Acoustic comparison of vowel sounds among adult females.

INTRODUCTION/OBJECTIVE: This study consist of an experiment comparing acoustic characteristics of vowel production among females. The aim of this investigation was to explore the stability among vowels with quantification of acoustic changes in the voice related to speech production associated with an ample variety of vowel sounds. An additional goal was to establish a systematic control of variables and standardization of the data collection procedures.

METHODS

All data were collected in a quiet environment using the Computerized Speech Lab (CSL; Kay Elemetrics, Montvale, NJ), a computer-based system designed to measure characteristics of voice. Jitter and shimmer, measures of perturbation that reflect characteristics of voice, were applied. Two additional acoustic measures were examined: (1) noise-to-harmonic ratio (NHR), a general evaluation of presence of noise in the voice signal; and (2) voice turbulence index (VTI), related to turbulence caused by abnormal adduction of vocal folds. A systematic methodology of data collection was organized, in an effort to establish a research protocol based on relevant literature, involving (1) keeping constant fundamental frequency (F0) and intensity, (2) positioning of participants and recording equipment, and (3) environmental noise.

RESULTS AND CONCLUSION

When measured acoustic parameters of vowels were compared, results revealed that the vowel sounds had a significant effect on shimmer and VTI. Furthermore, speech sounds classified as back vowels exhibited less perturbation and noise in the acoustic signal: the high-back vowels [u] and []; and the mid-back vowels [o] and [], demonstrated most of the statistically significant reduced values of shimmer and VTI among the 12 vowels compared. Further comparisons among front and back vowels grouped in clusters associated to more and less variability led to statistically significant differences in shimmer, NHR, and VTI. Overall, speech sounds classified as back vowels exhibited less variability and noise. Based on the results of this study, back vowels of the English language should be used in voice acoustic tasks in females, given the higher stability of their acoustic signal, as compared with other vowels tested.