Periodic amplitude fluctuations are perceived as stronger than equivalent random fluctuations.

Evidence

Sound 4, a 70-dB FM tone with small frequency deviation, produces only about 1/10 of the fluctuation strength of sound 1. This result is expected on the basis of the data displayed in Fig. 10.5. Sound 5 represents a narrowband noise with a bandwidth of 10 Hz. The fluctuation strength elicited by this narrow-band noise can be estimated as follows: as a first approximation, the narrow-band noise can be regarded as an AM tone at 1 kHz with 6.4 Hz modulation frequency (see Sect. 1.1). If an effective modulation factor of 40% for narrow-band noise is assumed, then according to the results displayed in Fig. 10.3, the fluctuation strength of narrow-band noise should be a factor of about 2.5 smaller than the fluctuation strength of AM tones with a 98% modulation factor. A comparison of the relative fluctuation strength of sound 3 and sound 5 in Fig. 10.7, however, reveals that the fluctuation strength of narrow-band noise is smaller by a factor of about 5 than the fluctuation strength of an AM tone. It is apparently the periodic fluctuation of AM tones, in contrast to the random amplitude fluctuations of the noise, that enhances the perceived fluctuation strength of the AM tone. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1419

Categorization

• XREF: Relates to general perceptual principle that periodic/rhythmic structure is processed more strongly than stochastic variation, echoing work on rhythm perception and auditory scene analysis.