Acoustic-articulatory correlations in a four-region model of the vocal tract: Experimental evidence for blade features

In the first part of this report, the formant frequencies F1–F4 and the quality (or gain) factors Q1–Q4 are correlated with the positions, areas, or area ratios formed by the four active articulators: tongue root, tongue body, blade, lips. Vowel area functions of ten speakers were taken from seven X-ray and MRI studies and fit to 27 equal-length tubes by means of cubic spline interpolation. Among the findings it was determined that (1) when the blade position (location of smallest constriction) moves toward the lips, F3 frequency shifts higher; (2) blade aperture (blade area normalized by lip area) is directly correlated with Q3. In the second part of the report, evidence for these two blade relations is provided using acoustic recordings of actual coronal speech sounds. To this end an auditorily-based estimator of Q3 is developed: the peak energy factor PE3. The asymptotic ERB (equivalent rectangular bandwidth) of the auditory filter is about one-sixth octave wide. Hence one-sixth octave is adopted as the unit of formant frequency resolution. Measured F3 frequencies are observed to span six one-sixth octaves (one octave). The six F3 distinctions are classified by the primary and secondary features of blade position [anterior posterior] and [AB RB], where AB and RB are advanced blade and retracted blade. Dentalveolars are [+anterior –posterior]; postalveolars are [–anterior +posterior]. Blade aperture is captured by the feature pair [elevated depressed]. Laminals are [+elevated –depressed]; apicals are [–elevated –depressed]. When the blade aperture increases from a small value (laminal) through a medium value (apical) to a large value (depressed), PE3 also increases. The coronal fricatives of American English, Toda, and Ubykh are examined as well as the coronal stops, nasals, and liquids of Central Arrernte. Both the palatographic evidence and the PE3 measures consistently show the laminality of [s ̪ s] and the apicality of [ʃ ʂ]. Furthermore, the [s ʃ] sounds are both found to be [+anterior]. In American English, for example, there is no statistically significant difference in F3 frequency between laminal [s] and apical [ʃ], which indicates very similar blade positions. ______________________________________________________________________________ *This paper is an abridged and updated version of a two-part report that had previously appeared in Vol. 11 of the IULC Working Papers. I wish to thank Jennifer Cole for suggesting a more thorough introduction to the concept of the distinctive feature than was presented earlier. Geoffrey Stewart Morrison pointed out that the supraglottal excitation of obstruents may lead to results different from those obtained with the original vowel glottal-source model. He also called attention to the possible presence of a sublingual cavity during sibilant production. Both concerns are now addressed in Sections 2.7 and 3.1, respectively. In 2013 Mark Tiede completed his Yale dissertation “An MRI-based morphological approach to vocal tract area function estimation” in which he measured the vowel area functions of 12 speakers – on the same order as the 10 analyzed here. A future paper will examine the acoustic-articulatory correlations of the four articulator regions using his data set.