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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 industrial noise too when assessing potential hearing damage at high levels. It also finds widespread use in audio equipment measurement, though arguably it is not the most suitable weighting for this purpose.



Loudness measurements
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 10kHz, and it must be assumed that a better match was not aimed for because steep filters were more dificult 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 20kHz low-pass filter to be combined with the weighting curve.



Environmental noise measurement
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 (sound pressure level) 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 our ears analyse sound. We are considerably more sensitive to noise in the region of 6kHz 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 0dB SPL, this is in fact very quiet indeed, and appliances are more likely to have noise levels of 30 to 40dB SPL.

Audio reproduction and broadcasting equipment


Human sensitivity to noise in the region of 6kHz 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 6kHz 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 is now maintained by the ITU is universally used by Broadcasters in Britain, Europe, and former countries of the British Empire such as Australia and South Africa. It looked set to take over from A-weighting in the 1970's, though it remained less well known in the USA where A-weighting still predominates.

It was widely adopted in the UK, Australia, South Africa and Europe, especially when it was adopted by the Dolby corporation who realised its superior validity for their purposes. Though now the weighting of choice in these and other countries, it's advantages over A-weighting seem to be less well understood in the USA, where the use of A-weighting predominates.

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 −96 dB relative to FS (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) ie −86 dB relative to FS.

The use of weighting curves is in no way to be regarded as 'cheating', provided that the proper curve is used. Nothing of relevance is being 'hidden', and even when, for example, hum is present at 50 or 100Hz at a level above the quoted (weighted) noise floor this is of no importance because our ears are very insensitive to low frequencies at low levels, so it will not be heard.