Morphological Variation in the Adult Vocal Tract 1 Running head: MORPHOLOGICAL VARIATION IN THE ADULT VOCAL TRACT Morphological Variation in the Adult Hard Palate and Posterior Pharyngeal Wall

Purpose: Adult human vocal tracts display considerable morphological variation across individuals, but the nature and extent of this variation has not been extensively studied for many vocal tract structures. There exists a need to analyze morphological variation and, even more basically, to develop a methodology for morphological analysis of the vocal tract. Such analysis will facilitate fundamental characterization of the speech production system, with broad implications from modeling to explaining inter-speaker variability. Method: A data-driven methodology to automatically analyze the extent and variety of morphological variation is proposed and applied to a diverse subject pool of 36 adults. Analysis is focused on two key aspects of vocal tract structure: the midsagittal shape of the hard palate and the posterior pharyngeal wall. Result: Palatal morphology varies widely in its degree of concavity, but also in anteriority and sharpness. Pharyngeal wall morphology, by contrast, varies mostly in terms of concavity alone. The distribution of morphological characteristics is complex, and analysis suggests that certain variations may be categorical in nature. Conclusion: Major modes of morphological variation are identified, including their relative magnitude, distribution and categorical nature. Implications of these findings for speech articulation strategies and speech acoustics are discussed. Introduction Vocal tract morphology is a fundamental consideration in characterizing the human speech production system because, as with any motor system, the physical size and shape of structures that comprise the vocal tract underlie many aspects of articulation and control. Morphology has additional importance for the speech production system due to Morphological Variation in the Adult Vocal Tract 3 its role in shaping speech sounds. The vocal tract's acoustical properties (e.g., resonant characteristics) are determined by its shape, which is determined not only by active shaping and articulation, but also by the vocal tract's inherent morphology. At the same time, morphology varies widely across individuals, which has at least two major implications. First, morphological variation is a potential source of variability in both the articulatory and acoustic domains. Second, a detailed understanding of morphological differences between individuals can facilitate fresh insights into many aspects of inter-speaker variability, speech motor control and speech production modeling. Many studies (e.g., Hiki & Itoh, 1986) have observed differences in palatal concavity (i.e., whether the palate is flat or has a high, domed shape), but little beyond concavity has been noted or quantified. Even with this basic understanding of morphological differences, it has become clear that palate shape influences …