Using Sound Change to Study Phonological Representations: The Case of American English Diphthongs

The question of diphthong phonematicity —do diphthongs consist of two monophthongs in one nucleus, or a single, dynamic vowel phoneme —is an important question when analyzing the phonemic inventory of a language with diphthongs. American English has a dearth of synchronic phenomena that could be used to test diphthong phonematicity, and phonetic studies have suffered from a lack of direct reference to phonological structures. This study takes a novel approach to diphthong phonematicity by examining the acoustic signature of American English sound changes. Specifically, it compares the vowels of speakers from the North and Midlands regions. When analyzed as biphonematic, the diphthong components of the Northern diphthongs are expected to shift in parallel with the Northern Cities Shift impacting the monophthongs. Other than a slight raising of the nucleus of /aI/, the study finds no significant shift of diphthong components parallel to the monophthongs. To rule out the possibility that this deviance could be explained as the result of contextual variation, the offglides are compared to their corresponding monophthongs. The back offglides are found to be significantly different from /U/ and each other. Similarly, the front offglides are nearly significantly different from /I/ and each other. This precludes the possibility of contextual effects and suggests a monophonemic representation of American English diphthongs. The paper concludes with a discussion of directions for future research such as further testing the results and development of a subsegmental formalism that accounts for diphthong monophonematicity.