An SS ANOVA (Smoothing Spline Analysis of Variance) study of high tone lowering in Tsua
This article examines the acoustic phonetic properties of consonant-tone interaction in the Khoisan language, Tsua, using speech production data from original field research in Botswana. The Tsua tonal melodies with Fundamental Frequency (F0) shapes having the most extreme excursions are the result of a rare consonant-tone interaction pattern with depressor types found in both African and East Asian tone languages. The Tsua depressor types are voiced obstruents, aspirated obstruents and the glottal fricative /h/. Statistical analysis via Smoothing Spline Analysis of Variance (SS ANOVA) reveals two striking generalisations: (i) only root-initial High tones followed by a non-High tone are depressed; and (ii) the overall shapes of the F0 curves are more important for tone melody identification than whether they are produced at a slightly higher or lower Hertz value, even if the difference is statistically significant. The first finding reflects the context-dependent nature of Tsua tonal depression. The second stands in contrast to studies of other tone systems that suggest the relative Hertz differential between adjacent tones is more important for identification. These findings expand our knowledge of tonal phonetics by showing what is possible in a typologically rare tone system, and highlight the importance of statistical methods in phonetic fieldwork.