Inverse filtering for the analysis of vocal function
Martin Rothenberg · The Journal of the Acoustical Society of America · 1989
The waveform of the volume velocity of the airflow through the larynx, as obtained by inverse filtering the volume velocity of the airflow exiting the mouth and nose, is a potential convenient noninvasive measure of the vibratory pattern of the vocal folds during voiced speech. Of special interest is a measure that is sufficiently robust for clinical use in the detection, monitoring, and treatment of voice disorders. The problems in this application are of three types: (1) The parameters of the inverse filter must be set to match the acoustic transfer function of the vocal tract; (2) glottal airflow is, in some way, not an accurate indicator of vocal fold motion; and (3) the analysis must be accurate under a wide range of normal and disordered voice qualities and possible nasal coupling. Because of these problems, it has been difficult to find a robust and accurate automatic procedure, even if the analysis is restricted to only the simplest case of open vowels. However, there appears to be a rather straightforward, although unconventional, solution to this inverse-filtering problem if (1) the vocal fold vibratory parameters of most interest are properly defined, (2) physiological restrictions on the possible vibratory patterns are taken into account, and (3) the nonlinear interaction between the glottal source and the vocal tract acoustics is used in the analysis procedure.