ACOUSTIC COUPLING IN PHONATION AND ITS EFFECT ON ...

19known as non-modal phonation. Non-modal phonation is observed in different degreesin normal and pathological subjects, and it is associated with different acoustic characteristics,such as breathy voice, irregular vibrations, and failure to phonate [110].The problem of breathiness was initially explored using a Norton equivalent of thevolume velocity source with a parallel glottal impedance [9]. It has been shown thatincomplete glottal closure can be related with a clear decay of harmonic energy inthe source spectra and an increase of the formant’s bandwidth [10–12], as well asthe introduction of turbulent noise [10,12,13]. A number of descriptors to identifypossible non-modal behavior from microphone recordings have been proposed [12].The effects of incomplete closure and posterior glottal opening on the glottal pulseswere studied [14,15] using previously described aerodynamic equations and geometricconsiderations [46]. These investigations provide the most insightful review on theeffects of subglottal coupling on the closed phase portion of the glottal airflow pulse.The studies showed that strong fluctuations could be present during the closed portionof the cycle and described the structure and origin of them. The acoustic couplingbetween the subglottal and supraglottal tracts is also a consequence of finite glottalimpedances. Subsequent studies on this topic illustrated that the acoustic couplingbetween the vocal tract and the subglottal system results in the introduction of polezeropairs in the vocal tract transfer function that correspond to resonances of theuncoupled subglottal tract [13,69,90,91]. Whether this relation holds in the oppositedirection, i.e., how the resonances of the vocal tract affect the subglottal ones, hasnot been explored.2.2.2 Source-filter interactions: Level 2In this second type of interaction, the subglottal and supraglottal systems canaffect the dynamics of vocal fold vibration. The general conditions where the tractimpedances favor or hamper the oscillation have been described in terms of the seriessuperposition of the tract reactances [17]. Even though a complete discussion of the