Importance of sentence-level and phoneme-level envelope modulations during vowels in interrupted speech

Speech interrupted by noise has been used as a simplified case for listening to speech in the presence of a fluctuating masker. The present study investigated the importance of overall vowel amplitude and intrinsic vowel amplitude modulation to sentence intelligibility. Eight young normal-hearing listeners participated in the experiment. Sentences were processed according to three conditions that replaced vowel segments with noise matched to the long-term average speech spectrum. Vowels were replaced with (1) low-level noise that distorted the overall sentence envelope, (2) segment-level noise that restored the overall syllabic amplitude modulation of the sentence, and (3) segment-modulated noise that further restored faster temporal envelope modulations during the vowel. Results demonstrated incremental benefit with increasing resolution of the temporal envelope. An additional seven listeners participated in a separate experiment that replaced vowels with a vowel masker instead of noise. The vowel masker was modified to have a flattened fundamental frequency at the mean of the replaced segment. The vowel masker either had a standard temporal envelope or was modulated by the envelope of replaced vowel. The listeners with the vowel maskers performed more poorly than those with the noise maskers. No benefit of segment modulation was observed with the vowel masker.