"(1) acoustics: of a periodic quantity: the smallest time-unit for which the quantity repeats itself. By extension, the term period is applied eben when successive cycles of oscillation are not identical. The extension means taht the term is applied to the time elapsing between corresponding points of two successive members of a series of more or less irregularly periodic sounds. In the last resort, as irregualrities increase, the only corresponding points will be the beginnings, hence the ultimate permissable extension of the term for bio-acoustic purposes: lapse of time between two successive homologous members of a series of emissions measured from the beginiing of one to the beginning of the other. Clearly, however, the concept of period has no meaning outside of a repetitive context, and for the definition just igven to habe any meaning it is necessary to take a mean of the time-lapses between successive members in the whole series, or in a fair sample of it.
(2) music, applied to ornithology by Sotavalta: the smallest indepedant unit of expression: a unit of higher order than the sentence, consisting of one or several sentences which describe the whole expression. Periods are generally coordinated, and separated from each other by a silent interval. A period can be an exact repetition of the preceding period, or be differnet form it; though it often has a simialr structure." 
- . Glossarial Index. Acoustic Behavior of Animals. Acoustic Behavior of Animals. Elsevier; 1963.
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