6.11 beats. Periodic variations that result from the superposition of two simple harmonic quantities of different frequencies f1 and f2. Beats involve the periodic increase and decrease of amplitude at the beat frequency which is the absolute value of (f1 – f2).
Annotation As the difference between frequencies f1 and f2 is gradually increased the beats, as heard, first appear as a slow fluctuation in loudness, then appear as a train of intermittent sounds (intermittence), and finally produce a sensation of roughness. When the difference between f1 and f2 is great enough, the subjective interaction disappears and the two are heard only as separate primary tones.