Phon

The phon is a unit of perceived loudness, which is a subjective measure of the intensity of a sound. At a frequency of 1 kHz, one phon is defined to be equal to one decibel of sound pressure level above the nominal threshold of hearing (1 picowatt/metre²).

At other frequencies, the phon departs from the decibel, but is related to it by a frequency weighting curve that reflects the frequency response of human hearing. The standard curve for human hearing is the A-weighted curve, but others are in use.