A unit which takes account of the ear’s nonlinear response to the loudness and frequency of a sound. The phon uses a decibellogarithmic, i.e., logarithmic, scale which is based on the level of intensity of a given sound that corresponds to dB rating of a pure tone at 1kHz, subjectively judged to be of the same loudness. An increase of one phon is about the smallest increment in loudness that can normally be perceived. The scale practically ranges from 0dB to 130dB, and its logarithmic nature means that a rise of three phons approximates a doubling in intensity. See equal loudness curves, SPL.