Two close tones progress through beats, roughness, then smooth separation as their gap widens

Evidence

For the majority of listeners beats are usually heard when the frequency difference between the tones is less than about 12.5 Hz, and the sensation of beats generally gives way to one of a “fused” tone which sounds “rough” when the frequency difference is increased above 15 Hz. As the frequency difference is increased further there is a point where the fused tone gives way to two separate tones but still with the sensation of roughness, and a further increase in frequency difference is needed for the rough sensation to become smooth. The smooth separate sensation persists while the two tones remain within the frequency range of the listener’s hearing. — David Howard, Jamie Angus, coustics and Psychoacoustics, 561

Categorization

• DEFINE: Defines the concrete perceptual sequence of two-tone interaction that underpins critical bandwidth.