A weighting filter is used to emphasize or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.
Audio applications
In each field of audio measurement, special units are used to indicate a weighted measurement as opposed to a basic physical measurement of energy level. For sound, the unit is the phon (1 kHz equivalent level).
Loudness measurements
In the measurement of loudness, for example, an A-weighting filter is commonly used to emphasize frequencies around 3–6 kHz where the human ear is most sensitive, while attenuating very high and very low frequencies to which the ear is insensitive. The aim is to ensure that measured loudness corresponds well with subjectively perceived loudness. A-weighting is only really valid for relatively quiet sounds and for pure tones as it is based on the 40-phon Fletcher–Munson equal-loudness contour. 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).