Amplitude-modulated tones produce three distinct sensations depending on modulation frequency

Evidence

Using a 100% amplitude-modulated I-kHz tone and increasing the modulation frequency from low to high values, three different areas of sensation are traversed. At very low modulation frequencies the loudness changes slowly up and down. The sensation produced is that of fluctuation. This sensation reaches a maximum at modulation frequencies near 4 Hz and decreases for higher modulation frequencies. At about 15 Hz, another type of sensation, roughness, starts to increase. It reaches its maximum near modulation frequencies of 70 Hz and decreases at higher modulation frequencies. As roughness decreases, the sensation of hearing three separately audible tones increases. This sensation is small for modulation frequencies near 150 Hz; it increases strongly, however, for larger modulation frequencies. This behaviour indicates that roughness is created by the relatively quick changes produced by modulation frequencies in the region between about 15 to 300 Hz. There is no need for exact periodical modulation, but the spectrum of the modulating function has to be between 15 and 300 Hz in order to produce roughness. For this reason, most narrow-band noises sound rough even though there is no periodical change in envelope or frequency. Roughness is again a sensation which we can consider while ignoring other sensations. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1435

Categorization

• DEFINE: Defines the three perceptual regimes: fluctuation (peak near 4 Hz), roughness (peak near 70 Hz), and separately audible tones (above about 150 Hz), a core psychoacoustic map.
• XREF: Connects to the reader’s interest in how fluctuation/roughness/perceived-tones form a continuous perceptual spectrum from temporal to spectral processing — parallels Zahorian’s temporal modulation spectrum work.