Isochrony

Isochrony is the postulated rhythmic division of time into equal portions by a language. Rhythm is an aspect of prosody, others being intonation, stress, and tempo of speech. Three alternative ways in which a language can divide time are postulated: The duration of every syllable is equal (syllable-timed); The duration of every mora is equal (mora-timed). The interval between two stressed syllables is equal (stress-timed). The idea was first expressed thus by Kenneth L. Pike in 1945, though the concept of language naturally occurring in chronologically and rhythmically equal measures is found at least as early as 1775 (in Prosodia Rationalis). This has implications for linguistic typology: D. Abercrombie claimed "As far as is known, every language in the world is spoken with one kind of rhythm or with the other ... French, Telugu and Yoruba ... are syllable-timed languages, ... English, Russian and Arabic ... are stress-timed languages." While many linguists find the idea of different rhythm types appealing, empirical studies have not been able to find acoustic correlates of the postulated types, calling into question the validity of these types. However, when viewed as a matter of degree, relative differences in the variability of syllable duration across languages have been found. In a syllable-timed language, every syllable is perceived as taking up roughly the same amount of time, though the absolute length of time depends on the prosody. Syllable-timed languages tend to give syllables approximately equal prominence and generally lack reduced vowels. French, Italian, Spanish, Romanian, Brazilian Portuguese, Icelandic, Cantonese, Mandarin Chinese, Armenian, Turkish and Korean are commonly quoted as examples of syllable-timed languages. This type of rhythm was originally metaphorically referred to as "machine-gun rhythm" because each underlying rhythmical unit is of the same duration, similar to the transient bullet noise of a machine-gun. Since the 1950s, speech scientists have tried to show the existence of equal syllable durations in the acoustic speech signal without success.