Just-noticeable amplitude modulation drops steeply as tone level rises

Evidence

The solid line, which holds for the I-kHz tone, shows that for low levels large degrees of modulation in the range of 20% are necessary to become just audible. At levels of about 40 dB, a degree of modulation of 6% becomes just noticeable. For even higher levels, this just-noticeable degree of amplitude modulation decreases further and reaches, for sound pressure levels of 100 dB, values of about 1 %. This dependence on level holds not only for the I-kHz tone but also for most other pure tones of different frequencies, if instead of the sound pressure level the loudness level (see Chap. 8.1) is used as the abscissa. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1013

Categorization

• QUESTION: The claim that loudness level, not pressure level, is the unifying abscissa across frequencies depends on Ch. 8.1’s loudness scaling — worth verifying when reaching that chapter whether this generalization fully holds.