Van Bergem, D. R. (1993). Acoustic vowel reduction as a function of sentence accent, word stress, and word class. Speech Communication, 12, 1–23.
Many previous studies have found that sentence accent, word stress and content word would usually expand a vowel in its pitch range, duration, and make it closer to the cardinal vowel positions. In this study, the interaction between such phenomenon and vowel reduction is further examined. Experimental stimuli were isolated syllables, (un-)stressed-(un-)accented content words, and also accented and nonaccented function words. 15 male speakers were recruited, and they were asked to read the stimuli with appropriate intonation. Measurement and computation were taken on the vowel formant (F1 and F2), formant curvature (the bigger the curvature, the less the influence from the environment), and duration. Results showed that the effect of accent and stress are both significant. Accented stress content words were the closest to isolated syllables, which represent the canonical representation, and unaccented syllables were the most different. The same pattern was also found in duration. The above results suggested that the degree of vowel reduction was influenced by accent and stress. Syllables that were [+accent, +stress] would be reduced the least, while those that were [–accent, –stress] would be reduced the most. Such reduction was not always a phenomenon of centralization. The reduction direction of vowels was found to be quite diverse, which might have something to do with the influence of surrounding consonants. A further examination on individual differences among speakers showed that some speakers had distinct vowel spaces for individual vowels, while others showed much overlapping. In order to investigate how such phenomenon influenced listeners’ perception, a second experiment was done, in which listeners were asked to identify each vowel spoken by different speakers in different accent and stress conditions. Results showed that listeners were more accurate in identifying vowels spoken by speakers who had distinct vowel space for each vowel. Also, [+accent, +stress] vowels were more identifiable than [–accent, –stress] ones and function words. To sum up, the influence of sentence accent, word stress, and word class on the realization and identifiability of vowel reduction was confirmed in this study.
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