The phon was proposed as a unit of perceived loudness level LN for pure tones by S. S. Stevens. The purpose of the phon scale is to compensate for the effect of frequency on the perceived loudness of tones. By definition, 1 phon is equal to 1 dBSPL at a frequency of 1 kHz. The equal-loudness contours are a way of mapping the dBSPL of a pure tone to the perceived loudness level in phons. These are now defined in the international standard ISO 226:2003, and it should be noted that the research on which this document is based concluded that earlier Fletcher–Munson curves and Robinson-Dadson curves were in error.

Although proposed as units, phons are not generally accepted according to the stringent criteria of metrology, and have not been accepted as standard units by the United States National Institute of Standards and Technology.

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.

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