The geometric vocal tract variables controlled for vowel production: proposals for constraining acoustic-to-articulatory inversion

Abstract

Studies on vocal tract shape estimation from the speech signal have thoroughly established that no unique inversion from the acoustic speech signal to the vocal tract area function is possible. However, by emphasizing the non-uniqueness of vocal tract shapes, these studies have exaggerated the possibilities of compensatory articulation, which in turn challenges the traditional classification in terms of place of articulation. We suggest, in the light of more recent studies, that this phonetic concept was too hastily discarded. These studies show that certain geometric variables are very important in the process of motor control and in the typology and description of vowel systems. The aim of this paper is to show the possibility and the value of deriving these controlled variables, rather than the entire vocal tract geometry, from acoustic parameters. For this purpose, using a statistical model of the vocal tract integrating articulatory constraints, we generated a 60 000 item look-up table of vocalic configurations. The analysis of 10 French vowels shows that, for the majority of them, it is possible to associate the formant patterns with the well known geometric parameters, namely the lip area and the location and dimension of the tongue constriction. This result seems to confirm that some geometric variables controlled during production can be inferred from the acoustic signal if articulatory constraints are taken into account in the inversion procedure.