A unit of loudness level which is related to the ears subjective response to frequencies at different levels of intensity. 

The range of phons is from 0 to 130 with 0 phons corresponding to a 0 db level at a frequency of 1000 Hz. At each phon level the actual sound pressure level varies with frequency change according to the subjective experience of equal loudness as derived from the equal loudness curves (Fletcher-Munson curves).