ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

5pSC33. A qualitative study of mechanisms of jitter-induced shimmer in the voice.

Darrell Wong

WJ Gould Voice Res. Ctr., Denver Ctr. for the Performing Arts, 1245 Champa St., Denver, CO 80204

Robert Lange

WJ Gould Voice Res. Ctr.

Ingo R. Titze

Univ. of Iowa

Chwen Geng Guo

WJ Gould Voice Res. Ctr.

Measurements of jitter and shimmer are typically applied to microphone signals to discern the level of perturbations in the oscillatory behavior of the vocal folds. Shimmer, the average cycle-to-cycle change in amplitude, is known to linearly increase as the extent of amplitude modulation of tissue displacement increases. Behavior under frequency modulation (FM), however, is nonlinear. This nonlinearity is often attributed to time aliasing, the superposition of jittered impulse responses (assuming the source-filter model of phonation). As a consequence, whenever jitter is present, any measurement of shimmer cannot be solely attributed to amplitude modulation of the vocal fold tissue. The present study attempts to characterize other sources of FM-induced shimmer by examining points earlier in the voice production process. An interactive computer simulation of the vocal fold and vocal tract system is used under conditions of FM1/2 subharmonic modulation of the tissue displacement. It appears that, in addition to time aliasing, shimmer may be generated from FM-induced perturbations in mucosal wave propagation and tissue displacement. The paper qualitatively describes and illustrates these jitter-induced shimmer mechanisms.