Cross-frequency interactions in band importance functions

The Speech Intelligibility Index (SII) is a metric of the amount of information available in a degraded or masked speech signal. The SII is used to predict speech recognition outcomes and is part of hearing aid prescription formulae. A critical assumption in the calculation of the SII is that frequency bands contribute independently to speech recognition, i.e., the importance of a band does not change based on the context of speech cues in other bands. Prior work has challenged this assumption by demonstrating that pairs of bands can contain synergistic or redundant information. The present work extends these findings by directly measuring pairwise interactions between the 21 frequency bands defined by the Critical Band Procedure of the SII. Forty-one participants with normal hearing identified words filtered to contain pseudorandom combinations of four or five bands. Pairwise interactions indicated both synergy and redundancy and accounted for substantial variability in recognition accuracy. The importance of individual bands decreased when pairwise interactions were considered, with the largest decreases for frequency bands above 1 kHz. The spectral proximity and envelope correlation between pairs of bands predicted whether their combination was synergistic or redundant. Interactions between bands play a critical role in speech recognition.