A Simulation Study on the Effect of Glottal Boundary Conditions on Vocal Tract Formants

In the source-filter theory, the complete closure of the glottis is assumed as a glottal boundary condition. However, such assumption of glottal closure in the source-filter theory is not strictly satisfied in actual utterance. Therefore, it is considered that acoustic features of the glottis and the subglottal region may affect vocal tract formants. In this study, we investigated how differences in the glottal boundary conditions affect vocal tract formants by speech synthesis simulation using speech production model. We synthesized five Japanese vowels using the speech production model in consideration of the source-filter interaction. This model consisted of the glottal area polynomial model and the acoustic tube model in the concatenation of the vocal tract, glottis, and the subglottis. From the results, it was found that the first formant frequency was affected more strongly by the boundary conditions, and also found that the open quotient may give the formant stronger effect than the maximum glottal width. In addition, formant frequencies were also affected more strongly by subglottal impedance when the maximum glottal area was wider.