Acoustic characteristics of tongue root position and vowel assimilation in Akan

Abstract

The purpose of this research was twofold: (1) To find an acoustic measure (or diagnosis) of tongue root position in a language where a difference of tongue root position is systematic and significant, and (2) apply this measure to cases of vowel assimilation between vowels of conflicting tongue root positions in order to determine whether tongue root positions were affected. Akan, a K wa language spoken in Ghana, exhibits a form of vowel harmony controlled by the tongue root in which the vowels in a particular word must be spoken with either an advanced or a retracted tongue root. The position of the tongue root [i.e., either advanced or retracted; hereafter referred to as either + or − ATR (advanced tongue root)] is determined by the word stem, and all affixes must agree with that tongue root position. In this paper, we will look at data from Kwawu, an Akan dialect, which has the following vowels: [i,e,u,o] are [+ ATR] vowels, while [ᶅ,ɛ,o,ɔ,a] are [− ATR] vowels. All [− ATR] vowels have variants when they occur in a syllable that precedes another word containing the high [+ ATR] vowels [i,u]. Two claims have been made about these variants. One asserts that the variants assimilate in tongue root position to the following [+ ATR] vowel and become [+ ATR] vowels themselves, while the other argues that the assimilation does not involve the tongue root but merely raising of the tongue blade. Resolving this dispute required that we first define a measure that would provide a reliable diagnosis of tongue root position. Only acoustic measures were considered as other means were not practical. Five kinds of measures were examined: formant frequency, formant bandwidth, duration, pitch, and amplitude of the first three harmonics. It was found that frequency bandwidth (more precisely the bandwidth of the first formant) provided the only reliable diagnosis of vowel type. The [+ ATR] vowels were found to have a greatly narrower first formant when compared with [− ATR] vowels. This measure was then used to examine the behavior of one of the [− ATR] vowels, the mid vowel [ɛ], in two contexts: before the high [+ ATR] vowels [i,u], and before the low [− ATR] vowel [a]. Bandwidth measurements of ɛ before the [+ ATR] vowels [i,u] do not show the narrower bandwidth found in [+ ATR] vowels, allowing us to preliminarily conclude that tongue root position is not assimilated.