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For other uses, see Phon (disambiguation).

The phon is a unit of loudness level for pure tones. Its purpose is to compensate for the effect of frequency on the perceived loudness of tones.[1] The phon was proposed in DIN 45631 and ISO 532 B by S. S. Stevens.[2]


By definition, the number of phon of a sound is the dB SPL of a sound at a frequency of 1 kHz that sounds just as loud.[3] This implies that 0 phon is the limit of perception, and inaudible sounds have negative phon levels.

The equal-loudness contours are a way of mapping the dB SPL of a pure tone to the perceived loudness level (LN) in phons. These are now defined in the international standard ISO 226:2003, and the research on which this document is based concluded that earlier Fletcher–Munson curves and Robinson–Dadson curves were in error.[citation needed]

The phon unit is not an SI unit in metrology. It is used as a unit of loudness level by the American National Standards Institute.

The phon model can be extended with a time-varying transient model which accounts for "turn-on" (initial transient) and long-term, listener fatigue effects. This time-varying behavior is the result of psychological and physiological audio processing. The equal-loudness contours on which the phon is based apply only to the perception of pure steady tones: tests using octave or third-octave bands of noise reveal a different set of curves, owing to the way in which the critical bands of our hearing integrate power over varying bandwidths and our brain sums the various critical bands.[clarification needed][citation needed]

See also[edit]


  1. ^ William M. Hartmann, Signals, Sound, and Sensation, American Institute of Physics, 2004. ISBN 1-56396-283-7.
  2. ^ UNSW Music Acoustics
  3. ^ Nave, C. R. (2002-02-10). Loudness Units: Phons and Sones. HyperPhysics. Retrieved on 2008-07-10 from

External links[edit]