Page 256 - DENG504_LINGUISTICS

Linguistics

Notes rhythm is a prosodic characteristic of a speaker's native language in much the same way as the
phoneme inventory and the phonotactics are characteristic of the native language at a segmental
level. Rhythm's phonological status therefore allows for the analysis of an aspect of prosody
which has neither a linguistic or affective function, and it is interesting therefore to investigate if
rhythm is compromised or spared in a speaker with RHD.

18.5 Defining and Measuring Rhythm

The definition of rhythm is somewhat nebulous, probably because rhythm works differently in
different languages, and as described below, acoustic cues to rhythm have been difficult to locate.
Trask however, defines rhythm as 'the perceptual pattern produced in speech by the occurrence at
regular intervals of prominent elements'. The prominent elements that Trask refers to may be
either stresses or syllables, and on this basis early descriptions of speech rhythm, such as that by
Pike, distinguish two types of rhythm known as stress-timing and syllable-timing. Abercrombie
takes this distinction one step further and states that all languages fall into one of these two
categories. For example, British English and Dutch are classified as being stress-timed. In stress-
timed languages, speakers seem to leave roughly equal durations between stressed syllables. This
gives rise to feet (another unit of rhythm, usually defined as consisting of one stressed syllable
followed by any number of unstressed syllables) of roughly equal duration, but individual syllables
within the foot may vary greatly in duration. Syllable-timed languages, such as French and Spanish,
on the other hand, tend to exhibit syllables which sound to be of roughly equal duration, but
display less of a durational alternation between stressed and unstressed syllables.
The chief problem with these classical descriptions of rhythm is that they rest heavily on the
impressionistic perception of the listener. Instrumental studies (such as those by Roach, 1982 and
Dauer, 1983), by contrast, have consistently found that feet are not isochronous (equally timed) in
so called stress-timed languages, and that syllables are not isochronous in syllable-timed languages.
As a result, researchers' views of rhythm have changed in two fundamental ways. Firstly, most
researchers, following Dauer, now see rhythm as a continuous variable. Instead of all languages
being classified as stress- or syllable-timed, they are now believed to fall on a continuum between
these two extremes. Secondly most authors now claim that languages exhibit only perceptual
isochrony, whereby syllables or feet sound to be of equal duration to the listener without being
equal acoustically. However, the basis of this perceptual isochrony still needs to be explained,
even if the acoustic measures of syllable and foot duration are inadequate for the task.
In recent years, researchers have begun to use new measures to investigate the basis of perceptual
isochrony. The two most developed of these proposals describe rhythm by using measures of the
relative durations of vowels and consonants. One proposal by Ramus, Nespor and Mehler
suggested the use of three measures: the standard deviation of vowel, and consonant durations,
and the proportion of the total utterance comprising vowel durations. These measures were
shown to be significantly different when applied to the perceptually and classically defined syllable-
and stress-timed languages. The Pairwise Variability Index (PVI) popularised by Low, Grabe and
Nolan makes use of a similar comparison to that of Ramus et al. Essentially the PVI compares the
duration of successive vocalic and intervocalic durations. Using the PVI, Low et al. showed that
Singapore English is more syllable-timed than British English, and Grabe and Low further
demonstrated that the PVI gives significantly different results when applied to those languages
classically described as syllable- or stress- timed.
These metrics of speech rhythm work on the assumption that rhythm arises from the phonological
structure of a language. The classically stressed timed languages will show greater variety in
vowel durations than syllable-timed languages because they have a greater degree of vowel
reduction. Because unstressed words will exhibit vowel reduction, and stressed words will not,
and because stressed and unstressed syllables tend to alternate in these languages, there should be
a large difference between successive vowel durations. In addition stress-timed languages will

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