David A. Berry Ingo R. Titze
Natl. Ctr. for Voice and Speech, Dept. of Speech Pathol. and Audiol., Univ. of Iowa, Iowa City, IA 52242
Hanspeter Herzel
Humboldt University, O-1040 Berlin, Germany
Because the vocal folds are nonlinear systems with many degrees of freedom, bifurcations and chaos should be expected for certain parameter configurations. Indeed, bifurcations and chaos appear in some of the simplest models of vocal fold vibration. They are also seen in more complex models, in models that incorporate left--right asymmetries, and in biomechanical simulations of vocal fold oscillations. Furthermore, an acoustical analysis of many types of rough voice (e.g., creaky voice, vocal fry, and newborn infant cries) reveals an intimate relationship between voice mechanics and bifurcations and chaos. Likely physiological mechanisms of the observed nonlinear phenomena are presented. Basic concepts and analysis techniques from nonlinear dynamics are reviewed and compared with traditional voice analysis schemes. Finally, it is shown how much of the terminology in the literature describing rough voice can be unified within the framework of nonlinear dynamics. [This research was supported by Grant. No. P60 DC00976 from the National Institute on Deafness and Other Communication Disorders.]