Beyond midpoints: Vowel dynamics of the low-back-merger shift

Abstract

The Low-Back-Merger Shift—the lowering/retraction of /æ/, /ɛ/, and /ɪ/—is a now-well-studied phenomenon of American English. However, it is primarily described by the relative position of the vowels using single-point acoustic measurements. This study seeks to fill an apparent gap in the literature by describing change in these vowels’ formant trajectories. Data come from 45 h of sociolinguistic interviews conducted in southwest Washington (ages 18–86). A generalized additive mixed-effect model (Wood, 2007) fit to pairs of formant measurements extracted at 11 timepoints per vowel token confirms that the lax vowels are shifting and uncovers changes in their formant dynamics in apparent time. For instance, the bulk of /æ/ lowering and retraction is a result of its onset shifting rather than the offset. For all three vowels, the amount of change in F2 over the vowel’s duration decreased over time, resulting in qualitative changes in the shape of the vowel trajectory (when visualized in the F1-F2 space). Furthermore, though the men lagged behind the women in relative position in the vowel space, both genders changed the shape of their trajectories at the same time. This study shows that there is more to vowel shifts than changes in their midpoints.