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The A-weighting curve is one of a family of curves defined in IEC179 and various other standards for use in sound level meters. Though originally intended only for the measurement of low-level sounds (around 40-phon) it is now commonly used for the measurement of environmental noise and industrial noise, when assessing potential hearing damage and other noise health effects at moderate to high intensity levels. A-weighting also finds widespread use in audio equipment measurement, though arguably it may not be the most suitable weighting for this purpose.
A-weighting is only really valid for relatively quiet sounds and for pure tones as it is based on the 40-phon Fletcher-Munson curves which represented an early determination of the equal-loudness contour for human hearing. The B and C curves were intended for louder sounds (though they are less used) while the D curve is used in assessing loud aircraft noise (IEC 537). Although the original Fletcher-Munson family of contours have been subject to fresh experimental determinations, notably by Robinson-Dadson, a recent survey by the ISO standards organisation suggests that the latter may have been less accurate than thought, and the survey comments that it is fortunate that in the latest standard set of contours, defined in ISO 226 (2003), the 40-phon curve comes closer to the original Fletcher-Munson, than to the Robinson-Dadson. Nevertheless, it will be noted that A-weighting would be a better match to the loudness curve if it fell much more steeply above 10 kHz, and it can be assumed that a better match was not aimed for because steep filters were more difficult to construct in the early days of electronics. Nowadays, no such limitation need exist, as demonstrated by the ITU-R 468 curve. If weightings such as these are used without further band-limiting it is possible to obtain different readings on different instruments if ultrasonic noise is present. Good design therefore requires a 20 kHz low-pass filter to be combined with the weighting curve.
A-weighted decibels are abbreviated dB(A) or dBA. When acoustic (calibrated microphone) measurements are being referred to, then the units used will be dB SPL referenced to 20 micropascals = 0 dB SPL. dBrn adjusted is a synonym for dBA.
While the A-weighting curve has been widely adopted for environmental noise measurement, and is standard in many sound level meters, it does not really give valid results for noise because of the way in which the human ear analyzes sound. The A-weighting system is used commonly in roadway noise and aircraft noise analyses. We are considerably more sensitive to noise in the region of 6 kHz than we are to tones of equivalent level (see ITU-R 468 weighting for further explanation).
A-weighting is also in common use for assessing potential hearing damage caused by loud noise, though this seems to be based on the widespread availability of sound level meters incorporating A-weighting rather than on any good experimental evidence to suggest that such use is valid. The distance of the measuring microphone from a sound source is often "forgotten", when SPL measurements are quoted, making the data useless. In the case of environmental or aircraft noise distance need not be quoted, as it is the level at the point of measurement that is needed, but when measuring refrigerators and similar appliances the distance should be stated; where not stated it is usually one metre (1 m). An extra complication here is the effect of a reverberant room, and so noise measurement on appliances should state "at 1 m in an anechoic chamber". Measurements made outdoors will approximate well to anechoic conditions.
A-weighted SPL measurements of noise level are increasingly to be found on sales literature for domestic appliances such as refrigerators and freezers, and computer fans. Although the threshold of hearing is typically around 0 dB SPL, this is in fact very quiet indeed, and appliances are more likely to have noise levels of 30 to 40 dB SPL.
Human sensitivity to noise in the region of 6 kHz became particularly apparent in the late 1960's with the introduction of compact cassette recorders and Dolby-B noise reduction. 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 sometimes one piece of equipment would even measure worse than another and yet sound better, because of differing spectral content.
ITU-R 468 noise weighting 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 BBC Research Department, and was standardised by the CCIR and later adopted by many other standards bodies (IEC, BSI) and, as of 2006, is maintained by the ITU. It was widely used in Europe, especially in broadcasting, especially when it was adopted by Dolby Laboratories who realised its superior validity for their purposes. Its advantages over A-weighting seem to be less well understood in the US, where the use of A-weighting predominates. It is universally used by broadcasters in Britain, Europe, and former countries of the British Empire such as Australia and South Africa.
Though the noise level of 16-bit audio systems (such as CD players) is commonly quoted (on the basis of calculations that take no account of subjective effect) as −98 dBFS (relative to full scale), the best 468-weighted results are in the region of −68 dB relative to alignment level (commonly defined as 18 dB below FS), or −86 dBFS.
The use of weighting curves can be regarded as "cheating" when used to hide less-than-ideal performance in the regions outside the weighted band, and since weighted measurements are better than the unweighted measurements used by competitors. (The weighting method used should therefore always be specified along with a measurement.) They are more correct from a psychoacoustic perspective, however, provided that the proper curve is used.
For instance, noise shaping achieves the same distortion benefits as dither noise, but moves the noise to inaudible high frequencies. This would measure differently in weighted and unweighted tests. In this case, the extra noise is both harmless (beneficial, actually) and inaudible, and a weighted measurement would be appropriate.
The gain curves are defined by the following s-domain transfer functions :
kA ≈ 7.39705×109
kB ≈ 5.99185×109
kC ≈ 5.91797×109
kD ≈ 91104.32
The k values are constants which 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.)