Phonological and acoustic properties of [ATR] in the vowel system of Akebu (Kwa)

The study examines phonological and phonetic properties of vowel [ATR] in Akebu (Kwa). The sum of descriptive evidence, including vowel harmony, vowel distribution in non-harmonising contexts, vowel reduction, and typological and etymological considerations indicates a rare vowel inventory with the [ATR] contrast in front/back vowels but a height contrast in the three redundantly [-ATR] central vowels /ᵻ, ə, a/. This analysis was checked against four common acoustic metrics of [ATR]: F1 and F2 frequencies, spectral slope, and F1 bandwidth size (B1). As expected, the results on the last three metrics were variable across speakers and vowel types and, therefore, inconclusive. The results on F1 were consistent but did not distinguish between [ATR] and vowel height. Two results still suggested the [-ATR] value of central vowels: the latter occupied the same belt of F1 frequencies and the same position of the observed over predicted B1 values as the front/back [ ATR] vowels.