At very high modulation rates, listeners detect AM only via average intensity change.

Evidence

For low modulation frequencies, the broken curve in Fig. 7.2, which represents the data for white noise, runs parallel to that for tones, but the increment corresponding to the square root of the modulation frequency continues towards higher modulation frequencies up to 500 Hz. At such high modulation frequencies and corresponding high degrees of modulation of about 0.4, another criterion is used to discriminate between modulated and unmodulated noise. The strong amplitude modulation increases the intensity averaged over some hundred milliseconds, and this change in overall intensity or overall sound pressure level is used by the subject to hear the difference. The fluctuations produced by the amplitude modulation, however, remain inaudible at such large frequencies of modulation. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1022

Categorization

• XREF: Relates to the book’s own models of temporal integration/roughness and to the earlier square-root law; connects to the reader’s broader psychoacoustics interest in detection criteria switching.