Envelope enhancement for improving hearing in reverberant spaces

Cochlear implants are neural prostheses which give a sense of hearing to deaf people by stimulating the auditory nerve. In many users they restore the ability to understand speech in quiet, but noise and reverberation cause severe problems. It seems that implant users have difficulties hearing out one source in a potpourri of sources. This so called “auditory scene analysis” relies, amongst other cues, on binaural information which is degraded with implants. Sound localization ability, directly dependent on access to binaural cues, is highly sensitive to the presence of noise or reverberation [1, 2]. These studies have shown that noise and reverberation affects localization ability of cochlear implant users already at a signal-to-noise ratio (or direct-to-reverberant ratio) about 6-12 dB more positive than that found with normal hearing listeners. Here we focus on the coding of binaural information to improve the perception of sound direction – an important aspect for locating and understanding speakers. Several ways exist to improve the coding of binaural cues in reverberation in cochlear implants. Most past approaches have attempted to reduce the energy of the interfering reverberation or noise. Our novel approach instead aims to increase the perceptual saliency of the cues used to locate sounds in reverberation. Studies of sound localization and the precedence effect have shown that information in the sound onset is important for correct localization in reverberant space [3, 4, 5, 6]. Onset information is transmitted in the modulated envelope of the electrode signals in cochlear implants. However, due to the bandbass filtering and compression stages in the implant, modulation depth and onset gradient inherent in envelope fluctuations are reduced – both are important factors for transmitting interaural time difference cues (ITDs) in the envelope [3].