Aim The aim of this study was to determine how deterioration of voice quality, such as breathiness, may impact on the intelligibility of speech.
Method Acoustic analysis was conducted on sustained vowel phonation (/i/ and /a/) and sentences produced by voice disordered speakers. Measures included: frequency and amplitude of the first two formants (F1, F2), singing power ratio (SPR), the amplitude difference between the first two harmonics (H1-H2), voice onset time (VOT), and energy ratio between consonant and vowel (CV energy ratio). A series of two-way (glottal closure by vowel) mixed design between and within-subjects Analysis of Variances conducted on these acoustic measures showed a significant glottal closure (complete and incomplete) or glottal closure by vowel interaction effect on the F2 frequency, H1-H2 amplitude difference, and singing power ratio. Based on findings in literature that reported a dominant first harmonic as a useful predictor of breathiness, the measure of H1-H2 amplitude difference was selected as a factor for investigation of the impact of voice quality on the perception of vowel intelligibility and clarity. Fixed-length vowel segments at five levels of H1-H2 amplitude difference were presented to 10 male and 10 female inexperienced listeners between the ages of 19 and 34 years.
Results It was expected that the tokens with a dominant first harmonic, indicative of a more breathy voice, would be associated with a lower rate of correct vowel identification and of being perceived as “clearer”. Although no linear relationship between breathiness and intelligibility was revealed, results indicated the presence of thresholds of intelligibility for particular vowels whereby once a level of breathiness was reached intelligibility would decline. Conclusion The finding of a change of the perceptual ratings as a function of the H1-H2 amplitude difference, identified in previous studies as a measure of breathiness, revealed thresholds of intelligibility for particular vowels below which breathiness would be tolerated with little impact on intelligibility but beyond which intelligibility ratings suffered markedly.