Intelligibility of Vowels Altered in Duration and Frequency

This study was conducted to examine the differential effects upon vowel intelligibility of various degrees of time compression (TC) and frequency division (FD), with and without time restoration. A male speaker and a female speaker were used. For a given percentage of distortion, FD degrades vowel intelligibility more severely than TC. Restoring time to normal for FD speech produced by slow playing via the Fairbanks speech compressor does not enhance intelligibility. Vowel confusions under TC are related to duration; those for FD conditions appear to be closely related to the perception of Vowel Formant 2 and to a lesser degree, Vowel Formant 1. Patterns of male and female vowel confusions are generally much alike for all conditions and types of distortion. Results tentatively indicate superior female vowel intelligibility under all conditions of distortion, the advantage being largest for FD, somewhat less for TC. These results suggest that over a limited range of frequency division (up to 40%), vowel phonemic quality is relatively unaffected by proportionate shifting of fundamental frequency and formant structure, indicating that a “relative-vowel” hypothesis of vowel phonemic quality may hold for limited shifts in the frequency of vowel spectra.