Glossary definition of 'A-weighting'The A-weighting curve is a filter designed to emulate the way the human ear hears noise and is defined in IEC61672:2003. It is designed to mirror the 40 phon equal-loudness-level of the human ear to pure tones as set out by Fletcher and Munson in their 1933 paper. In other words, each frequency is weighted according to the ear's sensitivity at that frequency.
Originally intended for assessing the perceived loudness of low levels of noise, this is now used widely for the measurement of all kinds of industrial and environmental noise. The ear is less sensitive to low frequencies and very high frequencies than frequencies in the speech range and this is particularly true at low levels. Using an A-weighting filter helps to make the measurements more likely to correlate with perceptions of loudness. Using the A-weighting curve at high levels artificially reduces the subjective effects of noise in those frequencies and is often used unscrupulously to make noise figures look better than they should.
The graph above shows an A-weighting filter. Below it in red is the spectrum of a device with some mains hum at 50Hz. Using an A-weighting filter reduces the effect of this mains hum by 30dB.