beats

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.