Effects of vocal fold asymmetry on pressure wave fluctuations in a mechanical model

In this paper, we numerically simulate speech production on the basis of vocal fold (VF) models with geometrical (effective VF vibrational depth, glottal half gap and VF thickness) or mechanical (Young's modulus, density and viscosity) asymmetries as a pathological VF model, and consider the effects of the asymmetries on the speech production process. The simulation shows asymmetric vibrations with a phase difference between the left and right VFs and fluctuations in the pressure wave within the larynx. To investigate the relationship between the fluctuations and VF asymmetry, we quantitatively estimate the fundamental frequency, amplitude and waveform fluctuations in the pressure wave by varying the asymmetry. For most cases, increasing the VF asymmetry increases the fluctuations. The fluctuations obtained from the simulation for symmetric models are in rough agreement with those of actual speech signals. However, with increasing asymmetry, the fluctuations exceed the range for actual speech data. This result suggests that the degree of VF asymmetry can be evaluated by estimating fluctuations in speech waves.