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 It is superseded by ISO 80000-8.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. | ||