Scale that indicates percieved loudness of a sound.
The Phon scale is developed to indicate how people percieve loudness, acontrary to Decibel that shows actual sound level. Low frequencies are percieved as more silent than high frequencies - even if they have the same actual level (Decibel). Only at 1kHz the scales overlap.
The scale is also created to represent a doubling or halfing of the percieved loudness for each 10 phon up or down.