The Formant Bandwidth as a Measure of Vowel Intelligibility in Dysphonic Speech

The current paper examined the impact of dysphonia on the bandwidth of the first two formants of vowels, and the relationship between the formant bandwidth and vowel intelligibility.

Speaker participants of the study were 10 adult females with healthy voice and 10 adult females with dysphonic voice. Eleven vowels in American English were recorded in /h/-vowel-/d/ format. The vowels were presented to 10 native speakers of American English with normal hearing, who were asked to select a vowel they heard from a list of /h/-vowel-/d/ words. The vowels were acoustically analyzed to measure the bandwidth of the first and second formants (B1 and B2). Separate Wilcoxon rank sum tests were conducted for each vowel for normal and dysphonic speech because the differences in B1 and B2 were found to not be normally distributed. Spearman correlation tests were conducted to evaluate the association between the difference in formant bandwidths and vowel intelligibility between the healthy and dysphonic speakers.

B1 was significantly greater in dysphonic vowels for seven of the eleven vowels, and lesser for only one of the vowels. There was no statistically significant difference in B2 between the normal and dysphonic vowels, except for the vowel /i/. The difference in B1 between normal and dysphonic vowels strongly predicted the intelligibility difference.

Dysphonia significantly affects B1, and the difference in B1 may serve as an acoustic marker for the intelligibility reduction in dysphonic vowels. This acoustic-perceptual relationship should be confirmed by a larger-scale study in the future.