Effects of vowel context and discriminability on band independence in nonsense syllable recognition

The Speech Intelligibility Index algorithm [(1997). ANSI S3.5-1997] models cues in disjoint frequency bands for consonants and vowels as additive, independent contributions to intelligibility. Data from other studies examining only consonants in single-vowel nonsense stimuli exhibit synergetic and redundant band contributions that challenge the band independence assumption. The present study tested the hypotheses that (a) band independence is present for multi-vowel stimuli, and (b) dependent band contributions are artifacts of confounding stimulus administration and testing methods. Data were measured in two experiments in which subjects identified filtered nonsense consonant-vowel-consonant syllables using a variety of randomly selected vowels. The measured data were used in simulations that further characterized the range of subject responses. Results of testing and simulation suggest that, where present, band independence is fostered by low broadband error, high vowel diversity, and high vowel discriminability. Synergistic band contributions were observed for confusable vowels that were most susceptible to filtering; redundant contributions were observed for the least susceptible vowels. Implications for intelligibility prediction and enhancement are discussed.