A-weighting is the most commonly used of a

family of curves In geometry, a family of curves is a set of curves, each of which is given by a function or parametrization in which one or more of the parameters is variable. In general, the parameter(s) influence the shape of the curve in a way that is more c ...

defined in the International standard

IEC The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...

61672:2003 and various national standards relating to the measurement of

sound pressure level Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone ...

. A-weighting is applied to instrument-measured sound levels in an effort to account for the relative

loudness In acoustics, loudness is the subjectivity, subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The rel ...

perceived by the human ear, as the ear is less sensitive to low audio frequencies. It is employed by arithmetically adding a table of values, listed by

octave In music, an octave ( la, octavus: eighth) or perfect octave (sometimes called the diapason) is the interval between one musical pitch and another with double its frequency. The octave relationship is a natural phenomenon that has been refer ...

or third-octave bands, to the measured sound pressure levels in dB. The resulting octave band measurements are usually added (logarithmic method) to provide a single A-weighted value describing the sound; the units are written as dB(A). Other weighting sets of values – B, C, D and now Z – are discussed below. The curves were originally defined for use at different average sound levels, but A-weighting, though originally intended only for the measurement of low-level sounds (around 40

phon The phon is a logarithmic physical unit, unit of loudness level for tones and complex sounds. Loudness is measured in sone which is a linear unit. Human sensitivity to sound is variable across different frequencies; therefore, although two diff ...

), is now commonly used for the measurement of

environmental noise Environmental noise is an accumulation of noise pollution that occurs outside. This noise can be caused by transport, industrial, and recreational activities. Noise is frequently described as 'unwanted sound'. Within this context, environmenta ...

and

industrial noise Occupational noise is the amount of acoustic energy received by an employee's auditory system when they are working in the industry. Occupational noise, or industrial noise, is often a term used in occupational safety and health, as sustained expo ...

, as well as when assessing potential

hearing damage Hearing loss is a partial or total inability to hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to acquire spoken l ...

and other

noise health effects Noise health effects are the physical and psychological health consequences of regular exposure to consistent elevated sound levels. Noise from traffic, in particular, is considered by the World Health Organization to be one of the worst environm ...

at all sound levels; indeed, the use of A-frequency-weighting is now mandated for all these measurements, because decades of field experience have shown a very good correlation with occupational deafness in the frequency range of human speech. It is also used when measuring low-level noise in audio equipment, especially in the United States. In Britain, Europe and many other parts of the world, broadcasters and audio engineers more often use the

ITU-R 468 noise weighting ITU-R 468 (originally defined in CCIR recommendation 468-4, therefore formerly also known as CCIR weighting; sometimes referred to as CCIR-1k) is a standard relating to noise measurement, widely used when measuring noise in audio systems. The ...

, which was developed in the 1960s based on research by the

BBC #REDIRECT BBC #REDIRECT BBC #REDIRECT BBC Here i going to introduce about the best teacher of my life b BALAJI sir. He is the precious gift that I got befor 2yrs . How has helped and thought all the concept and made my success in the 10th board ex ...

and other organizations. This research showed that our ears respond differently to random noise, and the equal-loudness curves on which the A, B and C weightings were based are really only valid for pure single tones.

History

A-weighting began with work by Fletcher and Munson which resulted in their publication, in 1933, of a set of

equal-loudness contours An equal-loudness contour is a measure of sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon and ...

. Three years later these curves were used in the first American standard for

sound level meter A sound level meter (also called sound pressure level meter (SPL)) is used for acoustic measurements. It is commonly a hand-held instrument with a microphone. The best type of microphone for sound level meters is the condenser microphone, whic ...

s. This

ANSI The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...

standard, later revised as ANSI S1.4-1981, incorporated B-weighting as well as the A-weighting curve, recognising the unsuitability of the latter for anything other than low-level measurements. But B-weighting has since fallen into disuse. Later work, first by Zwicker and then by Schomer, attempted to overcome the difficulty posed by different levels, and work by the BBC resulted in the CCIR-468 weighting, currently maintained as ITU-R 468 noise weighting, which gives more representative readings on noise as opposed to pure tones.

Deficiencies

A-weighting is valid to represent the sensitivity of the human ear as a function of the frequency of pure tones. The A-weighting was based on the 40-phon

Fletcher–Munson curves An equal-loudness contour is a measure of sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon and i ...

, which represented an early determination of the

equal-loudness contour An equal-loudness contour is a measure of sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon and ...

for human hearing. However, because decades of field experience have shown a very good correlation between the A scale and occupational deafness in the frequency range of human speech, this scale is employed in many jurisdictions to evaluate the risks of occupational deafness and other auditory problems related to signals or speech intelligibility in noisy environments. Because of perceived discrepancies between early and more recent determinations, the

International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...

(ISO) revised its standard curves as defined in ISO 226, in response to the recommendations of a study coordinated by the Research Institute of Electrical Communication, Tohoku University, Japan. The study produced new curves by combining the results of several studies, by researchers in Japan, Germany, Denmark, UK, and USA. (Japan was the greatest contributor with about 40% of the data.) This has resulted in the recent acceptance of a new set of curves standardized as ISO 226:2003. The report comments on the surprisingly large differences, and the fact that the original Fletcher–Munson contours are in better agreement with recent results than the Robinson-Dadson, which appear to differ by as much as 10–15 dB especially in the low-frequency region, for reasons that are not explained. The report also shows that the 40-phon Fletcher-Munson contour is in better agreement with the updated 60-phon contour incorporated into ISO 226:2003, which challenges the common assertion that A-weighting represents loudness only for quiet sounds. Nevertheless, A-weighting would be a better match to the loudness curve if it fell much more steeply above 10 kHz, and it is likely that this compromise came about because steep filters were difficult to construct in the early days of electronics. Nowadays, no such limitation need exist, as demonstrated by the ITU-R 468 curve. If A-weighting is used without further band-limiting it is possible to obtain different readings on different instruments when ultrasonic, or near ultrasonic noise is present. Accurate measurements therefore require a 20 kHz low-pass filter to be combined with the A-weighting curve in modern instruments. This is defined in IEC 61012 as AU weighting and while very desirable, is rarely fitted to commercial sound level meters.

B-, C-, D-, G- and Z-weightings

A-frequency-weighting is mandated by the international standard IEC 61672 to be fitted to all sound level meters and are approximations to the equal loudness contours given in ISO 226. The old B- and D-frequency-weightings have fallen into disuse, but many sound level meters provide for C frequency-weighting and its fitting is mandated — at least for testing purposes — to precision (Class one) sound level meters. D-frequency-weighting was specifically designed for use when measuring high-level aircraft noise in accordance with the IEC 537 measurement standard. The large peak in the D-weighting curve is not a feature of the equal-loudness contours, but reflects the fact that humans hear random noise differently from pure tones, an effect that is particularly pronounced around 6 kHz. This is because individual neurons from different regions of the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...

in the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

respond to narrow bands of frequencies, but the higher frequency neurons integrate a wider band and hence signal a louder sound when presented with noise containing many frequencies than for a single pure tone of the same pressure level. Following changes to the ISO standard, D-frequency-weighting should now only be used for non-bypass-type jet engines, which are found only on military aircraft and not on commercial aircraft. For this reason, today A-frequency-weighting is now mandated for light civilian aircraft measurements, while a more accurate loudness-corrected weighting

EPNdB Effective perceived noise in decibels (EPNdB) is a measure of the relative noisiness of an individual aircraft pass-by event. It is used for aircraft noise certification and applies to an individual aircraft, not the noise exposure from an airport. ...

is required for certification of large transport aircraft. Z- or ZERO frequency-weighting was introduced in the International Standard IEC 61672 in 2003 and was intended to replace the "Flat" or "Linear" frequency weighting often fitted by manufacturers. This change was needed as each sound level meter manufacturer could choose their own low and high frequency cut-offs (–3 dB) points, resulting in different readings, especially when peak sound level was being measured. It is a flat frequency response between 10 Hz and 20 kHz ±1.5 dB. As well, the C-frequency-weighting, with –3 dB points at 31.5 Hz and 8 kHz did not have a sufficient bandpass to allow the sensibly correct measurement of true peak noise (Lpk). G-weighting is used for measurements in the

infrasound Infrasound, sometimes referred to as low status sound, describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to perce ...

range from 8 Hz to about 40 Hz. B- and D-frequency-weightings are no longer described in the body of the standard IEC 61672:2003, but their frequency responses can be found in the older IEC 60651, although that has been formally withdrawn by the International Electrotechnical Commission in favour of IEC 61672:2003. The frequency weighting tolerances in IEC 61672 have been tightened over those in the earlier standards IEC 179 and IEC 60651 and thus instruments complying with the earlier specifications should no longer be used for legally required measurements.

Environmental and other noise measurements

A-weighted

decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...

s are abbreviated dB(A) or dBA. When acoustic (calibrated microphone) measurements are being referred to, then the units used will be dB

SPL SPL may refer to: Association football * Saudi Professional League * Scottish Premier League * SportPesa Premier League, Kenya * Singapore Premier League * RoboCup Standard Platform League, matches between autonomous robots Computing Program ...

referenced to 20 micropascals = 0 dB SPL. The A-weighting curve has been widely adopted for

measurement, and is standard in many sound level meters. The A-weighting system is used in any measurement of environmental noise (examples of which include

roadway noise Roadway noise is the collective sound energy emanating from motor vehicles. It consists chiefly of road surface, tire, engine/transmission, aerodynamic, and braking elements. Noise of rolling tires driving on pavement is found to be the biggest ...

, rail noise,

aircraft noise Aircraft noise pollution refers to noise produced by aircraft in flight that has been associated with several negative stress-mediated health effects, from sleep disorders to cardiovascular ones. Governments have enacted extensive controls that a ...

). A-weighting is also in common use for assessing potential

caused by loud noise, including noise dose measurements at work. A noise level of more than 85 dB(A) each day increases the risk factor for hearing damage. A-weighted SPL measurements of noise level are increasingly found on sales literature for domestic appliances such as refrigerators, freezers and computer fans. In Europe, the A-weighted noise level is used for instance for normalizing the noise of tires on cars. Noise exposure for visitors of venues with loud music is usually also expressed in dB(A), although the presence of high levels of low frequency noise does not justify this.

Audio reproduction and broadcasting equipment

Although the A-weighting curve, in widespread use for

noise measurement In acoustics, noise measurement can be for the purpose of measuring environmental noise or measuring noise in the workplace. Applications include monitoring of construction sites, aircraft noise, road traffic noise, entertainment venues and neighb ...

, is said to have been based on the 40-phon Fletcher-Munson curve, research in the 1960s demonstrated that determinations of equal-loudness made using pure tones are not directly relevant to our perception of noise. This is because the cochlea in our inner ear analyses sounds in terms of spectral content, each 'hair-cell' responding to a narrow band of frequencies known as a critical band. The high-frequency bands are wider in absolute terms than the low frequency bands, and therefore 'collect' proportionately more power from a noise source. However, when more than one critical band is stimulated, the outputs of the various bands are summed by

the brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ i ...

to produce an impression of loudness. For these reasons equal-loudness curves derived using noise bands show an upwards tilt above 1 kHz and a downward tilt below 1 kHz when compared to the curves derived using pure tones. This enhanced sensitivity to noise in the region of 6 kHz became particularly apparent in the late 1960s with the introduction of

compact cassette The Compact Cassette or Musicassette (MC), also commonly called the tape cassette, cassette tape, audio cassette, or simply tape or cassette, is an analog magnetic tape recording format for audio recording and playback. Invented by Lou Ottens ...

recorders and

Dolby-B noise reduction A Dolby noise-reduction system, or Dolby NR, is one of a series of noise reduction systems developed by Dolby Laboratories for use in analog audio tape recording. The first was '' Dolby A'', a professional broadband noise reduction sy ...

. A-weighted noise measurements were found to give misleading results because they did not give sufficient prominence to the 6 kHz region where the noise reduction was having greatest effect, and did not sufficiently attenuate noise around 10 kHz and above (a particular example is with the 19 kHz pilot tone on FM radio systems which, though usually inaudible is not sufficiently attenuated by A-weighting, so that sometimes one piece of equipment would even measure worse than another and yet sound better, because of differing spectral content.

was therefore developed to more accurately reflect the subjective loudness of all types of noise, as opposed to tones. This curve, which came out of work done by the

Research Department, and was standardised by the CCIR and later adopted by many other standards bodies (

, BSI) and, , is maintained by the ITU. It became widely used in Europe, especially in broadcasting, and was adopted by

Dolby Laboratories Dolby Laboratories, Inc. (often shortened to Dolby Labs and known simply as Dolby) is an American company specializing in Noise reduction#In audio, audio noise reduction, Audio data compression, audio encoding/compression, spatial audio, and ...

who realised its superior validity for their purposes when measuring noise on film soundtracks and compact cassette systems. Its advantages over A-weighting are less accepted in the US, where the use of A-weighting still predominates. It is used by broadcasters in Britain, Europe, and former countries of the British Empire such as Australia and South Africa.

Function realisation of some common weightings

The standard defines weightings (

A(f), C(f)

) in dB units by tables with tolerance limits (to allow a variety of implementations). Additionally, the standard describes weighting functions

R_X(f)

to calculate the weightings. The weighting function

R_X(f)

is applied to the amplitude spectrum (not the intensity spectrum) of the unweighted sound level. The offsets ensure the normalisation to 0 dB at 1000 Hz. Appropriate weighting functions are:

A

A(f) &= 20\log_\left(R_A(f)\right) - 20\log_\left(R_A(1000)\right) \\ &\approx 20\log_\left(R_A(f)\right) + 2.00 \end

B

B(f) &= 20\log_\left(R_B(f)\right) - 20\log_\left(R_B(1000)\right) \\ &\approx 20\log_\left(R_B(f)\right) + 0.17 \end

C

&\approx 20\log_\left(R_C(f)\right) + 0.06 \end

D

R_D(f) &= \frac \sqrt \\ D(f) &= 20\log_\left(R_D(f)\right). \end

Transfer function equivalent

The gain curves can be realised by the following s-domain

transfer function In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a function (mathematics), mathematical function that mathematical model, theoretically models the system's output for ...

s. They are not defined in this way though, being defined by tables of values with tolerances in the standards documents, thus allowing different realisations:

A

H_\text(s) \approx

:''k''_A ≈ 7.39705 × 10⁹

B

H_\text(s) \approx

:''k''_B ≈ 5.99185 × 10⁹

C

H_\text(s) \approx

:''k''_C ≈ 5.91797 × 10⁹

D

H_\text(s) \approx

:''k''_D ≈ 91104.32 The ''k''-values are constants that are used to normalize the function to a gain of 1 (0 dB). The values listed above normalize the functions to 0 dB at 1 kHz, as they are typically used. (This normalization is shown in the image.)

Notes

References

{{Reflist, refs= {{Cite journal , last=Meyer-Bisch , first=Christian , date=2005 , title= easuring noise, journal=Médecine/Sciences , volume=21 , issue=5 , pages=546–550 , doi=10.1051/medsci/2005215546 , issn=0767-0974 , pmid=15885208 , doi-access=free {{citation , url= http://storeycountywindfarms.org/ref3_Impact_Sound_Pressure.pdf , author-first1=Richard L. St. , author-last1=Pierre, Jr. , author-first2=Daniel J. , author-last2=Maguire , title=The Impact of A-weighting Sound Pressure Level Measurements during the Evaluation of Noise Exposure , date=July 2004 , access-date=2011-09-13 {{citation , title=Precise and Full-range Determination of Two-dimensional Equal Loudness Contours , url=http://www.nedo.go.jp/itd/grant-e/report/00pdf/is-01e.pdf , archive-url=https://web.archive.org/web/20070927210848/http://www.nedo.go.jp/itd/grant-e/report/00pdf/is-01e.pdf , archive-date=2007-09-27 {{cite journal , author-last1=Rimell , author-first1=Andrew , author-last2=Mansfield , author-first2=Neil , author-last3=Paddan , author-first3=Gurmail , s2cid=13997453 , title=Design of digital filters for frequency weightings (A and C) required for risk assessments of workers exposed to noise , journal=Industrial Health , volume=53 , date=2015 , issue=53 , pages=21–27 , doi=10.2486/indhealth.2013-0003 , pmid=25224333 , pmc=4331191 http://www.icao.int/Meetings/EnvironmentalWorkshops/Documents/NoiseCertificationWorkshop-2004/BIP_2_2_jb.pdf {{Bare URL PDF, date=March 2022 {{cite journal , author-last1=Lauer , author-first1=Amanda , author-last2=El‐Sharkawy , author-first2=AbdEl‐Monem M. , author-last3=Kraitchman , author-first3=Dara , author-last4=Edelstein , author-first4=William , title=MRI Acoustic Noise Can Harm Experimental and Companion Animals , journal=Journal of Magnetic Resonance Imaging , date=2012 , volume=36 , issue=3 , pages=743–747 , doi=10.1002/jmri.23653 , pmid=22488793 , s2cid=7436249 , doi-access=free {{cite journal , title=Researches in loudness measurement , journal=IEEE Transactions on Audio and Electroacoustics , volume=14 , issue=3 , pages=141–151 , doi=10.1109/TAU.1966.1161864 , date=1966 , author-last1=Bauer , author-first1=B. , author-last2=Torick , author-first2=E. {{cite book , title=IEC 61672-1:2013 Electroacoustics - Sound level meters - Part 1: Specifications , date=2013 , publisher=IEC {{cite web , publisher=Cross Spectrum , date=2004 , url-status=live , url=http://www.cross-spectrum.com/audio/weighting.html , title=Frequency weighting equations , archive-url=https://web.archive.org/web/20110617080228/http://www.cross-spectrum.com/audio/weighting.html , archive-date=2011-06-17{{Unreliable source?, date=March 2011 {{cite journal , author1-link=Ronald Aarts, url=https://www.aes.org/e-lib/browse.cfm?elib=7054 , date=1 March 1992 , journal=

Audio Engineering Society The Audio Engineering Society (AES) is a professional body for engineers, scientists, other individuals with an interest or involvement in the professional audio industry. The membership largely comprises engineers developing devices or products ...

, volume=40, issue=3, pages=142–146 , author-first=Ronald M. , author-last=Aarts , title=A Comparison of Some Loudness Measures for Loudspeaker Listening Tests , access-date=2022-10-27, archive-url=https://web.archive.org/web/20221027155213/https://www.aes.org/e-lib/browse.cfm?elib=7054 , archive-date=2022-10-27, url-status=live, url-access=subscription {{citation , url=http://www.ptpart.co.uk/show.php?contentid=70 , archive-url=https://web.archive.org/web/20080630083107/http://www.ptpart.co.uk/show.php?contentid=70 , archive-date=2008-06-30 , title=Noise Measurement Briefing , publisher=Product Technology Partners Ltd. {{cite web , title=Tieffrequente Geräusche inkl. Infraschall von Windkraftanlagen und anderen Quellen - Bericht über Ergebnisse des Messprojekts 2013-2015 , language=de , edition=3 , date=February 2020 , location=Karlsruhe, Germany , editor1-first=U. , editor1-last=Ratzel , editor2-first=O. , editor2-last=Bayer , editor3-first=P. , editor3-last=Brachat , editor4-first=M. , editor4-last=Hoffmann , editor5-first=K. , editor5-last=Jänke , editor6-first=K.-J. , editor6-last=Kiesel , editor7-first=C. , editor7-last=Mehnert , editor8-first=C. , editor8-last=Scheck , editor9-first=C. , editor9-last=Westerhausen , editor10-first=K.-G. , editor10-last=Krapf , editor11-first=L. , editor11-last=Herrmann , editor12-first=J. , editor12-last=Blaul , orig-date=2016-02-26 , pages=10–11, 13, 17, 22–24, 27–28, 32–33, 38–39, 43–44, 49, 90 , publisher=

Landesanstalt für Umwelt, Messungen und Naturschutz Baden-Württemberg The ''Landesanstalt für Umwelt, Messungen und Naturschutz Baden-Württemberg'' (LUBW) or Baden-Württemberg State Institute for the Environment, Survey and Nature Conservation is a central institution of the Germany, German States of Germany, fed ...

(LUBW), Referat 34 – Technischer Arbeitsschutz, Lärmschutz , url=https://pudi.lubw.de/detailseite/-/publication/84558 , access-date=2021-06-07 , quote-page=90 , quote=Für den Bereich des Infraschalls gibt es eine eigene Frequenzbewertung, die so genannte G-Bewertung. Entsprechend bewertete Pegel werden als dB(G) – „Dezibel G“ – angegeben. Bekannter ist die A-Bewertung von Geräuschen als dB(A) – „Dezibel A“ –, die dem Hörempfinden des Menschen nachempfunden ist. Die G-Bewertung hat ihren Schwerpunkt bei 20 Hz. Zwischen 10 Hz und 25 Hz werden Pegel verstärkt, darunter und darüber fällt die Bewertungskurve rasch ab. Zweck der G-Bewertung ist es, eine Situation im Hinblick auf tiefe Frequenzen bzw. Infraschall mit einer einzigen Zahl zu charakterisieren. Ein Nachteil ist, dass Frequenzen unterhalb 8 Hz und oberhalb 40 Hz kaum mehr einen Beitrag leisten.}

(104 pages)

External links

Noise Measurement Briefing
Archived fro
the original
on 2013-02-25.

Circuit diagrams
AES pro audio reference definition of "weighting filters"

A-weighting in detail

Equation and online calculation
Researches in loudness measurement by CBS using noise bands, 1966 IEEE Article

Comparison of some loudness measures for loudspeaker listening tests (Aarts, JAES, 1992)
PDF containing algorithm for ABCD filters Noise pollution Sound Audio engineering Noise Articles containing video clips Acoustics de:Bewerteter Schalldruckpegel fr:Décibel A ja:A特性