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The sone is a unit of perceived loudness N after a proposal of S. Smith Stevens in 1936. In acoustics, loudness is a subjective measure of the sound pressure. At a frequency of 1 kHz, 1 phon is defined to be equal to 1 dB of sound pressure level Lp above the nominal threshold of hearing, the sound pressure level SPL of 20 µPa (micropascals) = 2 · 10-5 pascal (Pa). One sone is equivalent to 40 phons, which is defined as the loudness level NL of a 1 kHz tone at 40 dB SPL. The number of sones to a phon was chosen so that a doubling of the number of sones sounds to the human ear like a doubling of the loudness, which also corresponds to increasing the sound pressure level by 10 dB, or increasing the sound pressure by a factor 3.16 =. At frequencies other than 1 kHz, the measurement in sones must be calibrated according to the frequency response of human hearing, which is of course a subjective process. The study of apparent loudness is included in the topic of psychoacoustics. To be fully precise, a measurement in sones must be qualified by the optional suffix G, which means that the loudness value is calculated from frequency groups, and by one of the two suffixes F (for free field) or D (for diffuse field).
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