vowel quality in persian: stable or unstable?

this paper addresses the validity of two hypotheses concerning changes of vowel quality in persian. the theory of centralization claims that there is a strong correlation between vowel duration and vowel quality. as vowel duration is shortened in unstressed contexts, the size of the vowel inventory reduces toward the center, leading to considerable overlap between different categories and a reduced, crowded acoustic space. in contrast, the dispersion theory of speech perception maintains that there is no correlation between vowel duration and vowel quality, and that vowels are arranged in the acoustic vowel space so as to minimize the potential for perceptual confusion between the distinct vowel categories. changes in vowel quality were examined in unstressed syllables using duration and f1 and f2 spectral values. results indicated that, contrary to the prediction of the centralization hypothesis, there was no strong correlation between duration and vowel quality differences, and that vowels displayed no significant spatial reduction in the direction of the center of the vowel space. the results are more consistent with the dispersion theory as they show that differences in quality in persian are limited to small positions around the area in the acoustic space occupied by vowel categories. in light of our findings, we may well classify persian with languages which do not possess the process of vowel reduction.