Dissecting coarticulation: How locus equations happen

Abstract

A programmatic series of studies aimed at expanding our understanding of coarticulation in V 1·CV 2 sequences is presented. The common thread was examining coarticulatory dynamics through the prism of locus equations (LEs). Multiple experimental methodologies (articulatory synthesis, X-ray film, Principal Component Analysis, and extraction of time constants for F2 transitions), guided by a few theoretical assumptions about speech motor planning and control, were used to uncover the articulatory underpinnings responsible for the trademark acoustic form of LE scatterplots. Specific findings were: (1) the concept of a stop consonantal ‘target’ was quantitatively derived as a vowel-neutral, ‘deactivated,’ tongue contour; (2) the linearity of LEs is significantly enhanced by the uniformity of F2 transition time constants, which normalize with respect to F2 transition extents, and an inherent linear bias created by the smaller frequency range of [F2 onset−F2 vowel] relative to F2 vowel frequencies; (3) realistic LE slopes and y-intercepts were derived by modeling different extents of V 2 overlap onto stop consonantal target shapes at closure; and (4) a conceptually simple model, viz. interpolation between successive articulatory target shapes, followed by derivation of their formant values expressed as LEs, came surprisingly close to matching actual LEs obtained from our speaker.