According to Stevens' definition, the

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

to a phon was chosen so that a doubling of the number of

sounds to the human ear like a doubling of the loudness, which also corresponds to increasing the sound pressure level by approximately 10 dB, or increasing the mean square sound pressure by a factor 10 (since due to the major property of logarithms for any given sound pressure level L=10*log10(p2/p02) dB the following holds: L + 10 dB = L + 10*1 dB = L + 10*log10(10) dB = 10*log10(p2/p02) dB + 10*log10(10) dB = 10*log10((p2/p02) * 10) dB). At frequencies other than 1 kHz, the measurement in sones must be calibrated according to the frequency response of human hearing, which is a subjective process. The study of apparent loudness is included in the topic of psychoacoustics and employs methods of psychophysics.
To be fully precise, a measurement in

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).
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