Coherent modulation enhancement: Improving performance in noise for hearing aids and cochlear implants

Abstract

Difficulty hearing in noise is a pervasive problem for hearing-impaired listeners. One approach is to digitally reduce noise using a modulation filter, which can selectively modify the time envelope of selected spectral bands. Previous approaches to modulation filtering, such as those based upon a Hilbert transform magnitude, increased signal distortion. A new coherent approach was used to determine a fixed and an optimal adaptive modulation filter. The optimal adaptive filter used information from the target speech to design the modulation filter. Threshold signal-to-noise ratio was measured adaptively using a spondee-in-noise task [Turner et al., J. Acoust. Soc. Am. 115, 1729 (2004)]. Subjects selected the spondee heard in a forced-choice task with a two-talker babble background. The spondee level was fixed at the listeners most comfortable level and noise level adjusted adaptively using a 2 dB step size, with threshold based on 10 reversals. Sensorineural hearing-impaired listeners completed three conditions: unprocessed speech; fixed modulation filter; and optimal adaptive modulation filter. Normal-hearing subjects heard a parallel set of conditions processed to simulate a 6-channel cochlear implant. There was significant improvement in threshold signal-to-noise ratio with the optimal adaptive filter for both groups. There was no improvement for the fixed modulation filter relative to unprocessed speech.