Nonlinear behavior of vocal fold vibration: The role of coupling between the vocal folds

Coupling between the vocal folds is one of the nonlinear mechanisms allowin g regulation and synchronization of mucosal vibration. The purpose of this study was to establish that modulations such as diplophonia and abnormaliti es observed in vocal signals that may be observed in some cases of laryngea l pathology can be considered as nonlinear behavior due to the persistence of some physical interaction (coupling). An experimental model using excise d porcine larynx was designed to create tension asymmetry between the vocal folds and to obtain vocal signals with modulations. Signals were analyzed by spectral analysis and the phase portrait method. Results were compared w ith computer-generated synthetic signals corresponding to nonlinear combina tions of sinusoid signals. Under these conditions, evidence of nonlinear be havior was detected in 85% of experimental signals. These findings were int erpreted as a demonstration of vocal fold interaction. Based on these findi ngs, the authors conclude that (1) coupling must be taken into account in p hysical models of laryngeal physiology, and that (2) methods of nonlinear d ynamics may be used for objective voice analysis.