Beats transition to roughness to smoothness as partials separate on the basilar membrane.

Evidence

The simplest form of beats are heard as undulations of amplitude (loudness) occurring between two sinusoidal partials if they differ by less than about 10 Hz. As the difference increases, the sensation of beats eventually gives way to a “fused” tone, which sounds “rough” when the frequency difference goes above 15 Hz. If we increase the frequency difference further, the fused tone remains rough but becomes discernible as two separate tones. Still further, the rough sensation finally becomes smooth. This happens when the two tones “get out of each other’s way” on the basilar membrane: each pure tone affects a nonoverlapping critical band. (See section 26.1 for more details about these effects, which figure strongly in our sense of dissonance and therefore our choices of musical temperament.) — Eric J. Heller, Why You Hear What You Hear- An…, 2216

Categorization

• DEFINE: Precise threshold definitions: beats under ~10 Hz separation, rough fusion above ~15 Hz, smooth once critical bands don’t overlap.
• XREF: Directly connects to musical dissonance and choices of temperament — this is a core mechanism for why some intervals sound agreeable and others harsh.
• ADDENDUM: The roughness/dissonance mechanism grounds the sensory side of taste in music, complementing the social framing of the immodesty article.