Variability in the production of quantal vowels revisited

Abstract

Articulatory and acoustic variability in the production of five American English vowels was examined. The data were movement records for selected fleshpoints on the midsagittal tongue surface, recorded using the x-ray microbeam. An algorithm for nonlinearly transforming fleshpoint positions to a new Cartesian space in which the x and y axes represent, respectively, the distance of the fleshpoint along the opposing vocal tract wall and the distance perpendicular to the tract wall, is described. The transformation facilitates a test of Quantal Theory in which variability in the two dimensions is compared over many productions of a given vowel type. The data provide some support for the theory. For fleshpoints near ‘‘quantal’’ constriction sites, the primary variability was in the x dimension (constriction location). The y-dimension values were more tightly constrained, and the formant frequencies were more significantly correlated with the y values than with the x values. The greater variability in constriction location than in degree was not an artifact of the greater distances traversed in the x dimension between the vowel and constrictions in neighboring consonants, since the pattern was preserved when pellet values were translated to take into account a ‘‘context-free’’ vowel target (the average values in the context of preceding and following labial consonants). Moreover, the observed correlations between formant values and pellet positions in the two dimensions for [i] and [u] were duplicated in an articulatory-to-acoustic modeling test using values for constriction length and cross-sectional area estimated from the data. The model showed smaller second formant variability in the x dimension than in the y dimension for equal-sized excursions near the constriction sites for these close vowels, in keeping with the interpretation that speakers exercise less precise control in just the dimensions and regions where quantal stability is available. However, the articulatory pattern was seen not just in vowels which clearly have consonantlike constrictions (the quantal vowels [i], [u], and [ɑ ]), but also in nonquantal vowels such as [æ].