PhonThe phon is a unit of loudness for pure tones. Human sensitivity to sound is variable across different frequencies; therefore, although two different tones may have identical sound intensity, they may be psychoacoustically perceived as differing in loudness. The purpose of the phon is to provide a standard measurement for perceived intensity. The phon is psychophysically matched to a reference frequency of 1 kHz. In other words, the phon matches the sound pressure level (SPL) in decibels of a similarly perceived 1kHz pure tone. For instance, if a sound is perceived to be equal in intensity to a 1kHz tone with an SPL of 50dB, then it has a loudness of 50phons, regardless of its physical properties. The phon was proposed in DIN 45631 and ISO 532 B by S. S. Stevens.
By definition, the number of phon of a sound is the dB SPL of a sound at a frequency of 1kHz that sounds just as loud.} This implies that 0phon 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.
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 (ANSI).
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.