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Modeling dynamic trajectories of front vowels in the American South.

Regional variation in American English speech is often described in terms of shifts, indicating which vowel sounds are converging or diverging. In the U.S. South, the Southern vowel shift (SVS) and African American vowel shift (AAVS) affect not only vowels' relative positions but also their formant dynamics. Static characterizations of shifting, with a single pair of first and second formant values taken near vowels' midpoint, fail to capture this vowel-inherent spectral change, which can indicate dialect-specific diphthongization or monophthongization. Vowel-inherent spectral change is directly modeled to investigate how trajectories of front vowels /i eɪ ɪ ɛ/ differ across social groups in the 64-speaker Digital Archive of Southern Speech. Generalized additive mixed models are used to test the effects of two social factors, sex and ethnicity, on trajectory shape. All vowels studied show significant differences between men, women, African American and European American speakers. Results show strong overlap between the trajectories of /eɪ, ɛ/ particularly among European American women, consistent with the SVS, and greater vowel-inherent raising of /ɪ/ among African American speakers, indicating how that lax vowel is affected by the AAVS. Model predictions of duration additionally indicate that across groups, trajectories become more peripheral as vowel duration increases.