Effects of vocal fold vibration on the turbulent sound generation in voiced sibilant fricatives

Voiced fricatives involve complex aerodynamic interactions between turbulent noise generation in the oral cavity and vocal fold vibration. In this study, aeroacoustic simulations combining a simplified vocal tract model of the alveolar fricative /s/ with a two-mass vocal fold model were conducted to investigate the effects of vocal fold vibration and tongue motion on fricative sound production. The compressible Navier-Stokes equations were solved for a vocal tract model with the tongue dynamically lowered from the fricative configuration to the vowel /u/ configuration. Comparisons between vibrating and non-vibrating conditions showed that vocal fold vibration reduced high-frequency fricative noise above 4 kHz by approximately 5 dB under identical subglottal pressure. Flow field revealed reduced turbulent velocity fluctuations around the teeth in the vibrating condition, indicating that part of the aerodynamic energy was transferred to sustaining vocal fold oscillation. During tongue lowering, the glottal airflow rate increased more gradually in the vibrating condition than in the non-vibrating condition. Consequently, the fricative noise decayed earlier in the voiced condition, suggesting that vocal fold vibration facilitates an earlier transition from fricative noise to vowel production, potentially contributing to smoother consonant–vowel transitions in voiced fricatives.