Isochrony

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Isochrony is the idea that a language rhythmically divides time into equal portions. Three types of divisions are postulated:


 * 1) the temporal duration between two stressed syllables is equal (stress-timed);
 * 2) The duration of every syllable is equal (syllable-timed);
 * 3) the duration of every mora is equal (mora-timed).

The idea was first expressed by Ken Pike in 1945. 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.

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 stress and generally lack reduced vowels.

Finnish, Icelandic, Cantonese Chinese, French, Italian and Spanish 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 such as Japanese, Gilbertese or Luganda 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 long vowels or diphthongs but double vowels, so that CVV takes twice the time as CV. A final /N/ also takes as much time as a CV syllable, and, at least in poetry, so does the extra length of a geminate consonant. However, colloquial language is less settled than poetic language, and the rhythm may vary from one region to another or with time.

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. Stress-timing is sometimes called Morse-code rhythm. Stress-timing is strongly related to vowel reduction processes. English, German, Danish, Swedish, Norwegian, Faroese, Dutch and Portuguese are typical stress-timed languages, as are some southern dialects of Italian.

European Portuguese is more stress-timed than the Brazilian. The latter has mixed characteristics and varies according to speech rate, gender and dialect. At fast speech rates, Brazilian Portuguese is close to British English, while in slow speech rates, it is close to Peninsular Spanish. The dialects of Rio Grande do Sul and Bahia are considered to sound more syllable-timed than the others, while the southeastern dialects such as the mineiro, in Minas Gerais, are more stress-timed. Also, male speakers of Brazilian Portuguese speak faster than female speakers and speak in a more stress-timed manner.

Origin of the differentiation
This difference comes from the human's two senses of rhythm.
 * When a human hears a fast rhythm, typically faster than 330 milliseconds (ms) per beat, the series of beats is heard as one solid noise. For example, a human can imitate a machine gun sound, but hardly count its beats.
 * Conversely, when a slow rhythm is heard, typically slower than 450 ms per beat, each beat is separately understood. The speed of a slow rhythm can be controlled beat by beat, such as hand clapping in music.

If a language has a simple syllable structure, the difference in time length between the simplest and the most complicated syllables in the language is not wide, and it is possible to say any syllable in less than 330 ms. This includes languages that have very few consonants in each syllable. Thus we can use the fast syllable-timed rhythm.

If a language has complex syllables such as ones with consonant clusters, the difference in time length between syllables can be very wide, such as the words a and strengths in English. In this case, the language has slow stress-timed rhythm.