The effect of glottal opening on the acoustic response of the vocal tract
Résumé
In the classical theory of vowel production it is standard to assume linear separability of the voicing source, located at the glottis, from the vocal tract downstream. In this paper we consider an effect of relaxing this assumption and investigate how the open phase of the glottal cycle may affect the acoustic response of the vocal tract. A mechanical model of the larynx and vocal tract is used to make measurements of the formant frequencies with both a static glottis and a time-varying glottal area. For the static glottis the vocal tract was excited by an external sound source. The first and second formant frequencies increased with increasing glottal width. For the widest glottis investigated the upward shift in the first formant was 13% of the value found with a closed glottis. The direction of the shift is well-modelled by a theoretical transmission line model of the vocal tract for which the value of the glottal impedance can be varied, although the F1 values are underestimated by approximately 5%. For the time-varying glottis the acoustic excitation came from the periodic interruption of a steady air-flow. The first formant frequency increased with both increasing glottal width and increasing glottal open quotient. Our findings have implications for realistic modelling of modal voice, especially where there is a permanent glottal leak, and breathy voice including pathological cases.