Investigating the relationship between glottal area waveform shape and harmonic magnitudes through computational modeling and laryngeal high-speed videoendoscopy

The glottal open quotient (OQ) is often associated with the amplitude of the first source harmonic relative to the second (H1*H2*), which is assumed to be one cause of a change in vocal quality along a breathy-to-pressed continuum. The association between OQ and H1*-H2* was investigated in a group of 5 human subjects and also in a computational voice production simulation. The simulation incorporated a parametric voice source model into a nonlinear source-filter framework. H1*-H2* and OQ were measured synchronously from audio recordings and high-speed laryngeal videoendoscopy of “glide” phonations in which quality varied continuously from breathy to pressed. Analyses of individual speakers showed large differences in the relationship between OQ and H1*-H2*. The variability in laryngeal high-speed data was consistent with simulation results, which showed that the relationship between OQ and H1*-H2* depended on mean glottal area, a parameter associated with the degree of source-filter interaction and not directly measurable from high-speed video of the vocal folds. In addition, H1*-H2* may change with increasing glottal gap size; this change contributes to the observed variability in the relationship between H1*-H2* and OQ.