Few phonological phenomena have so captured the attention of theorists and continued to baffle them as the phenomenon of tonal downstep. Downstep is the lowering of the tonal register that sometimes occurs between adjacent, otherwise identical tones. It is cumulative, and successive occurrences of the phenomenon result in ever lower settings of the tonal register. The present work reports on an instrumental study of downstep in Bimoba, a Gur language spoken in the Northern Region of Ghana. So far as I am aware, the present work is the only description in existence of tone in Bimoba.
Bimoba is a good candidate for a study of downstep, because it is a ‘three-tone’ language in which both Low tones (floating and non-floating) and Mid tones cause High tones to be downstepped. A number of phonological studies (based on auditory impressions only) of three-tone languages with downstep of High tone have claimed that the difference in pitch between a High and a following Mid is equivalent to that between a High and a following downstepped High. These include Supyire (Carlson 1983), Babanki (Hyman 1979), Moba (Russell 1986) and Kagoro (van de Kolk 1992). This issue is important to tone theorists because a number of proposals on the theory of downstep formally equate a downstepped High with one type of Mid tone. These include Clements (1983), Hyman (1986, 1993), Inkelas (1987) and Snider (1990). On the other hand, a number of other proposals account for downstep with phonetic implementation rules. These include Pierrehumbert (1980), Poser (1984), Beckman & Pierrehumbert (1986) and Pierrehumbert & Beckman (1988). ‘If register shift is indeed a rule of phonetic implementation, there is no reason, a priori, for the interval of register shift to be equivalent to that found between two phonemic tones’ (Snider 1990: 470). So far as I am aware, no instrumental study has ever addressed this issue. The present study therefore undertakes to help fill this lacuna, and concludes that in Bimoba a Mid tone is phonetically indistinguishable from a downstepped High tone in comparable environments.
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