A-weighting

A-weighting is the most commonly used of a family of curves defined in IEC179 and various other standards relating to the measurement of perceived loudness, as opposed to actual sound intensity. The others are B, C, and D, weighting (see below).Loudness is not the same thing as sound intensity, and there is not even a simple relationship between the two, because the human hearing system is more sensitive to some frequencies than others, and furthermore, its frequency response varies with loudness, as has been demostrated by the measurement of equal-loudness contours. In general, low frequency and high frequency sounds appear to be less loud than mid-frequency sounds, and the effect is more pronounced at low levels, with a flattening of response at high levels. Sound level meters therefore incorporate weighting filters, which reduce the contribution of low and high frequencies to produce a reading of loudness which corresponds approximately to what we hear. Four curves, with varying levels of severity are 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) is now commonly used for the measurement of environmental noise and industrial noise, as well as 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 because it is now known 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. (see ITU-R 468 weighting).
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A Weighting curve is a graph that is used to 'weight' measured values of a variable according to their importance in relation to some outcome. The most commonly know example is in sound level measurement where a specific set of weighting curves known as A, B, C and D weighting are often used. Measurements of sound intensity do not correspond to perceived loudness because the human ear is less sensitive at low and high frequencies, with the effect more pronounced at lower sound intensities. The four curves are applied to the measured intensity, for example by the use of a weighting filter in a sound level meter, to arrive at readings of loudness in Phons or in deciBells (dB) (see A-weighting).
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