ISO 31-7

ISO 31-7 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to acoustics. It is superseded by ISO 80000-8.

Its definitions include:

Quantity			Unit		Remarks
Name	Symbol	Definition	Name	Symbol
period, periodic time	T	time of one cycle	second	s
frequency	f, ν	f = 1/T	hertz	Hz	1 Hz = 1 s−1
. . .
loudness level	LN	LN = ln(peff/p0)1 kHz where peff is the root-mean-square value of the sound pressure of a pure tone of 1 kHz, which is judged by a normal observer under standardized listening conditions as being as loud as the sound under investigation, and where p0 = 20 μPa	phon		The loudness level is 1 phon if 20 log10(peff/p0)1 kHz=1, that is for a pure tone of frequency 1 kHz, 1 phon = 1 dB sound pressure level. This is not a purely physical quantity and unit, as it involves the subjective evaluation by humans.
loudness	N	A normal observer's auditory estimate of the ratio between the strength of the sound considered and that of a reference sound having a loudness level of 40 phons	sone		A standard relation between loudness in sones and loudness level in phons has been adopted for practical use in ISO 131.