Evaluation of an ITD-to-ILD Transformation as a Method to Restore the Spatial Benefit in Speech Intelligibility in Hearing Impaired Listeners

TL;DR

This study explores transforming low-frequency ITDs into ILDs to restore binaural benefits and improve speech intelligibility in hearing-impaired listeners, showing promising results for hearing aid applications.

Contribution

It introduces a novel ITD-to-ILD transformation method that enhances speech understanding for hearing-impaired individuals by compensating for reduced ITD sensitivity.

Findings

Transforming low-frequency ITDs into ILDs improves speech reception thresholds.

Removing ITDs decreases speech intelligibility slightly, but substitution can enhance it.

Adding ILDs while preserving ITDs benefits speech understanding across all directions.

Abstract

To improve speech intelligibility in complex everyday situations, the human auditory system partially relies on Interaural Time Differences (ITDs) and Interaural Level Differences (ILDs). However, hearing impaired (HI) listeners often exhibit limited sensitivity to ITDs, resulting in decreased speech intelligibility performance. This study aimed to investigate whether transforming low-frequency ITDs into ILDs could reintroduce a binaural benefit for HI listeners. We conducted two experiments with HI listeners. The first experiment used binaurally phase-shifted sinusoids at different frequencies to evaluate the HI listeners ITD sensitivity threshold. All subjects had an increased ITD threshold at higher frequencies, with different ITD sensitivities between the subjects in the lower frequencies. In the second experiment, Speech Reception Thresholds (SRTs) were measured in different binaural configurations by manipulating Head-Related Transfer Functions (HRTFs). The results showed that, despite the decreased ITD sensitivity, removing ITDs decreased SRTs by approximately 1 dB compared to the unprocessed baseline, where ITDs and ILDs are available. Furthermore, substituting low-frequency ITDs with ILDs yielded an improvement for a lateral target speaker. Adding the low-frequency ILDs while preserving the ITDs caused a significant improvement for speakers in all directions. These findings suggest that the proposed transformation method could be effective in restoring binaural benefits in HI listeners. The results of this study suggest the use of such transformation techniques to be implemented in hearing aids and cochlear implants, directly benefiting HI listeners.