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Two sounds of equal intensity do not have the same loudness, because of the frequency sensibility of the human ear. A 80 dB sound at 100 Hz is not as loud as a 80 dB sound at 3 kHz. A new unit, the phon, is used to describe the loudness of a harmonic sound. X phons means ?as loud as X dB at 1000 Hz?. Another tool is used : the equal loudness curves, a.k.a. Fletcher curves.
Another scale currently used is the sone, based upon the rule of thumb for loudness. This rule states that the sound must be increased in intensity by a factor 10 to be perceived as twice as loud. In decibel (or phon) scale, it corresponds to a 10 dB (or phons) increase. The sone scale?s purpose is to translate those scales into a linear one.
log(S) = 0,03(Lph ? 40)
Where S is the sone level, and Lph the phon level. The conversion table is as follows:
| Phons | Sones |
|---|---|
| 100 | 64 |
| 90 | 32 |
| 80 | 16 |
| 70 | 8 |
| 60 | 4 |
| 50 | 2 |
| 40 | 1 |
