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The sone is a unit of perceived loudness. In acoustics, loudness is a subjective measure of the intensity of a sound. One sone is equivalent to 40 phons, which is defined as the loudness 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 intensity by 10 dB, or increasing the sound power by a factor 10. At frequencies other than 1 kHz, the measurement in sones must be calibrated according to the frequency response of human hearing.

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).