Reconceptualizing the vowel space in analyzing regional dialect variation and sound change in American English

Vowel space area (VSA) calculated on the basis of corner vowels has emerged as a metric for the study of regional variation, speech intelligibility and speech development. This paper gives an evaluation of the basic assumptions underlying both the concept of the vowel space and the utility of the VSA in making cross-dialectal and sound change comparisons. Using cross-generational data from 135 female speakers representing three distinct dialects of American English, the first step was to establish that the vowel quadrilateral fails as a metric in the context of dialect variation. The next step was to examine the efficacy of more complete assessments of VSA represented by the convex hull and the concave hull. Despite the improvement over the quadrilateral, both metrics yielded inconsistent estimates of VSA. This paper then explores the possibility that regional variation can be characterized more effectively if formant dynamics and the resulting spectral overlap were also considered in defining the space. The proposed formant density approach showed that the working space may be common to all dialects but the differences are in the internal distribution of spectral density regions that define dialect-specific "usage" of the acoustic space. The dialect-inherent distribution of high and low density regions is largely shaped by sound change.