Beat rate is set by frequency difference, not by the true tone periodicity.

Evidence

The case of adding 27 Hz and 25 Hz sinusoids is shown in figure 24.4. The GCD of these two frequencies is 1 Hz, and this is indeed the true periodicity of the tone. However, we expect to hear a 2 Hz beating, since the partials differ by 2 Hz, and we do. In fact, there are two loudness maxima per second, but as close inspection of figure 24.4 reveals, adjacent maxima are very slightly different. We can’t hear that slight difference; rather, we hear the maxima. Mathematically, this happens because the cosine oscillates at half the difference in frequency, or 1 Hz. However, there are loudness maxima at 2 Hz, because they happen whether cosine is near 1 or –1. — Eric J. Heller, Why You Hear What You Hear- An…, 2221

Categorization

• QUESTION: Worth chasing: why does the doubling of maxima (cos ±1) mask the true 1 Hz periodicity in perception — is this a general envelope-detection property?