Isochrony is a linguistic analysis or hypothesis assuming that any spoken language's utterances are divisible into equal rhythmic portions of some kind. Under this assumption, languages are proposed to broadly fall into one of two categories based on rhythm or timing: syllable-timed or stress-timed languages (or, in some analyses, a third category: mora-timed languages). In syllable-timing, there are pauses of roughly equal length between every syllable, while in stress-timing, there are pauses of roughly equal length only between every stressed syllable.
While isochrony may be a useful theoretical framework in some contexts, studies suggest that, at best, syllable structure in natural speech has only a somewhat regular rhythm and perfectly regular speech may in fact be less intelligible. Careful linguistic analysis instead reveals that isochrony may be a subjective feature of language: more a listener perception than an inherent part of a language, making it a difficult hypothesis to test. Empirical studies have been mixed: unable to directly or fully support the hypothesis but sometimes lending support to it as a general or average tendency of languages; therefore, the concept remains controversial in linguistics. According to British phonetician Peter Roach: there is no language which is totally syllable-timed or totally stress-timed - all languages display both sorts of timing; languages will, however, differ in which type of timing predominates. Secondly, different types of timing will be exhibited by the same speaker on different occasions and in different contexts.
History Rhythm is an aspect of prosody, others being intonation, stress, and tempo of speech. Isochrony refers to rhythmic division of time into equal portions by a language. 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). Soames (1889) attributed the idea to Curwen. 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.
Alternative division of time 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).
Syllable timing 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, Singlish, Cantonese, Mandarin Chinese, Armenian, Turkish, Korean and many Indian languages (such as Hindi, Urdu, Punjabi, Tamil, Odia, Bengali, etc.; to name a few) 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. More recent research claims that the duration of consonantal and vocalic intervals is responsible for syllable-timed perception.
Mora timing Some languages like Japanese, Gilbertese, Slovak and Ganda also have regular pacing but are mora-timed, rather than syllable-timed. In Japanese, a V or CV syllable takes up one timing unit. Japanese does not have diphthongs but double vowels, so CVV takes roughly twice the time as CV. A final /N/ also takes roughly as much time as a CV syllable, as does the extra length of a geminate consonant. Ancient Greek and Vedic Sanskrit were also strictly mora-timed. Classical Persian was also mora-timed, though most modern dialects are not. Mora-timing is still common when reciting classical Persian poetry and music.
Stress timing
In a stress-timed language, syllables may last different amounts of time, but there is perceived to be a fairly constant amount of time, on average, between consecutive stressed syllables. Consequently, unstressed syllables between stressed syllables tend to be compressed to fit into the time interval: if two stressed syllables are separated by a single unstressed syllable, as in delicious tea, the unstressed syllable will be relatively long, while if a larger number of unstressed syllables intervenes, as in tolerable tea, the unstressed syllables will be shorter. Stress-timing is sometimes called "Morse-code rhythm," but any resemblance between the two is only superficial. Stress-timing is strongly related to vowel reduction processes. English, Thai, (non-Swiss) German, Russian, Danish, Swedish, Catalan, Norwegian, Faroese, Dutch, European Portuguese, and (Iranian) Persian are typical stress-timed languages. Some stress-timed languages, for example Arabic, retain unreduced vowels.
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