Synthesis of Subharmonic Voice with Kinematic Vocal Fold Model

Objective: Frequent subharmonic vocal fold vibration is a common sign of a voice disorder but is also occasionally present among those without voice concerns. This motivates investigation into how such irregular vibration translates to the radiated acoustic signal and to our perception. A useful tool for studying subharmonic vibration is the kinematic vocal fold model which is coupled aerodynamically and acoustically with the wave-reflection vocal tract model. This model grants a user with a fine-grained control over the irregular vocal fold motion while the nonlinear interaction of acoustic waves and aerodynamic quantities induces a realistic combination of subharmonic tones in the output acoustic signal. This presentation will describe the construction of the model and its application to relate the source modulation extent to the output subharmonics-to-harmonics ratio.

Methods: A kinematic vocal fold model [Titze, 1984. JASA 75(2), 570-580] is driven by purely sinusoidal lateral displacement function with varying amplitude and phase depending on the sagittal and vertical locations of the vocal fold surface. To introduce subharmonic motion to this model, the sinusoidal function is replaced by an amplitude- or frequency-modulated function. The frequency of the modulation signal is locked to an integer division of the fundamental frequency, thereby simulating the entrained subharmonic oscillation.

The subharmonics-to-noise ratio (SHR) was estimated from a 50-millisecond steady-state segment with varying modulation extents, subharmonic periods, and vowel vocal tract models. The amplitude-modulated source was assessed in a Monte Carlo experiment (100 trials each) with randomized initial phases and turbulent noise.

Results and Conclusions: Overall, logarithmic increase in the modulation extent yielded linear increase in the SHR in decibels. However, substantial SHR variation (>20 dB) was observed at a fixed extent. Some of the variability was explained by the vowels and initial modulation phase while the subharmonic period had a little effect on the SHR. The proposed model and SHR findings are valuable assets towards assessing acoustic analysis tools and parameters. The model can also be used to create audio stimuli for perceptual studies.