Waveform beat rate depends on the lower partial’s frequency, not just the mistuning amount

Evidence

We notice that both 201 + 400 Hz and 200 + 401 Hz differ from perfection by 1 Hz, but the waveform beating is different. The waveform shape is the way two sinusoids of different period are combining: Are crests adding with crests, and the like? A key point is that a given crest in a plot of sin(ax + b) shifts at a rate inversely proportional to a as b changes: Δxc/Δb = –1/a, where xc is the position of a crest. So, for example, as the phase b = 2πΔft advances in the term sin(2πft + 2πt)—(Δ f = 1)—it advances the sinusoidal peaks in proportion to 1/f. This is why 200 + 401 Hz has waveform beats half as often as 201 + 400 Hz. — Eric J. Heller, Why You Hear What You Hear- An…, 2228