This acoustic study examines sound (vowel) change in apparent time across three successive generations of 123 adult female speakers ranging in age from 20 to 65 years old, representing three regional varieties of American English, typical of western North Carolina, central Ohio and southeastern Wisconsin. A set of acoustic measures characterized the dynamic nature of formant trajectories, the amount of spectral change over the course of vowel duration and the position of the spectral centroid. The study found a set of systematic changes to /I, ε, æ/ including positional changes in the acoustic space (mostly lowering of the vowels) and significant variation in formant dynamics (increased monophthongization). This common sound change is evident in both emphatic (articulated clearly) and nonemphatic (casual) productions and occurs regardless of dialect-specific vowel dispersions in the vowel space. The cross-generational and cross-dialectal patterns of variation found here support an earlier report by which found this recent development in these three dialect regions in isolated citation-form words. While confirming the new North American Shift in different styles of production, the study underscores the importance of addressing the stress-related variation in vowel production in a careful and valid assessment of sound change.
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