LARYNGEAL ADJUSTMENT IN THE PRODUCTION OF VOICELESS CONSONANTS: II. PHYSICAL MODELLING. Annemie Van Hirtum , Nicolas Ruty , Xavier Pelorson , Susanne Fuchs , Pascal Perrier TUE - Technical University Eindhoven, Eindhoven, The Netherlands ICP - Institut de la Communication Parl´ ee, UMR CNRS 5009 - INPG, Grenoble, France ZAS - Centre for General Linguistics, Berlin, Germany ABSTRACT In this paper, we present an attempt to describe flow past the glottis and to predict the pressure inside the vocal tract during phonation. Different theoretical models to describe the pressure distribution inside the whole vocal tract will be presented and compared. The- oretical models will be validated on ‘in-vitro’ measurements per- formed on a mechanical replica of the human phonatory system. Next, an application of this theoretical approach to the simulation of vowel-plosive-vowel (VCV) sequences is presented. It is shown that even using a very crude mechanical model for the vocal folds (such as a 1 or 2-mass model) one can already replicate some im- portant features with a surprising accuracy. This will be illustrated by examples of predictions for the onset-offset glottal pressure and of the fundamental frequency of oscillation. Lastly, the simula- tions are compared to ‘in-vivo’ observations as e.g. presented in a companion paper. 1. INTRODUCTION In this paper we discuss the effect of the vocal tract on the vocal folds oscillations. While such a study has already been carried out extensively in the case of vocal coupling [1, 2, 3, 4], very little is known about the behaviour of flow past the glottis. The possibility of having a pressure recovery downstream of the glottis is particu- larly crucial during the production of the voiceless consonant such as a plosive. Due to the presence of a closure of the vocal tract the supraglottal pressure increases which can explain, in a part, the offset of the vocal folds oscillations. Previous attempts to simulate such voiceless consonants tend indeed to schow what a precise co- ordination between the glottal source and the constriction is crucial [5, 6]. As the matter of fact, without such a description, acceptable acoustic results can only be simulated using unrealistic glottal ges- tures. In this paper, we present an attempt to predict the pressure inside the vocal tract during phonation. Different theoretical models to describe the pressure distribution inside the whole vocal tract will be presented and compared. A particular attention will be devoted to the relative balance between inertia and viscosity inside the vo- cal tract. These theoretical models will be compared to ‘in-vitro’ measure- ments performed on a mechanical replica of the human phonatory system. This set-up consists of a pressure reservoir (‘the lungs’), a self-oscillating mechanical ‘glottis’ [7] and a ‘vocal tract’ mod- eled by pipes with varying sections and length. Using this set-up, the generation of a voiced-plosive sequence can thus be simulated by closing one part of the ‘vocal tract’. Using pressure sensors placed along the replica, the pressure distribution can be measured (subglottal, supraglottal and inside the vocal tract). A systematic study of the acoustical coupling and of the flow recovery will be presented and compared to the theoretical predictions. Next, we present an application of this theoretical approach to the simula- tion of vowel-plosive-vowel (VCV) sequences. It is shown that even using a very crude mechanical model for the vocal folds (such as a 1 or 2-mass model) one can already replicate some important features with a surprising accuracy. This will be illustrated by ex- amples of predictions for the onset-offset glottal pressure and of the fundamental frequency of oscillation. Lastly, the simulations are compared to ‘in-vivo’ observations as e.g. presented in a companion paper [8]. 2. MODELING VOCAL FOLDS DYNAMICS The interaction of expiratory airflow with the vocal folds tissues is known to be the primary source of human voiced sound pro- duction. The airflow through the larynx induces instability of the vocal folds. The resulting vocal fold vibrations modulate the air- flow giving rise to a periodic sequence of pressure pulses which propagates through the vocal tract and is radiated as voiced sound. Consequently physical modeling of the 3D fluid-structure interac- tion between the living vocal folds tissues and the expiratory air- flow is essential in the study of phonation. Simplifications of the physical reality are favoured due to a historical interest for speech control and synthesis applications requiring a limited number of physiological meaningfull and measurable parameters. Therefore physical models like vocal fold two-mass models strive to repre- sent the main features of phonation while assuming severe sim- plifications in the biomechanical structure and fluid mechanical flow modeling. The description of the aerodynamics in the glot- tis assumes a simplified one-dimensional quasi-stationary incom- pressible flow as described by the stationary Bernoulli‘s equation. Usually several corrections are applied accounting for 1) flow sep- aration using Liljencrants ‘ad-hoc’ criterium, 2) viscosity in the glottis (Poiseuille flow), 3) inertance of air [9, 10] and 4) down- stream pressure recovery. The possibility of having a pressure recovery downstream of the glottis is particulary crucial during the production of voiceless con- sonants concerning the onset and offset of vocal fold oscillation. In [9] the pressure recovery is estimated by evaluating the quasis- teady momentum equation depending on the area ratio at the po- sition of flow separation in the glottis and the vocal tract area past the glottis. The same area ratio is presented in [11, 12] as a geo- metrical basis for quantifying the pressure recovery in a diffuser.