Perception and Acoustic Studies of Vowel Intelligibility in Dysphonic Speech.

Purpose: This study aimed to (1) examine the effect of dysphonia and background noise on the identifiability of vowels, and (2) evaluate the relationship between the degree of vowel intelligibility and vowel-related acoustic measurements.

Method: A speech perception experiment was conducted with speech samples collected from 10 adult females with healthy voices and 10 adult females with dysphonia. The speech material was 11 vowels of American English in /h/-vowel-/d/ format. Cafeteria noise was added to these samples at a signal-to-noise ratio of -6 dB. A total of 10 adults with normal hearing participated in a speech perception experiment, in which the vowels were presented with and without the noise. F1 and F2 frequencies of the vowels were measured, and their relationships with the vowel intelligibility were statistically evaluated.

Results: The group-level analysis showed that vowel intelligibility was lower in dysphonic speech than normal speech, both in quiet and at signal-to-noise ratio of -6 dB. The intelligibility was higher for the high vowels than the low vowels. In general, the vowel confusion pattern was similar between normal and dysphonic speech. However, data from a speaker with severe diplophonia showed a distinct confusion pattern. Voice quality ratings did not significantly correlate with the vowel intelligibility. There was a significant correlation between F2 and the vowel intelligibility in quiet. A post-hoc acoustic analysis revealed that vowels of the speakers with lower vowel intelligibility had lower acoustic energy above 1 kHz.

Conclusion: Dysphonia negatively affects vowel intelligibility. Low vowels were more vulnerable to the effect of dysphonia than high vowels. Among different types of dysphonic voice qualities, diplophonia appears to be particularly detrimental to vowel intelligibility. F2 significantly correlated with vowel intelligibility; however, this result requires a careful interpretation. Given that the acoustic energy above 1 kHz resulted in better intelligibility in noise, a treatment strategy that targets this frequency region may help improve intelligibility in noise. Future studies should examine the generalizability of this finding to different age and gender groups, and intelligibility as a whole.