The sone is a unit of perceived loudness proposed by Stanley Smith Stevens in 1936. In acoustics, loudness is the subjective perception of sound pressure. It is a non-SI unit.
According to Stevens' definition, a loudness of 1 sone is equivalent to the loudness of a signal at 40 phons, the loudness level of a 1 kHz tone at 40 dB SPL. But phons scale with level in dB, not with loudness, so the sone and phon scales are not proportional. Rather, the loudness in sones is, at least very nearly, a power law function of the signal intensity, with an exponent of 0.3. With this exponent, each 10 phon increase (or 10 dB at 1 kHz) produces almost exactly a doubling of the loudness in sones.
At frequencies other than 1 kHz, the loudness level in phons is calibrated according to the frequency response of human hearing, via a set of equal-loudness contours, and then the loudness level in phons is mapped to loudness in sones via the same power law.
The study of apparent loudness is included in the topic of psychoacoustics and employs methods of psychophysics.
Read more about Sone: Formulas, Examples of Sound Pressure, Sound Pressure Levels, and Loudness in Sone
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