Articulatory modelling of nasals: data and models
Résumé
The problem of nasality is complex and has given rise to a large number of studies, from both perception and production points of view. The nasality feature is related to the velum position: lowering the velum, and thus opening the velopharyngeal port, is a simple gesture that induces strong and complex changes in the vocal tract acoustical behaviour. The realisation of nasality involves (1) an articulatory level that deals with the shape of the articulators and their degrees of freedom, and (2) a control level that deals with the coordination of these articulators. The present contribution describes our attempts to characterise at both levels the nasality as realised by one specific subject. The first part is devoted to the development of a 3D articulatory model of the velum that complements the jaw, tongue and lips models already built from MRI data collected on the same subject (Badin et al., 2002). The corpus consisted of a set of artificially sustained phonemes designed as to cover the maximal range of velum positions: the four French nasal vowels and their oral counterparts, and the consonants [p t m n] in three symmetrical contexts [a i u]. Following a method already proven for orofacial articulatory modelling (Badin et al., 2002), the 3D coordinates defining the velum geometry extracted from the MRI data were submitted to decomposition in linear components. Two parameters were found to control the velum, roughly corresponding to vertical and horizontal movements of the uvula, and explaining 74 % of the data variance, with an RMS reconstruction error of 0.13 cm. In addition, a detailed description of the 3D geometry of the nasal and paranasal cavities of the subject was obtained from EBCT scans, and associated with the velum and tongue models to reconstruct area functions. The use of 3D is particularly important, as it appears that the subject's uvula is often in contact with both the back of the tongue and the pharyngeal wall, creating an occlusion in the midsagittal plane, though lateral canals remain open. The influence of tongue upon velum was also studied in detail. The whole model provides finally a link between the oropharyngeal port area and the velum control parameters. The second aspect of the work was the study of the effects of spatial and contextual coarticulation on the velum position in vowels and consonants, and on the associated acoustic outputs. We present an articulatory analysis of velar movements obtained by ElectroMagnetic Articulography for consonants and vowels recorded for the MRI subject uttering the same corpus of VCV sequences. These findings increase our knowledge of the articulatory control of nasality, and open the way to the articulatory synthesis of nasal vowels and consonants. Badin, P., Bailly, G., et al. (2002). Three-dimensional articulatory modeling of tongue, lips and face, based on MRI and video images. J. Phonetics, 30(3), 533-553.