Loudness doubles per 10 phon except below 40 phon where it drops steeply.

Evidence

Because an increment of 10 phon corresponds to a doubling of loudness, an increment of 20 phon corresponds to an increment in loudness by a factor of 4. Therefore, the parameter of 60 phon can be transformed to a loudness of 4 sone, 80 phon to a loudness of 16 sone, 100 phon to a loudness of 64 sone, and so on. At loudness levels below 40 phon, the loudness decreases more quickly. This leads to the fact that the loudness corresponding to 20 phon is more than a factor of 4, indeed a factor of 6.6 lower than the loudness corresponding to 40 phon. The loudness value corresponding to 20 phon is 0.15 sone. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1188

Categorization

• DEFINE: Establishes the phon-to-sone conversion (10 phon = 2x loudness) and its breakdown at low levels — a core psychoacoustic scaling fact.
• XREF: Connects to Stevens’ power law scaling in psychophysics; also relevant to any audio engineering discussion of loudness normalization (LUFS standards).