Modulation frequency determines whether sound is heard as fluctuation, roughness, or multiple tones

Evidence

If the level of sound fluctuates relatively slowly over a certain range of rates, between about 1 Hz and 16 Hz, a significant fluctuation strength may be perceived. When the modulation rate far exceeds 16 Hz, our hearing is unable to follow the level of sound, and the sound is perceived as rough, associated with the psychoacoustic quantity roughness. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217

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. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1435

Categorization

• DEFINE: Establishes the operational boundary between the three temporal sensations (fluctuation ~4 Hz peak, roughness ~70 Hz peak, separate tones above ~150 Hz) that recurs through the roughness chapter.
• corroborated: 4 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217; Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1435
• XREF: connects to: An amplitude-modulated tone produces a three-line spectrum; Frequency-modulated tones grow louder only above roughly 20 Hz modulation — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1297; Low modulation frequencies produce fluctuation sensation while higher ones produce roughness — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1381 (+10 more)

Broader Connections

• Fast amplitude modulations within a critical band produce roughness perception #corroborates (0.85) — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1237
• Frequency separation between two tones systematically shifts fusion, roughness, then resolution #corroborates (0.80)
• Amplitude-modulated sounds split into slow-varying envelope and fast-varying fine structure #expands (0.75) — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 997