Phonetically grounded phonology and sound change: The case of Tswana labial plosives

A widely held assumption in phonology is that phonology should be phonetically grounded. Under a strict version of this view, productive phonological processes that counter phonetic naturalness should not be possible. Traditional grammars of Tswana describe it as having a productive process of post-nasal devoicing of voiced plosives (/mb/→[mp]), counter to phonetic expectations. The vocal fold settings during the nasal should promote, not inhibit, voicing in a following consonant. Leakage through the velic valve during the initial part of the post-nasal oral closure should inhibit buildup of intra-oral air pressure and hence promote voicing. The unexpectedness of the Tswana pattern leads to questioning the accuracy and reliability of the traditional descriptions of this language. In this paper, we report an acoustic study of 12 Tswana speakers, showing that there is evidence that at least some speakers have an active, productive process of post-nasal devoicing. However, our data also show evidence that this phonetically unnatural system is unstable, and is in fact in the process of changing towards a more natural system. We consider the relevance of these results for views about the relationship between phonetics and phonology, and for theories of language change.