expands and passes into each baffled chamber before finally opening to the atmosphere.  The silencer volume is perhaps 20 times the volume of the barrel. 

Silencers do not actually silence but slightly attenuate the noise emitted.  They remove the 'crack' sound but a 'thump' sound remains. 


Hand Arm Vibration 

Hand-arm vibration (HAV) can cause the permanent and painful condition 'vibration white finger'.  Harm arm vibration can also cause numbness and tingling and painful joints and muscle damage.  HAV is also linked to carpel tunnel syndrome.


Extensive exposure to hand-arm vibration can lead to permanent health effects which are incurable and can be debilitating.  

Usually the cause of HAV is use of hand held tools such as portable grinders and compressed air tools which repeatedly impact on the surface of the hands and put energy into the hands and beyond in to the wokers arms. 

You should only purchase tools that have been designed and constructed to reduce the risk of vibration, and are suitable for their intended use. Train workers to use them safely and keep them properly maintained.

Tools should be assessed and exposures of workers considered.  It might be that a lower impact tool is available or that the tool can be used in a different way to keep the impact safe.  For example, sharing the job across two staff might keep both staff within safe limits.  Staff must be trained and measures should be put in place to eliminate or reduce risks from exposure to hand-arm vibration.

Vibration white finger has various symptoms including nerve damage, numbness, and reduced dexterity. 

It is named after the vascular manifestations where a lack of blood supply causes the fingertips of a worker to turn white.  The fingers can also turn blue due to lack of oxygen.

The issue is exacerbated in the worker has cold hands. 

Blue Tree Acoustics are able to undertake Hand Arm vibration assessments.  

Speed of sound in water

The speed of sound in air is 340m/s 344m/s somewhere around there depending on the temperature.  Sound passes through other materials too, and it travels at different speeds depending on the material.  The speed in water is around 1500m/s.  Sound waves travel faster in denser substances generally because neighboring particles will bump into each other more easily.  It is the dumping of particles that passes the sound wave along through the medium.  Water has around 830 times more particles per volume than air, so sound waves can pass through it more easily. 

The actual speed varies depending on the temperature, the salinity and the pressure/depth.

Mackenzie says 

c = 1448.96 + 4.591T - 5.304 * 10^(-2)*T^2 + 2.374 * 10^(-4)*T^3 + 1.340 (S-35) + 1.630 * 10^(-2)*D + 1.675 * 10^(-7)*D2 - 1.025 * 10^(-2)*T(S - 35) - 7.139 * 10^(-13)*TD^3


T = temperature in degrees Celsius

S = salinity in parts per thousand

D = depth in metres

Coppens says:

c = c(0,S,t) + (16.23 + 0.253t)D + (0.213-0.1t)D2 + [0.016 + 0.0002(S-35)](S - 35)tD

c(0,S,t) = 1449.05 + 45.7t - 5.21*t^2 + 0.23*t^3 + (1.333 - 0.126t + 0.009*t^2)(S - 35)


t = T/10 where T = temperature in degrees Celsius

S = salinity in parts per thousand

D = depth in kilometres

 Typically these two equations agree to around 0.1m/s

There are many other complicated equations and the results vary a little.  There are also limitations where one equation might not be accurate in certain conditions.

Wind speed in the UK 

Most sound measurements are made in the UK when wind speeds are no greater than 5m/s which is 18km/h or 11mph.  

For a normal location in the UK historic data suggests that this speed is exceeded for around 25% of the time.  This suggests surveys could be undertaken 75% of the time in the UK.  This figure seems high, but the data does not set out how the exceedances are spread through each day.  A 3 hour survey could not be successfully undertaken if any period of the 3 hours exceeds 5m/s.  The true likelihood of a given period being suitable is probably less than 50%.  Rainfall would additionally reduce the window of good weather further.  

1950s Sound Insulation

Studying old texts, the first element that jumps out is the 'correct' use of dB but Hz is usually described as c/s cycles for second. In the 1930s the International Electrotechnical Commission (IEC) decided to name cycles per second after Heinrich Hertz, but clearly the change had not propagated through to all documents by the 1950s.

In 1865, Maxwell published the paper “A Dynamic Theory of the Electromagnetic Field”.  In this paper he explained electric and magnetic fields and the waves they form.  He stated that light was an electromagnetic wave.  He also predicted radio waves before they had been witnessed.  

In the 1880s, Hertz proved the existence of electromagnetic waves and was able to determine wavelengths.  His practical work agreed with  Maxwell's predictions.

Marconi transmitted radio signals for the first time in 1895.

In the 1950s the following was understood regarding sound insulation in buildings:

a) Continuous air paths enable sound to travel with very little attenuation. i.e. corridors, holes, gaps, keys holes etc will readily permit sound to transmit from one area into another  

b) The more massive the structure, the lower the transmission of sound in air into the structure.  i.e. mass law, increase mass equates to increased sound insulation.

c) Any break in the continuity of the structure provides an effective barrier and prevents sound passing across it, however the thin layer of air in cavity walls acts as a stiff cushion which will transmit some sound through to the other leaf.

d) Sound wave sin air and vibrations in structures can be transmitted with little attenuation along insignificant paths.  e.g. one wall tie might undermine the sound insulation properties of the whole structure.  

In the 1950s sound insulation and sound absorption were understood.  Sound absorption being used to reduce sound reflections form a surface, whereas sound insulation is a function of how much sound passes through the structure.  

Layouts should be considered with quiet rooms should be grouped together both vertically and horizontally and noise sources should be kept remove from these area.  

A rough consideration of 9 inch brick walls sound insulation performance states around 50dB insulation for 9 inch brick (229mm).  
Wide open windows around 10dB.  Slightly open windows around 15dB.  Closed windows around 20dB.  Increasing to sealed double glazed windows of 1/4 inch plate glass (6mm) 47dB.  

Generally in the 1950s the following sound insulation was stated for separating floors:
concrete floor 52dB,  timber joist floor 43dB, with heavy ceiling 48dB

Sand pugging and slag-wool pugging were commonly used in timber joist floors.  

Underfloor heating systems were in use in the 1950s in the UK and this is considered in various designs.   

Sound insulation failures

There are many reasons a site may fail a sound test.  The primary reason is that either the design is not sufficient or the installation is poor.

The design might be simply missing and the build was just created and built as it went along with no real plan or consideration of sound insulation and Approved Document E compliance.  It might be that the designer was trying to sell their product and is not actually competent to advise on sound insulation and acoustics. It might be that the designer was working without understanding the various terms used, DnT,w, Ctr DnT,w  + Ctr , Rw, L'nT,w etc are all precise and require understanding.  

Advice should come from MIOA and ANC firms who have experience, qualification, integrity and competence.

Flanking routes are commonly not considered correctly.  On-site reductions and workmanship is often not considered.  Bridging of building elements, replacement and substitution of products.  Incorrect wall ties, failure to ensure the integrity of isolating and resilient products.  Flanking via ventilation systems and ducting, or by curtain walling mullion and transom flanking routes.  Flanking ober wall heads where closing has not been successfully achieved.  Stopping missing or in the wrong place.  Insufficient mass or number of layers of material.  Lack of acoustic sealing.  Penetrations in critical elements.  

Sound insulation issues will be experienced by occupants and complaints and claims will follow.  It is much better to correctly design and build initially with a competent acoustican giving approval of the design or improving it where necessary, then having a UKAS or ANC testing firm confirm the performance of thew structure on completion.      
It is estimated that around 1/3 of those working in and around acoustics in the UK are not sufficiently qualified and insufficiently experienced.       

External Influences

Collecting meaningful data is not as simple as it seems. 

Temperature can affect measurements and can affect microphones.  This depends on the kit being used.

Humidity may change how a microphone behaves and can deteriorate the quality of the meter over time.  

Directionality - microphones have directionality particularly at high frequencies.  Large diaphragms are more sensitive to sound but are more directional.  Standards should be followed in they prefer a direction for microphone diaphragm.  

Reflections - sound reflects from surfaces and incorrect location of a microphone will result in unreliable data being collected due to reflections.  Which reflections can or should be in? Which should be out?

Background noise (residual noise) can completely destroy the quality of the data being collected.  A great deal of care is necessary to ensure conditions are correct and controlled.

Self made noise is where a sound level meter cannot read 0dB (or less) but will tend to read 20dB or so even if the true value is 10dB, as the self made noise of the system prevents an accurately low reading to be made.  

Wind noise can give false readings particularly in Z-weighted and very low frequency measurements.  Correct wind shields should be used and measurements should be taken at the correct times.

Dynamic range if incorrectly set will declare 40dB for a true reading of 30dB or 90dB for a true reading of 100dB.

Faulty meter- the measurement chain might be broken or impaired and both calibration and experience of what range should be experienced will need to be used to ensure reliable data is gathered.    


Yachting cannon

Various american texts describe using yachting cannons firing blank shells for an impulse source to measure reverberation time.  This seems extreme, but there might be very large spaces where something of this size is needed.  Cyril Harris was a fan of this method.  We have various methods of measuring reverberation time, to date we don't have a yachting cannon.

Wind and temperature gradients

Sound travels faster in warmer air.  Sound also travels faster in downind conditions rather than upwind conditions.  Air molecules have more energy at higher temperatures compared to colder temperatures and  can vibrate faster and allow sound waves to travel through the air more quickly.

If the temperature and wind speed are the same on the ground as they are high up in the sky then there is no gradient.  If however the the wind speed at height is greater than it is on the ground then it is possible for addition sound to be pushed into the downwind direction from the vertical and upwind directions.  It is possible for an additional 2dB to be added in this way downwind, and possible for up to 10dB to be attenuated upwind.  In the upwind direction there can be a shadow zone where sound is significantly attenuated as a result of the wind gradient with height.  Sound will bend towards to the slower wind speed.

If temperature decreases with height compared to the ground condition then sound will tend to bend upwards towards the colder air.  Shadow zones can occur where the sound which would usually be experienced has been bend upwards overhead rather than passing straight from source to receiver.

An inversion occurs when air is warmer at height and colder on the ground.  In this case sound can bend back towards the ground towards the cold.  Additional sound will be heard and measured as sound which would have passed overhead is now being pushed back down.    

It is very difficult to determine the temperatures at each height, the wind speeds at each height, and the overall effect of the two potential gradients.   Usually measurements are made when ground wind speeds are low and probably gradient effects are also low.     

Cloud cover can affect the temperature at height as it can insulate and prevent hot air escaping, particularly at night.

Environmental Impact Assessment (EIA)

Various types of noise assessment can be required, depending on the type and scale of the project.  Often a formal EIA is required by legislation, such as the Town and Country Planning (Environmental Impact Assessment) Regulations.  The aim of the EIA is to protect the environment by assessing the likely significant effects that may result from a proposed development and to inform the decision-making process.  Local planning authorities and developers should carefully consider if a project should be subject to an Environmental Impact Assessment.  If required, they should limit the scope of assessment to those aspects of the environment that are likely to be significantly affected.

Noise and vibration is often a key aspect of an EIA.

EIA Screening and Scoping

Under the EIA process there is an initial screening stage to determine whether a proposed project falls within the remit of the Regulations, or whether it is likely to have a significant effect on the environment and therefore requires an assessment.

Following this there is a scoping stage to determine the extent of issues to be considered in the assessment and reported in the EIA.  The developer or applicant can ask the local planning authority for its opinion on what information needs to be included (this is called a ‘scoping opinion’).  Usually, noise is primary environmental factor and therefore is usually within the scope, whereas vibration can often be scoped out of the assessment depending on the specific circumstances of the proposed development.

The main aspects of noise that are usually considered in the EIA are to establish the baseline conditions to enable the significance of noise created by the development to be assessed.  This typically includes noise impact during the construction phase and operational phase, as well as from changes in noise levels resulting from additional traffic flows associated with the development.


In the 1980s Lindqvist undertook research on noise level propagation in large factories.  The work was primarily focused on applying absorption and determining how sound levels would reduce with distance in different scenarios.  This was before ray tracing acoustic software was commonplace.  Some of the work is published in Acustica - international journal of acoustics.  

At this time scale models were often used but here a room was used as a scale model of 10.8m by 5.51m by 3.36m which was assumed to be roughly a 1:4 scale of a factory.    

As expected the decay is not quiet 6dB per doubling of distance, as there are reflections and as expected the more absorption, the closer the decay gets to 6dB per doubling distance.  

Ideal reverberation shapes

Various research suggests slightly different ideals but the likes of Morris and Nixon and also Richmond and Heyda in the 1930s and 1940s propose around 1.5 times the 1kHz reverberation time at 100Hz increasing to twice or 1.75 times at 60Hz.  Generally above 1kHz is flat at 1 times the 1kHz value.  Speech benefits from low RTs and there is little benefit for an increase at low frequencies. 

Morris and Nixon's NBC studio design work sets out various interesting bits of information and approaches to studio design.    

Parts of the ear

The Pinna is the outermost part of the ear, it is the part you can grab - it collects sound and funnels it into the head.  The ear canal is the tube which allows the sound to pass inwards.  These two parts are the outer ear.  The outer eat collects sound.

The middle ear consists of the ear drum which sound strikes, and the connected bones the hammer anvil and stirrup.  These pass the energy in the soundwaves in air and set up vibrations in the fluid in the inner ear.  The bones are the middle ear ossicles.  The middle ear converts energy from one form to another.

The inner ear is primarily the cochlea.  This is a fluid filled snail shell shape which has tiny hair type cells which sense movement in the fluid as the vibrations move the liquid.  The bones of the middle ear are connected to the oval window which transmits the movements of the bones in to the fluid.  The sensing information is called to the brain via the cochlea nerve.  There are also three semi-circular canals in the inner ear that are used to sense orientation and are used for balance.   

The Eustachian tube connects the rear of the ear drum to the back of the nose and allows pressure to be equalised.  This is the mechanism that you use when you fly to stop your ears feeling unwell.

Critical bandwidth

The basilar membrane in the middle of the cochlea allows the ear to sense amplitude and frequency of sound.  The ear acts as a spectral analyser and something between a 1/3 and 1/5 octave sound level meter at mid frequency acting as a series of fixed bandwidth overlapping filters.  The accuracy of the frequency determination is the critical bandwidth.    This is the range of frequencies passed by each filter.  Moore details more about the aural tuning curves used by the brain to sharpen the frequency perception.     

Subjective appraisal of acoustic quality

In 1974 the BBC reported some research into subjective assessment of acoustic quality types compared to objective measurement of those terms.  

The eleven terms were: warmth; definition; colouration; fullness of tone; liveness; intimacy; hardness; timbre; brilliance; string tone; overall.

Each being rated as between (in each case)  colder or warmer; muddier or clearer; more coloured or less coloured; thin or full; dead or live; less or more; hard or mellow; poorer or better; duller or more brilliant; worse or better; and dislike or like.

Various relationships between observed and calculated values with a good fit found. 

Critical bandwidth

The basilar membrane in the middle of the cochlea allows the ear to sense amplitude and frequency of sound.  The ear acts as a spectral analyser and something between a 1/3 and 1/5 octave sound level meter at mid frequency acting as a series of fixed bandwidth overlapping filters.  The accuracy of the frequency determination is the critical bandwidth.    This is the range of frequencies passed by each filter.  Moore details more about the aural tuning curves used by the brain to sharpen the frequency perception.     

Lateral position

The information arriving to the two ears allows the brain to make judgements about the location of the source.  The sound pressure level will be higher on the side exposed to the sound and lower on the side in shadow.  Sound pressure levels will be equal alone the centre line.  This is particularly obvious at high frequencies as these are most attenuated by barriers.  The timing of the arrival of the sound will also be different if the sound source of offset to one side.  The closer ear will receive sound first.  There will also be a phase difference in the signal.  The brain is able to put together the information and determine the likely source location angle relative to the head.  The listener can also adjust the head in order to get multiple readings of the location.  This would be useful for example if the sound might be direct in front or direct behind or overhead.  A few movements of the head will make this clear by supplementing the initial hearing with additional datasets.  The brain also combines visual images and previous knowledge to combine the information together to make a best guess.  A dog bark will be assumed to be on the ground not overhead for example.       

Duct-borne noise

Noise in mechanical services ducting systems impacting on a building fall into three groups.  Noise made outside the building being allowed into the building via the ducting; noise transmitted from one area of the building to another via the duct; and noise and vibration made by the system itself and transferred through the ducting.  The fourth and fifth issues relate to noise escaping the building and impact on other places.  Fourth being music noise or similar escaping the building via the ducting and fifth being plant noise escaping a building and impacting on another place.  

Various building types have these issues more or less at play and more or less critical.  A recording studio is very sensitive to external noise ingress and also cross-talk between areas.  A night-club is sensitive to noise egress via mechanical services.  Apartments in a busy city area might be concerned with cross-talk, self made noise and cross-talk between spaces.         


The time domain and the frequency domain are the main ways to display data.  The time domain has time on the x axis and signal level in the y axis.  This might be RMS dB level for example.

The frequency domain would have frequency in the x axis and dB SPL in the y axis.  The x axis might be linear but would probably be logarithmic.  To transform from time to frequency a Fourier transform is needed else a series of band pass filters to find the content within each particular band.        


Filters are usually classified by their frequency domain behaviour.  Low pass (i.e. excludes all but low frequency content), band pass (i.e. excludes all but a range of frequencies content), band stop (i.e. passes all content apart from at a range of frequencies) and high pass (i.e. permits only high frequency content to pass).  The ideal shape of the filter would be rectangular with straight edges, so for example a high pass set at 2kHz would allow 2kHz content and anything above with unity gain, and any content at 1999Hz or below would be attenuated to zero.  Filters introduce delay as a number of samples are required to perform the manipulation.  This is tolerated, but there would ideally be no phase distortion.  Not all theoretical filters can be realised.     


Four Thirty Three

Four Thirty Three is the 1952 John Cage composition which involves no instruments being played over three movements and so is essentially the sound of the room being listened to for the full performance of the piece.  Frank Zappa recorded it in the 1990s around the time of Cage's death.  Four Thirty Three was played by the Berlin Philharmonic as the last piece before their Covid lockdown in 2020 in part protest.

Cage was interested in the lines between silence and also noise and music.  Cage is quoted as saying that noises are sounds that have not yet been intellectualised and that there is no such thing as silence.

It is true that there is no such thing as silence.  Silence only exists when nothing else exists.           

Microphone sensitivity

Sensitivity relates to the output voltage of a microphone for a given input sound pressure.  For a 1 Pascal pressure input at 1kHz, which is 94dB, what is the output voltage.  That is the question.  

For example a Shure SM-57 dynamic mic states a sensitivity of 1.6 mV per Pascal (94dB SPL).  So for a 1 Pascal input (which is 94dB as a sound pressure level and equal to the usual calibration tone), the output will be 1.6mV which is 1.6 millivolts.
The Shure SM-57 also has a stated sensitivity of -56 dBV/Pa. 

1V = 0dBV (decibel volts) so minus 56dBV is less than 1 volt output per 1 Pascal.

It is 20*log(x/1) that gives the dBV value where x is the sensitivity of the microphone in terms of volts per Pascal.  Negative values of dBV mean less than 1 Volt is produced.  

A more sensitive microphone might be 30mV per Pascal which would be -30dBV.

Condenser microphones might be 20dB or 30dB more sensitive than dynamic microphones.


Mean Free Path

The mean free path is 4V/S where V is the volume of a space and S is the surface area of the inner faces of the space.

In 1960 Kosten explains and derives the equation and explains that there is 1/2 which comes from the fact that for any individual path, it strikes two surfaces, so the must be a divide by 2 required.  The second 1/2 comes from the average cos theta of the projected angle over the 4pi space solid angle i.e. the full sphere of possible angles.

The 4V/S is found to hold true to this day for diffusely reflecting surfaces.  It remains a good estimate for all rooms.         

T20 RT

Reverberation times are difficult and there is really no correct reverberation time.  We estimate a 60dB decay and assume an even and linear decay.  Depending how the data is used, the reverberation time found can vary.  How is the source level determined? How is the background level determined? How is the slope determined? Is T20 or T30 used? How to these relate to the true T60?  Of course all of this occurs at all frequencies and can alter results significantly.


Middle ear

The middle ear allows sound to transfer from the outer to the inner ear.  It is an impedance matching system which allows energy to transmit rather than be reflected.  The second function of the middle ear as stated by Barany in 1938 is to allow external sound to be accepted into the cochlea while self made noise is largely rejected.  Road traffic noise, rail noise, speech, and any other sound made outside of the body is encouraged to pass into the inner ear for detection, but the noise of chewing or blood flow and any other internal noise is generally rejected due to the action of the ossicles.  The system is set up so that as the skull vibrates there is little differential movement and little transfer of energy.

The muscles in the ossicles can contract when exposed to intense pressure levels middle ear reflex.  This is an attempt to reduce the transmitted energy and protect the inner ear. The action is slow and poor at protecting against impulsive sound like gunshots.  It is activated when speech is produced - so self made speech is attenuated.             

Harmonic series

Fundamental frequencies and harmonics on a string take this form which is used in some areas of nature such as shells.  The nodes are the places where the string stays in the centre - such as at the two ends of the string.  The anti-nodes or loops are the places there the string can move away from the middle.

f = 1/2L * (T/(m/L))^1/2
where f is fundamental frequency of the string, L is the length of the string in cm, T is the tension in dynes which is gcm/s2, m is the mass  in grams 


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Tinnitus is a symptom.  It is a ringing sensation in the ear that is not really there.  Some people describe it as a buzzing or a rustling or a buzzing or a rushing.  It might sound like an engine or a generator or maybe even a cricket chirping.  

Known causes include ear infections which can be treated.  Stress which will hopefully also be temporary.  Circulation  and blood disorders are known to cause tinnitus.  Drugs can cause tinnitus and this can be listed as a side effect for medication.  

Meniere's disease can cause tinnitus.

Noise exposure is a common cause of tinnitus.  Noise at Work regulations sets limits to protect staff and reduce exposure to noise.  


Sound Exposure Level LAE

LAE or SEL is an LAeq normalised or compressed into 1 second.

LAE = LAeq +10log(t) where t is the time in seconds of the measurement of the event.


LCE = LCeq +10log(t)

The above is the encoding of the sound exposure level.

Below is the the method to use the sound exposure level to find a sound pressure level over a time time period.

LAeq = LAE +10log(n) - 10log(T)


LCeq = LCE +10log(n) - 10log(T)

where n is the number of events and T is the time over which the sound is being considered.


Reverberation times in complex rooms

Sabine and Eyring have limitations.  

Kang provides an alternative for long spaces such as underground stations which is of interest and improves accuracy in some cases:

Td = (150 / 850M - 10 log[   (d/(d+850) ) * (1- alpha) ^25.6* (1/H + 1/W )* (d+425)^ (1/2 ] 

Where d is the source–receiver distance in meters
W is the room width in meters
H is the room height in meters 
αlpha is the average absorption coefficient of all boundaries 
M is the air absorption coefficient in dB per meter


Vibration Dose Value Units

Vibration Dose Value VDV has unusual units m/s^1.75

The 'a' in this equation is acceleration  and so has units m/s^2  or m/s/s or ms^-2.
Raised to the 4th power becomes m^4/s^8.  The integral with respect to time t (dt) means that the s^8 moves to s^7 in the same way acceleration m/s^2 would move to m/s integrating from acceleration to velocity moving the power up 1.

The m^4/s^7 is now raised to the 1/4 power, this becomes m/s^(7/4).  This is m/s^1.75.