Phon (unit)

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Physical unit
Unit name phon
Unit symbol
Physical quantity (s) Volume level
Formula symbol (Level)
In SI units
Named after ancient Greek φωνητικός phōnētikós
See also: Mel , Sone

The phon (according to the new German spelling also Fon ) is the unit of measurement of the psychoacoustic variable volume level . The volume level in Phon describes - like the loudness in Sone - the perceived volume with which a person perceives a sound event as an auditory event . The Phon was first introduced in 1925 by Heinrich Barkhausen . The phon is not an SI unit or legal unit in the EU, but is defined by ISO 532 and used as a standard in jurisprudence.

The value in Phon indicates what sound pressure level (in dB ) a sinus tone with a frequency of 1000  Hz has, which is perceived as loud as the actual sound event, which has a different frequency. By comparing the perceived volume of any sound signal with the volume of a reference signal (the sine tone at 1000 Hz), it is possible to describe the hearing sensation with a level value that is independent of the spectrum of the signal.


Listening area with curves of equal volume level (isophones) - here the older version; the new standard is ISO 226: 2003.

The unit phon of the volume level results from curves of equal volume perception (isophones) in a diagram with the axes sound pressure level and frequency. The lowest of these curves describes the course of the hearing threshold and is 3 phons, since a 1000 Hz tone (the reference signal for the phon definition) can only be perceived from a sound pressure level of 3 dB. The pain threshold is around 130 phons. Loud noises are perceived as pain and can lead to hearing damage even if they are exposed for a short time .

Volume level versus loudness

Relationship between volume level in Phon and loudness in Sone

See also: → Main article: Volume level , loudness

The size of the volume level in Phon is used to link the perceived volume with level values. A quantitative comparison of different volume levels is only possible to a limited extent using the Phon scale:

  • On the one hand, the phon value is not proportional to the perceived volume : a sound event with 80 phons corresponds (scalar) not to double the volume as a sound event with 40 phons, but 16 times as loud. The volume level is, mathematically speaking, “not proportionately scaled”.
  • On the other hand, an identical change in the phon value does not lead to an identical change in the perceived volume over the entire level range. An increase in the phon value from 10 to 20 phon results in a six-fold increase in the perceived volume. An increase in the phon value from 50 to 60 phon, however, only leads to a doubling of the perceived loudness.

In this respect, a further variable is necessary for psychometrizing the volume, which records the most linear possible connection between numerical value and stimulus intensity, namely the size of the so-called loudness with its unit sone :
A sound event that has a volume level of 40 phons is defined as loudness " 1 sone “assigned. A sound event that is perceived twice as loud has twice the loudness (ie 2 sones) etc. Below 40 phon there is a logarithmic relationship between sone and phon (see picture); an increase in the volume level by 10 phons corresponds to a doubling of the perceived volume. For example, a sound event with a volume of 100 phons is perceived 64 times as loud as a sound event with 40 phons.

At the same time, even with sone values, one cannot speak of a supposedly ' loudness perceived n times as loud '. - Although one can speak of n-fold loudness on a scalar basis , it cannot be assumed that test subjects have a concept of proportional 'loudness intervals' (“sounds n-fold as loud”).

Noise measurement technology

The phone is of little importance in noise measurement technology. To measure noise, either the weighted sound pressure level is measured (most legal guidelines require a measurement with the A- weighting filter), in which the systematic errors of this method are accepted, or the loudness in sone is determined from the sound event .

However, the phone plays a role when the sound pressure level and loudness are to be compared. For example, For example, the loudness measurement method according to Zwicker allows you to determine both the loudness in sone and the volume level in phon. The volume level in Phon then allows a direct comparison with weighted sound pressure level measurements in dB (A) and an easier evaluation of differences.

Difference threshold

A volume difference of about 1 phon is the difference threshold just at the limit of recognizability. Therefore it is neither necessary nor useful to specify fractions of a phon.

Web links

Wiktionary: Phon  - explanations of meanings, word origins, synonyms, translations
  • Conversion of volume level L (phon) into loudness N (sone) and loudness N (sone) into volume level L (phon)