ACOUSTIC COUPLING IN PHONATION AND ITS EFFECT ON ...

212.3 Bifurcations in voice productionVoice instabilities and bifurcations, also referred to as voice breaks, have beena long-standing topic of interest in speech science. Even though these singularitiesare encountered in different circumstances, including the male voice at puberty [117],singing [16,50], and many different voice pathologies [118], they are not completelyunderstood. In the following sections, the main aspects behind voice bifurcations aredescribed with the aim of differentiating the role that source-filter coupling may playin this phenomenon.Irregular vocal fold vibration and voice bifurcations are generally considered tobe produced by desynchronization between vibratory modes [103, 119, 120], strongasymmetries between left and right vocal folds and/or excessively high subglottalpressure [92,93], changes in vocal fold tension [121–123], chaotic behavior near limitcycles between registers [124], and nonlinear acoustic coupling [17,49,63,125,126].Titze [49] proposes differentiating the origin for voice bifurcation between sourceinducedand acoustically-induced. This classification was adopted in this thesis research,noting that it is unlikely that the two can be truly separated as unique factorsleading to the unstable behavior. Even though it is likely that a combination of componentscontribute in all cases, it is considered that “source” or “acoustic” ones maybe more dominant in certain cases.2.3.1 Source-induced bifurcationsObservations of unintentional register transitions have been consistently reportedin the frequency range of 150 Hz - 200 Hz in males and 300 Hz - 350 Hz in bothmales and females [127]. Different studies have suggested that instabilities may occurgiven a physiologic limit in the maximum active stress in the thyroarytenoid(TA) muscle, which controls the medial surface shape of the glottis and thus itsmain modes of vibration [121, 122, 127]. In this context, a bifurcation explanationfrom the theory of nonlinear dynamics was proposed to justify jumps occurred when