Mistuned musical partials produce slow timbre or quality beats distinct from loudness beats

Evidence

A closely mistuned partial, say, off by 0.25 Hz, as in 100.25 and 200 Hz, can be considered to be a perfectly tuned partial with its phase drifting by 2π once every 4 seconds (see equation 24.3). Such a slow drift would allow the listener to establish the timbre at all times, and if the timbre depends slightly on the relative phase, the timbre should cycle through a change once per 4 seconds. This would be interpreted as a sensation of slow, weak beating, not so dramatic as loudness beating. In fact, beats are heard going through a complete cycle once every 2 seconds—see the following. This phenomenon has generated lengthy discussions as to its cause and a prodigious amount of experimentation. A plot of the waveform over some interval (say, 10 seconds) reveals that it cycles through “shapes’” at the same frequency as the beats that are heard. This phenomenon has earned the names waveform beats, beats of mistuned consonances, and quality beats. — Eric J. Heller, Why You Hear What You Hear- An…, 2227

Categorization

• DEFINE: Defines waveform beats / beats of mistuned consonances / quality beats — slow phase-driven timbre cycling.