Nonlinear source–filter coupling in phonation: Vocal exercises

TLDR

Nonlinear source–filter coupling has been shown in simulations, excised larynx experiments, and physical models, yet it has not been consistently demonstrated in natural human phonations. This study aimed to quantify the proportion of source instabilities attributable to nonlinear source–tract coupling and hypothesized that vocal fold vibration is most destabilized when fundamental frequency crosses the first formant, causing a dramatic acoustic load change. Eighteen adult subjects performed three vocal exercises combining various fundamental frequency and formant glides, with expected manifestations of source–filter interaction including sudden frequency jumps, subharmonic generation, or chaotic vocal fold vibrations coinciding with F0–F1 crossovers. Results showed that bifurcations occur more frequently when F0 crosses F1, indicating that nonlinear source–filter coupling partly drives source instabilities, and that male subjects exhibit more bifurcations in such phonations, likely due to less exposure to these crossovers in normal speech. A companion paper provides the theoretical underpinnings.

Abstract

Nonlinear source–filter coupling has been demonstrated in computer simulations, in excised larynx experiments, and in physical models, but not in a consistent and unequivocal way in natural human phonations. Eighteen subjects (nine adult males and nine adult females) performed three vocal exercises that represented a combination of various fundamental frequency and formant glides. The goal of this study was to pinpoint the proportion of source instabilities that are due to nonlinear source–tract coupling. It was hypothesized that vocal fold vibration is maximally destabilized when F0 crosses F1, where the acoustic load changes dramatically. A companion paper provides the theoretical underpinnings. Expected manifestations of a source–filter interaction were sudden frequency jumps, subharmonic generation, or chaotic vocal fold vibrations that coincide with F0–F1 crossovers. Results indicated that the bifurcations occur more often in phonations with F0–F1 crossovers, suggesting that nonlinear source–filter coupling is partly responsible for source instabilities. Furthermore it was observed that male subjects show more bifurcations in phonations with F0–F1 crossovers, presumably because in normal speech they are less likely to encounter these crossovers as much as females and hence have less practice in suppressing unwanted instabilities.