Pronunciation: (fon), [key]
a unit for measuring the apparent loudness of a sound, equal in number for a given sound to the intensity in decibels of a sound having a frequency of 1000 cycles per second when, in the judgment of a group of listeners, the two sounds are of equal loudness.
var. of phono- before a vowel: phonic.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.