Perturbation Measurements on the Degree of Naturalness of Synthesized Vowels.

OBJECTIVE: To determine the impact of jitter and shimmer on the degree of naturalness perception of synthesized vowels produced by acoustical simulation with glottal pulses (GP) and with solid model of the vocal tract (SMVT).

STUDY DESIGN: Prospective study.

METHODS: Synthesized vowels were produced in three steps: 1. Eighty GP were developed (20 with jitter, 20 with shimmer, 20 with jitter+shimmer, 20 without perturbation); 2. A SMVT was produced based on magnetic resonance imaging (MRI) from a woman during phonation-/ε/ and using rapid prototyping technology; 3. Acoustic simulations were performed to obtain eighty synthesized vowels-/ε /. Two experiments were performed. First Experiment: three judges rated 120 vowels (20 humans+80 synthesized+20% repetition) as "human" or "synthesized". Second Experiment: twenty PowerPoint slide sequences were created. Each slide had 4 synthesized vowels produced with the four perturbation condition. Evaluators were asked to rate the vowels from the most natural to the most artificial.

RESULTS: First Experiment: all the human vowels were classified as human; 27 out of eighty synthesized vowels were rated as human, 15 of those were produced with jitter+shimmer, 10 with jitter, 2 without perturbation and none with shimmer. Second Experiment: Vowels produced with jitter+shimmer were considered as the most natural. Vowels with shimmer and without perturbation were considered as the most artificial.

CONCLUSIONS: The association of jitter and shimmer increased the degree of naturalness of synthesized vowels. Acoustic simulations performed with GP and using SMVT demonstrated a possible method to test the effect of the perturbation measurements on synthesized voices.