Vocal Exercises to determine Nonlinear Source-Filter Interaction

Ingo R. Titze, Tobias Riede, Peter Popolo

Nonlinear source-filter interaction has been demonstrated in computer simulations, in excised larynx experiments, and in physical models, but not in a consistent and unequivocal way in natural phonations. A promising way to demonstrate nonlinear interactions is to drive the system toward suddenly changing or unusual acoustic loading conditions by the vocal tract and observe source instabilities. An example would be to move the source fundamental frequency (F0) or a prominent harmonic through a formant. But instabilities can be purely source generated and not the result of any nonlinear interaction. The goal of this study was to pinpoint the proportion of source instabilities that are due to nonlinear source-tract interactions. We hypothesized that F0 — F1 crossing changes the acoustic load dramatically and that it maximally destabilizes vocal fold vibration and therefore increases the rate of instabilities in the vocal output. Eighteen subjects (9 adult males and 9 adult females) performed three vocal exercises that represented a combination of various fundamental frequency and formant glides. Expected manifestations of a source-filter interaction were sudden frequency jumps, subharmonic generation, or chaotic vocal fold vibration that coincides with F0 — F1 crossovers. Results indicated that the bifurcation occur more often in phonations with F0 — F1 crossovers, suggesting that nonlinear source-filter interaction are 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 don’t encounter these crossovers as much as females and hence have less practice in suppressing unwanted instabilities.

Ingo R. Titze, Ph.D.1,2 Tobias Riede, Ph.D.2 Peter Popolo, MS1.2
The University of Iowa1
Hawkins Drive
Iowa City, IA 52242 USA
National Center for Voice and Speech2
The Denver Center for the Performing Arts
1101 13th Street
Denver, CO 80204 USA
1+ 303-893-6487 FAX 1+ 303-893-6024
ititze@dcpa.org