Acoustic-perceptual relationships in variants of clear speech

Objective: The acoustic basis of intelligibility associated with varied clear speech instructions was studied. Methods: Twelve healthy speakers read 18 sentences in 'habitual', 'clear', 'hearing impaired' and 'overenunciate' conditions. The latter 3 conditions are varieties of clear speech. Acoustic measures included tense and lax vowel space area, a measure of vowel spectral change, articulation rate and sentence-level vocal intensity. Sentences were mixed with multitalker babble to prevent ceiling effects and were orthographically transcribed by 40 listeners. Percent-correct scores were obtained for each speaker and condition. Regression analyses were used to quantify relationships between acoustic measures and intelligibility. Results: Univariate regressions indicated that greater magnitudes of acoustic change in nonhabitual conditions were associated with greater increases in intelligibility. Multivariate regression analysis further indicated that lax vowel space, articulation rate and vocal intensity were significant predictors of intelligibility. Conclusions: Acoustic variables associated with intelligibility differed depending on whether relationships were examined using univariate or multivariate statistics. Multivariate statistics indicated that articulation rate was the strongest predictor of improvements in intelligibility above and beyond all other variables studied. The findings have implications for optimizing therapeutic use of clear speech for clinical populations.