Frequency and amplitude perturbation in the voices of female speakers.

Vocal jitter and shimmer during the sustained phonation of /a/, /i/, and /u/ were investigated for twenty adult female speakers using an automatic analysis program. The results showed overall average shimmer of 0.25 dB and overall average jitter of 0.84%. The critical value for shimmer was 0.48 dB. Significant differences between males and females were found.