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In certain languages, vowel quality changes in unstressed syllables. In acoustic phonetics, this characteristic, namely vowel 'undershoot' under certain conditions and the tendency to centralize, is referred to as 'vowel reduction'. Previous researches have shown that Persian vowel space reduces and is centralized in unaccented syllables and as a result have considered the change in vowel quality to be an acoustic correlate of lexical stress in Persian. The present paper is an investigation of the effect of stress on the duration and quality of simple vowels in accented and unaccented open syllables of bisyllabic words in the speech of 14 female and male speakers of Persian. Results showed a reduction of vowel duration in unaccented syllables, but no reduction or centralization of vowel space. Rather, the vowel space was larger in unaccented syllables. Vowel quality changed, but this change was not statistically significant for all vowels. Previous research has shown that a reduction in vowel duration is not necessarily accompanied by vowel undershoot, as articulators can move faster to achieve ideal vowel quality under time limitations. Thus, vowel reduction cannot be considered as an acoustic correlate of lexical stress in Persian, since besides stress, many other factors, including speaking style, syllable type, word length and type, influence vowel quality.

In rhythmic typology, languages are categorized into stress-timed and syllable-timed types. Earlier studies have highlighted the isochrony of interstress intervals and syllables in stress-timed and syllable-timed languages, respectively. Recent studies suggest that rhythm type depends on the characteristics of the phonological system of each language. Lexically marked stress, robust correlates of stress, stress-sensitive segmental rules, and complex syllable structures are the characteristics of stress-timed languages. On the other hand, the characteristic properties of syllable-timed languages are non-lexical stress, relatively weak correlates of stress, non-application of stress-sensitive segmental rules, and simple syllable structures. The present article investigated these phonological properties in Persian. Persian has been classified as a syllable-timed language. We hypothesized that the rhythmic type of Persian is the direct consequence of its phonological properties. Evidence shows that stress plays a weak role in Persian phonology. Hence, it seems to belong to the type of syllable-timed languages. [ABSTRACT FROM AUTHOR]