"(1) music: rhythmic pulsation, as e.g. three beats to the bar, etc; Sotavalta extends for bird song as follows: the smallest rhythmic unit; a single rhythmic accent that can be repeated after successive intervals, regular or irregular. Between beats may be sub-beats of a differnet order.
(2) physics: the periodic vriations of amplitude resulting from the addition of two periodic quantities of the same kind but slightly different frequency." 
- . Glossarial Index. In: Acoustic Behavior of Animals. Acoustic Behavior of Animals. Elsevier; 1963.
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