Across much of the United States, the phonemic contrast between the vowels of cot and caught is being lost through a sound change known as the low back vowel merger. This paper examines the spread of this merger in the state of Missouri. Acoustic examination of F0, F1,F2, F3, and vowel duration reveals that speakers in the greater St. Louis area maintain the phonemic distinction between /a/ andɔ/, while in much of the state, this distinction is being lost or diminished. In addition to static formant measures, the formant trajectories of the two vowels are examined, and it is found that in St. Louis speech, the VC consonantal transition of F2 is accomplished more quickly for/ɔ/ than for /a/. Although the F2 transitions of the two vowels differ, their overall spectral shapes are more similar than other comparable vowel pairs such as /æ/ and /g=e/, which are not undergoing widespread merger. The dynamic similarity between /a/ and /ɔ/ is posited as a partial explanation for why this particular merger is spreading so rapidly throughout the United States.

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