• A unit which takes into account the non-linear response of human hearing in describing loudness levels of sounds or noises. To define a phon scale, a listener is subjected to a reference tone of 1 kHz at a sound pressure of 2.0 × 10-5 pascal, while listening to another sound alternately. The reference tone is increased to match the sound, and the number of decibels the reference tone needs to be augmented is equated to the number of phons. That is, an increase in 20 decibels of the reference tone means that the other sound has an intensity of 20 phons.