A Numerical Experiment on Waveform Fluctuation within the Larynx

An analysis of fluctuations of speech wave, i.e., pitch, amplitude, and waveform, is a subject of growing interest for improvement in the quality of synthesized voice. However, little information with respect to the sound source itself in speech production is available, since it is difficult to measure fluid and acoustic motions within the larynx. The purpose of the present study is to clarify the relationship between unsteady motion within the larynx and generated speech wave. We numerically simulate speech production in a non-pathological speech organ based on our proposed glottal source model. The simulation shows waveform fluctuations in glottal jet velocity and pressure waves within the larynx caused by unsteady fluid motion. In order to investigate the unsteady motion effects on phonation, a harmonic-to-noise ratio (HNR) in terms of measures of the waveform fluctuations is estimated by varying lung pressure. The HNR decreases with the lung pressure. The HNR of pressure wave indicates the greatest fluctuation near the glottis, although the HNR does not show the fluctuation faraway distance from the glottis. This result suggests that the unsteady vortex motions within the larynx do not greatly affect speech waves radiating from the mouth. INTRODUCTION A better understanding of complex phenomena within the larynx is of interest from biomechanics and speech information processing standpoints. Recently, with improvements in computing power, a numerical simulation has become an efficient tool for clarifying the speech production [1][2][3]. However, little information with respect to effects of unsteady flow within the larynx on speech production is available. In a previous study [4], we numerically simulated glottal flow based on a rigid wall model of the larynx and reported that, rather than being a steady symmetric laminar flow, the glottal flow is an unsteady asymmetric flow. In order to simulate voiced sound including the unsteady flow effect, we have proposed a numerical glottal source model, which is constructed from a twodimensional unsteady flow model and a distributed parameter model for the vocal folds [5]. However the effects of unsteady motion of the glottal flow on the voiced sound have not yet been investigated. The purpose of the present study is to clarify the effects of the unsteady flow on pressure wave within the larynx. In order to investigate the unsteady motion effects on phonation, a harmonicto-noise ratio (HNR) in terms of measure of the fluctuation is estimated using numerical simulations. MODEL AND COMUPATIONAL METHODS A two-dimensional larynx model in the coronal (z-x) plane is shown in Fig. 1 (a). We assume that the configuration in the y direction is uniform, and that the initial configuration is symmetric about the glottal midline (z-axis). The initial size parameters of the larynx are shown in Table 1. The vocal tract attached to the larynx is approximated by a uniform rigid duct. A detailed description of the larynx shape is presented in the previous paper [4].