A-weighting
A-weighting is the most commonly used of a
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
History
A-weighting began with work by
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
Because of perceived discrepancies between early and more recent determinations, the International Organization for Standardization (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.[3]
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.[citation needed] 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.[4] 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 in the inner ear 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.[citation needed]
Following changes to the ISO standard, D-frequency-weighting by itself 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 is required for certification of large transport aircraft.[5] D-weighting is the basis for the measurement underlying EPNdB.
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[citation needed]. It is a flat frequency response between 10 Hz and 20 kHz ±1.5 dB.[6][failed verification] 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 range from 8 Hz to about 40 Hz.[7]
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
The A-weighting curve has been widely adopted for
A-weighted sound power levels LWA are increasingly found on sales literature for domestic appliances such as refrigerators, freezers and computer fans. The expected sound pressure level to be measured at a given distance as SPL with a sound level meter can with some simplifications be calculated from the sound power level. 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, 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.[8] 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.[citation needed] The high-frequency bands are wider in absolute terms than the low-frequency bands, and therefore 'collect' proportionately more power from a noise source.[citation needed] However, when more than one critical band is stimulated, the outputs of the various bands are summed by the brain 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
Function realisation of some common weightings
The standard[9] defines weightings () in dB units by tables with tolerance limits (to allow a variety of implementations). Additionally, the standard describes weighting functions [9] to calculate the weightings. The weighting function 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:[10]
A
B
C
D
Transfer function equivalent
The gain curves can be realised[12] by the following s-domain transfer functions. They are not defined in this way though, being defined by tables of values with tolerances in the standards documents, thus allowing different realisations:[citation needed]
A
- kA ≈ 7.39705 × 109
B
- kB ≈ 5.99185 × 109
C
- kC ≈ 5.91797 × 109
D
- kD ≈ 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.)
See also
- Noise
- Signal noise
- ITU-R 468 noise weighting
- M-weighting
- Psophometric weighting
- Audio quality measurement
- Noise pollution
- Noise regulation
- Headroom
- Rumble measurement
- Weighting filter
- Weighting curve
- Luminous efficiency function, the light equivalent
- LKFS
Notes
- dBrn adjustedis not a synonym for dB(A), but for dBa. (In telecommunications dBa denotes "decibels adjusted", i.e. weighted absolute noise power, which has nothing to do with A-weighting.)
References
- PMID 15885208.
- ^ Pierre, Jr., Richard L. St.; Maguire, Daniel J. (July 2004). "The Impact of A-weighting Sound Pressure Level Measurements during the Evaluation of Noise Exposure" (PDF). Retrieved 2011-09-13.
- ^ "Precise and Full-range Determination of Two-dimensional Equal Loudness Contours" (PDF). Archived from the original (PDF) on 2007-09-27.
- S2CID 13997453.
- ^ "BIP_2_2_jb ZIP file" (PDF).
- S2CID 7436249.
- ^ Ratzel, U.; Bayer, O.; Brachat, P.; Hoffmann, M.; Jänke, K.; Kiesel, K.-J.; Mehnert, C.; Scheck, C.; Westerhausen, C.; Krapf, K.-G.; Herrmann, L.; Blaul, J., eds. (February 2020) [2016-02-26]. "Tieffrequente Geräusche inkl. Infraschall von Windkraftanlagen und anderen Quellen - Bericht über Ergebnisse des Messprojekts 2013-2015" (in German) (3 ed.). Karlsruhe, Germany: Landesanstalt für Umwelt, Messungen und Naturschutz Baden-Württemberg (LUBW), Referat 34 – Technischer Arbeitsschutz, Lärmschutz. pp. 10–11, 13, 17, 22–24, 27–28, 32–33, 38–39, 43–44, 49, 90. Retrieved 2021-06-07. p. 90:
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.
[1] (104 pages) - .
- ^ a b c d IEC 61672-1:2013 Electroacoustics - Sound level meters - Part 1: Specifications. IEC. 2013.
- ^ "Frequency weighting equations". Cross Spectrum. 2004. Archived from the original on 2011-06-17.
- ^ Aarts, Ronald M. (1992-03-01). "A Comparison of Some Loudness Measures for Loudspeaker Listening Tests". Audio Engineering Society. 40 (3): 142–146. Archived from the original on 2022-10-27. Retrieved 2022-10-27.
- ^ "Noise Measurement Briefing". Product Technology Partners Ltd. Archived from the original on 2008-06-30.
Further reading
- Audio Engineer's Reference Book, 2nd Ed 1999, edited Michael Talbot Smith, Focal Press
- An Introduction to the Psychology of Hearing 5th ed, Brian C. J. Moore, Elsevier Press
External links
- Noise Measurement Briefing. Archived from the original on 2013-02-25.
- A-weighting filter circuit for audio measurements Archived 2016-12-31 at the Wayback Machine
- Weighting Filter Set Circuit diagrams
- AES pro audio reference definition of "weighting filters"
- Frequency Weighting Equations
- A-weighting in detail
- A-Weighting 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