Beat Notes

Zooming out further, we see a sinusoid with a low-frequency sinusoidal amplitude envelope.

**Figure 3:** Sinusoids at 18 and 22 Hz.
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An audio version of this note (with frequencies at 218 and 222 Hz) can be heard here.

The result is a sound that comes in and out of prominance, usually described as beating.

Why does this effect occur?

``Music 171: Amplitude Modulation'' by Tamara Smyth, Department of Music, University of California, San Diego (UCSD).