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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.
Loudness N in sones (for LN > 40 phon):
or loudness level LN in phons (for N > 1 sone):
Corrections are needed at lower levels, near the threshold of hearing.
To be fully precise, a measurement in sones must be specified in terms of the optional suffix G, which means that the loudness value is calculated from frequency groups, and by one of the two suffixes D (for direct field or free field) or R (for room field or diffuse field).
|Source of sound||sound pressure||sound pressure level||loudness|
|pascal||dB re 20 µPa||sone|
|threshold of pain||100||134||~ 676|
|hearing damage during short-term effect||20||approx. 120||~ 256|
|jet, 100 m away||6 ... 200||110 ... 140||~ 128 ... 1024|
|jack hammer, 1 m away / discotheque||2||approx. 100||~ 64|
|hearing damage during long-term effect||6×10−1||approx. 90||~ 32|
|major road, 10 m away||2×10−1 ... 6×10−1||80 ... 90||~ 16 ... 32|
|passenger car, 10 m away||2×10−2 ... 2×10−1||60 ... 80||~ 4 ... 16|
|TV set at home level, 1 m away||2×10−2||ca. 60||~ 4|
|normal talking, 1 m away||2×10−3 ... 2×10−2||40 ... 60||~ 1 ... 4|
|very calm room||2×10−4 ... 6×10−4||20 ... 30||~ 0.15 ... 0.4|
|leaves' noise, calm breathing||6×10−5||10||~ 0.02|
|auditory threshold at 1 kHz||2×10−5||0||0|