ACOUSTIC COUPLING IN PHONATION AND ITS EFFECT ON ...

17few high-order models are known to combine vocal fold tissue mechanics, laryngealaerodynamics, and acoustic interactions with the subglottal and supraglottal tracts[17, 107, 108]. Predictions using these models confirm the trends observed in lowordermodels. The effects of incomplete glottal on the source-filter coupling, energytransfer, and glottal airflow have not been explored with these higher-order models.2.2 Source-filter coupling in voice productionDue to the nonlinear nature of the glottal impedance, any source-filter interactionis a nonlinear phenomenon. A possibly “linear source-filter coupling” has beendescribed [17], where the glottal impedance is much higher than those of the vocaltract and subglottal system and the respective acoustic pressures have no influenceon the glottal flow or driving forces for the vocal fold oscillation. This classificationessentially describes a no-coupling scenario and thus will not be used in this study.It has been suggested that humans have the ability to operate their voices with andwithout source filter interactions by changing the degree of adduction of the vocalfolds and narrowing of the epilarynx tube [17]. Two possible types of source-filterinteractions have been proposed [17]: a “level 1” of interaction, one where only flowand sound interacts, and a “level 2” of interaction, where the tissue is now part of athree-way interaction that affects the tissue motion.2.2.1 Source-filter interactions: Level 1Early studies noted that source-filter interactions essentially affected the sourceproperties introducing skewing of the glottal pulse (a delay in its peak with respectto that of the glottal area) and ripples in the open phase [4,5,45,46,81]. Due to thiscoupling, the interactive airflow is also referred to as the “true glottal source” [6,7].Nonlinear coupling theory [17] summarizes these observations based on the relationbetween the tract impedances and the harmonic composition of the glottalsource, i.e., those harmonics located within the inertive portion of the total tract