Above a threshold, modulation is heard as separate sideband tones rather than as flutter.

Evidence

The roll-off above the maximum is due to the audibility of sidebands so that instead of listening for a kind of modulation or roughness, one listens for additional tones as the modulation frequency is increased. The frequency selectivity of the ear, discussed in Sect. 4.3 on masking, is the reason for this different criterion being used. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1020