Phon

Phon is a unit of loudness level used in psychoacoustics to express perceived loudness. One phon represents the loudness level of a sound that matches the perception of a 1 kilohertz pure tone at a given sound pressure level. In other words, the phon of a sound is the decibel level, measured at 1 kHz, that would produce the same subjective loudness as the target sound. The phon is a non-SI unit and is used to quantify perceived loudness rather than physical sound pressure alone.

Loudness varies with frequency, a property captured by equal-loudness contours. These contours show, for each frequency, the sound pressure level needed for a sound to be perceived as equally loud as a reference at 1 kHz. The concept of phons arose from Fletcher and Munson’s work on human hearing, and modern practice relies on standardized contours such as ISO 226. These standards enable conversion between physical measurements (dB SPL at various frequencies) and perceptual loudness levels across the audible spectrum.

Relation to other measures: the phon measures perceived loudness level, while the sone provides a more linear scale of loudness. Roughly, 40 phons is defined as 1 sone, and the approximate relationship is Sones ≈ 2^((Phon − 40)/10) for mid-frequency sounds, though exact values depend on frequency.

Usage and significance: phons are used in psychoacoustic research, audio engineering, and hearing-related applications to describe how loud a sound is felt, not merely how intense it is physically.