Locus equations were investigated as a phonetic index for children’ s production of stop + vowel tokens. Locus equations are straight-line regression fits to data points formed by plotting onsets of F2 transitions along the ordinate and their corresponding midvowel nuclei along the abscissa. Such functions for adult speech have been found to be extremely linear with slope and y-intercept values contrastively distinctive across place of articulation. Sixteen children, aged 3–5 years, produced /bVt/, /dVt/, and /gVt/ tokens embedded in a carder phrase and repeated in randomized order a minimum of three times. Six medial vowel contexts were used [i, I, ae, ٨ , a, u]. Both individual and group mean scatterplots were extremely linear and highly remniscent of adult prototypes. While labial and velar slopes exhibited some degree of overlap, labial versus alveolar and alveolar versus velar slopes were significantly different. All y-intercepts as a function of place of articulation were significantly different. Compared to adult norms, intersubject variability of slope and y-intercept ranges were greater for children. Locus equations can provide a phonetic descriptor for a child’ s attainment of stop place categories seeking to achieve the adult standard of a balance between coarticulatory adjustments and contrastive distinctiveness.
A previous study [Sussman et al., J. Acoust. Soc. Am. 90, 1309–1325 (1991)] of American English CV coarticulation showed a remarkably linear relationship between onset frequencies of F2 transitions (plotted along the ordinate) in relation to midvowel target frequencies (plotted along the abscissa) in CVC tokens with initial /bdg/ followed by ten medial vowels. Slope and y intercept of regression functions (‘‘locus equations’’) fitted to these coordinates systematically varied as a function of stop place. Discriminant analyses, using as predictor variables, slope and y intercept, yielded 100% correct classification of stop place categories. Locus equations provide a systematic lawfulness to coarticulatory variation and a relationally invariant phonetic index for stop place classification. The present study extends the metric to three additional languages−Thai, Cairean Arabic, and Urdu. Resulting scatterplots were extremely linear and varied as a function of stop place. Plotting slope X y intercept yields a derived map of CV phonetic space with which to relationally compare 2, 3, and 4-stop place languages. Within-language stop place contrasts were consistly divergent in CV space. Variability of labial, alveolar/dental, and velar coordinates across five languages showed fairly broad clustering of stop place categories, rather than narrowly focused ‘‘phonetic hot spots.’’ The data are discussed in relation to quantal and adaptive dispersion theories. [Work supported by NSF.]
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