We subjectively experience humans to speak with a certain regularity – which creates perceived rhythm within speech – at the same time as we expect them to display variation, mostly for emphasis and to satisfy personal preferences. Synthesized speech that does not exhibit these perceptual qualities is often classified as “robotic” and “unnatural”. The search for the objective bases of the perceived regularity in speech is old and has produced less than satisfactory results. In 1977, Ilse Lehiste, in an extensive review of the issue of isochrony (acoustic evidence for rhythmicity in speech) came to the conclusion that there were no direct acoustic correlates of rhythmicity . This view, supported by a number of further studies, has formed the consensus for spontaneously produced speech since then. However, Robert Port and his colleagues have in recent years suggested that some parts of perceived regularity may actually be directly dependent on the suddenness and the relative strength of voice onsets (so-called “beats”). This hypothesis was examined here with respect to continuous speech by a series of analyses performed in two languages, and it was found that indeed, beats do provide a minor temporal organizational effect within the speech phrase, but that the effect is so minor that it is of no or only circumscribed value to such applications such as speech synthesis or speech recognition.
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