AM tones change perceived frequency spread, giving extra detection cues that noise lacks.

Evidence

Interestingly, the detection threshold of an amplitude-modulated tone depends on its level, but such dependence is not found with noise. This can be understood when the excitation pattern of the tone on the basilar membrane is investigated in the cases of amplitude and frequency modulation. Schematic diagrams of these cases are shown in Figure 10.14. In the case of amplitude modulation, the shape of the excitation pattern depends on the level such that at maximum levels of the signal a much broader frequency range is covered than with the lowest levels. This implies that when the level is modulated, the frequency range is also modulated, which provides an additional cue to detect the modulation. The frequency coverage is, on the other hand, constant in the case of frequency-modulated tones, and such an additional cue is not available. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1164

Categorization

• XREF: Connects to the loudness/level-discrimination mechanism discussions in earlier chapters of this book on the basilar membrane excitation patterns.