Assimilatory effects in a vowel harmony system: an acoustic analysis of advanced tongue root in Akan

Akan has a vowel harmony system which contrasts two sets of vowels, distinguished by whether the tongue root is advanced or retracted. This paper first examines acoustic correlates of the distinguishing phonetic feature of Advanced Tongue Root (ATR), in order to find a reliable diagnostic, which was then used to examine two questions about the nature of assimilation between vowels of the contrasting harmony sets: Is vowel assimilation complete or partial? And is assimilation implemented gradiently over more than one syllable? Addressing these questions first required that we find a measure which would provide a reliable diagnosis of vowel type. Four acoustic measures were considered: formant frequency, formant bandwidth, vowel duration, and relative amplitude of spectral components. The two measures most strongly correlated with [ATR] set affiliation are F1, frequency and F1, bandwidth. However, as shifts in the frequency of the first formant could also be ascribed to differences in tongue height, bandwidth differences are more reliably an indication of the difference between [ATR] sets. This measure was then used to examine the behavior of the mid, [− ATR] vowel /e/ in assimilation and non-assimilation environments. Bandwidth measurements of /e/ in the assimilation environment suggest that it is produced in the same manner as [+ ATR] vowels, that is, assimilation for tongue root position was complete. Analysis of the vowels in the word /adaka/ in various frames shows that assimilation is limited to the syllable immediately preceding the assimilation environment.