Numerical simulation of aeroacoustic sound generation during tongue articulation and velopharyngeal closing*

The velopharyngeal air leakage to a nasal cavity is known to cause problematic sound generation for oral consonant pronunciation. In this study, effects of the velopharyngeal closing ratio during tongue articulation of fricative [s] on the sound generation are investigated by compressible flow simulations. The vocal tract geometries of /usu/ were extracted from a four-dimensional computed tomography scan of a subject who has no speech disorder, and the articulation process was simulated from the end of /u/ to the middle of /s/. The opening ratio of velopharyngeal valve was changed for each simulation. The turbulent airflow near the tongue constriction was modeled with the large eddy simulation, and sound propagation was directly computed by solving compressible Navier-Stokes equations. The moving immersed boundary method for the hierarchical structured grid was adopted to reduce the computational costs. When the velum and tongue moved normally as measured with the subject, the co-articulation process between /u/ and /s/ was observed, and the acoustic prediction agreed with the measurement. When the velum was not closed with the tongue elevation, the sound amplitude significantly decreased, suggesting the capability of the simulation to be utilized for the prediction of velopharyngeal insufficiency.