The phon is a unit of perceived loudness, which is a subjective measure of the strength (not intensity) of a sound. At a frequency of 1 kHz, 1 phon is defined to be equal to 1 dB of sound pressure level above the nominal threshold of hearing, the sound pressure level SPL of 20 �Pa (micropascal) = 2 · 10-5 Pascal (Pa). Our ears as sensors cannot convert sound intensities and powers, they can only use the sound pressure.
At other frequencies, the phon departs from the decibel, but is related to it by a frequency weighting curve (equal-loudness contour) that reflects the frequency response of human hearing. The standard curve for human hearing is the A-weighted curve (the equal-loudness contour for a 40 dB stimulus at 1kHz), but others are in use.
The "unit" phon has been largely replaced by the dB(A) (A-weighted decibel), though many old textbooks and instructors continue to use the phon.