ACOUSTIC COUPLING IN PHONATION AND ITS EFFECT ON ...

11. INTRODUCTIONThis chapter introduces the motivation for the proposed research, indicating theproblem and its significance. The objectives and hypotheses of the research andits expected contributions are stated. The structure of the thesis document is thendescribed.1.1 MotivationLinear source-filter theory, as applied to voice production [1], assumes that theglottal impedance is much larger than the acoustic input impedance of the vocal tractand subglottal system, with no interaction or coupling between components taken intoaccount. Different studies have elucidated the importance of the interaction, notingthat it affects glottal airflow introducing skewing and ripples in the open phase ofits pulses [2–5], which has been referred to as “true glottal source” [6–8]. Sourcefilterinteraction has also been related to the effects of incomplete glottal closure,which provides the basis of the current understanding of breathy and female voices.Indeed, it was shown that incomplete glottal closure is related to: changes in theharmonic decay of the source spectra and changes in the vocal tract formant andbandwidths [9–12], the introduction of turbulent noise [10,13], and fluctuations duringthe closed phase of the cycle [14,15].More recently, researchers have revised principles governing source-filter interaction[16,17] based on enhanced vocal tract geometries [18–22] that suggest the tracthas an impedance comparable or even larger than that of the glottis [23]. A recenttheory [17] proposed that irregularities, instabilities, and nonlinear phenomena invoice production can be highly affected or triggered by the acoustic coupling between