Effect of the vocal tract yielding sidewall on inverse filter analysis of the glottal waveform

The inverse filter is a serial cascade of filter elements with a transfer function that cancels the effect of the poles of the vocal tract transfer function on the acoustic waveform to reveal the underlying glottal volume velocity waveform. Inaccuracies in the glottal wave reconstruction derived from an all-zero inverse filter can be attributed to deviations of the vocal tract transfer function from an all-pole model. Presented is an analysis of the error stemming from the effect of the yielding vocal tract sidewalls on the vocal tract transfer function. Predictions about the resulting artifacts in the estimated glottal volume velocity are derived from an acoustic model. These predictions are confirmed by applying a linear predictive coding (LPC) inverse filter analysis method to vowels synthesized using a transmission line model of the vocal tract containing yielding sidewall parameters as well as natural productions of nonnasalized vowels.