Vowel variation in Southern Sotho: an acoustic investigation

Abstract

For the development of Human Language Technologies such as automatic speech recognition systems or text-to-speech systems, exact acoustic and phonological information of the language in question is essential. In the case of Southern Sotho, the current study is a first step in the direction of providing such information. Measurements of the frequencies of the first two formants of the vowels represented by the orthographic symbols i e a o u are made in several contexts in order to motivate particular phonetic and phonemic representations of these vowels, regardless of the conventional orthography. Our measurements corroborate some of the earlier impressionistic research into Southern Sotho. In particular, the four variants of the vowels orthographically represented by 'e' and 'o' respectively are clearly present, indicating that there are at least seven distinct (phonemic) vowels. These variants are distinguished mainly by vowel height, that is, by F1 differences. On the other hand, our results oppose the established knowledge in some important ways. Most notably, we find that harmonic vowel raising produces a larger change in vowel height than the height difference between the pairs of phonemically distinct middle vowels (/openo/ and /o/, /ε/ and /e/), respectively. The finding is seemingly reliable as this tendency is constant across all words investigated. An interesting finding is a significant modification of the vowels /u/ and /a/ in certain contexts. These are vowels previously believed not to have any allophones at all. Lastly, a number of important unresolved issues are highlighted.