Required number of tokens to determine representative voice perturbation values.

Acoustic perturbation analyses of prolonged vowels are used in determining phonatory stability characteristics. When a number of tokens are analyzed, the average perturbation values create a stability profile of the voice. How many tokens are needed to obtain a representative perturbation value? In this study, five perturbation measures were considered, namely, jitter, shimmer, harmonics-to-noise ratio, coefficient of variation for amplitude, and coefficient of variation for frequency. Subject groups were chosen on the basis of individual average perturbation values. Results indicate that, except for the harmonics-to-noise ratio, generally, the less stable the voice, the greater is the number of tokens needed to obtain representative averages. For highly stable voices, at least six tokens are suggested: for voices with normal to high levels of instability, at least 15 tokens are recommended. Regardless of vocal stability, at least 10 tokens are suggested for the harmonics-to-noise ratio measure.