The phon is a logarithmic unit of loudness level for tones and complex sounds. Loudness is measured in sones, a linear unit. Human sensitivity to

sound In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...

is variable across different frequencies; therefore, although two different tones may present an identical

sound pressure Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophon ...

to a human ear, they may be psychoacoustically perceived as differing in loudness. The purpose of the phon is to provide a logarithmic measurement (like

decibels The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a ...

) for perceived sound magnitude, while the primary loudness standard methods result in a linear representation. A sound with a loudness of 1 sone is judged equally loud as a 1 kHz tone with a sound pressure level of 40 decibels above 20 micropascals. 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

Stanley Smith Stevens Stanley Smith Stevens (November 4, 1906 – January 18, 1973) was an American psychologist who founded Harvard's Psycho-Acoustic Laboratory, studying psychoacoustics, and he is credited with the introduction of Stevens's power law. Stevens aut ...

Definition

By definition, the loudness level in phons of a sound is the sound pressure level (in dB SPL) of a 1 kHz pure tone that is judged as having the same loudness. The phon unit is not an

SI unit The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...

in metrology. It is defined as a unit of loudness level by the

American National Standards Institute The American National Standards Institute (ANSI ) is a private nonprofit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organiz ...

in the ''Acoustical Terminology'' standard ANSI/ASA S1.1-2013. Because the phon is a unit associated with a subjective percept, it is obtained by presenting the considered sound to a group of normal-hearing human listeners and by taking the median of the loudness levels they report. Such measurements have been performed for known sounds, such as pure tones at different frequencies and levels. The

equal-loudness contour An equal-loudness contour is a measure of sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon an ...

s are a way of mapping the dB SPL of a pure tone to the perceived loudness level in phons (see loudness for details).

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Explanation with definitions

Hearing Human-based units of measurement Units of sound Acoustics Sound

Definition

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Further reading

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