Voice instabilities due to source-tract interactions

Interactions of voice source and vocal tract resonances are of particular interest with respect to register transitions and bioacoustics. There is experimental evidence that singing into a tube generates nonlinear phenomena around matching frequencies of the pitch and the first formant. In the present study, we analyze a simplified two-mass model coupled to a straight tube. The essential parameters of the systems are varied systematically to localize instability regions in parameter space. The resulting bifurcation diagrams reveal subharmonic vibrations and deterministic chaos if the pitch and the first formant coincide. The computer simulations closely resemble published experimental observations. Furthermore, we demonstrate the role of coupling strength as the cause of instability. Small tube cross sectional areas and sufficiently large pressures are responsible for additional instability regions around second pitch harmonic and formant resonance. The results indicate that under certain circumstances source-tract interactions induce quite complex voice instabilities, even for symmetric vocal folds. These include frequency jumps, subharmonic regimes and chaos. The possible relevance for the soprano whistle voice and animal vocalization is discussed.