Beats are a form of amplitude modulation. As two frequencies are brought closer together, the beats will gradually slow down and disappear when they become identical. If two tones are about 15 Hertz or less apart, interference will result from their similar though not exactly identical frequencies. Gradually they will move out of phase until at 180° destructive interference results, producing diminished loudness. When they move back into phase, constructive interference will produce increased loudness. Beats arising from the mistuned unison are called first-order beats, and are both an acoustic and psychoacoustic phenomenon. Beats also may be heard between pure tones that are nearly an octave, fifth or fourth apart. These are called secondary or second-order beats. (Source: Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)
The interaction between two simple tones, resulting in a rhythmic interference interaction between them. The rate of beating is equivalent to the difference in frequency between the two tones. (smaller frequency difference = faster beating).
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