Acoustic correlates of the advanced vs. retracted tongue root feature in Igbo

Vowel harmony based on the advanced tongue root (±ATR) feature is a well-known attribute of the languages of the Macro-Sudan belt. Igbo (< Benue-Congo), one of these languages, on basis of which first instrumental studies of ±ATR articulation were conducted, has an asymmetric vowel system /i, ɪ, e, a, ɔ, o, ʊ, u/, so that root ±ATR harmony exists within three pairs and a single vowel /e/. This paper describes an acoustic realization of ±ATR in Igbo. According to the hypothesis that the value of the first formant in combination with the distribution of energy over the spectrum combine into a single perceptual cue that allows to distinguish vowels opposed by ±ATR we investigate (along with the description of the formant space) three spectral parameters that were shown to be acoustic correlates of ±ATR in other Macro-Sudan languages. The results of the instrumental analysis indicate that acoustic correlates of ±ATR harmony in Igbo are the value of the first formant, its bandwidth and normalized center of gravity. In addition, all parameters relevant for the ±ATR contrast in our data divided /ɔ/ utterances in different roots into two groups with contrasting values of the feature.