phon

   a logarithmic measure of sound loudness closely related to the decibel.

   Decibels are used for objective measurements, that is, they measure the actual pressure of the sound waves as recorded using a microphone. Phons are used for subjective measurements, that is, measurements made using the ears of a human listener. A sound has loudness p phons if it seems to the listener to be equal in loudness to the sound of a pure tone of frequency 1 kilohertz and strength p decibels. A measurement in phons will be similar to a measurement in decibels, but not identical, since the perceived loudness of a sound depends on the distribution of frequencies in the sound as well as the pressure of the sound waves. In the U.S., sound loudness is frequently measured in sones rather than phons: a sound of loudness s sones has loudness 10 log² s + 40 phons. In English the unit is pronounced "fon" rather than "phone."