Spectral feature enhancement for people with sensorineural hearing impairment: Effects on speech intelligibility and quality

Abstract
People with sensorineural hearing loss often have difficulty understanding speech in background noise at speech-to-noise ratios (0 to +6 dB) for which normally hearing people have little difficulty. Spectral analysis of speech in noise at these ratios typically shows that the major spectral prominences in the speech (formants) are well represented, but the spectral valleys between the formants are filled with noise. Hearing impaired people have a reduced ability to pick out the spectral prominences, and are more affected by the noise filling in the valleys, partly because of their reduced frequency selectivity. This paper describes a 16-channel bandpass filter bank, implemented in analog electronics, that attempts to enhance spectral features of speech in noise to improve intelligibility for the hearing impaired. Each channel generates an 'activity function' that is proportional to the magnitude of the signal envelope in that channel, averaged over a short period of time. A positively weighted activity function from the nth channel is combined with negatively weighted functions from channels n-2, n-1, n+1, and n+2, giving a correction signal used to control the gain of the bandpass signal in the nth channel. Recombining the bandpass signals results in an enhancement of spectral features of the speech in noise. Two different experiments are described here, one using the activity function as described, and the other using a non-linear transform of the activity function. In both experiments, several different weighting patterns were used in calculating the correction signal. The intelligibility of speech in noise processed by the system was measured for subjects with moderate sensorineural hearing loss. In both experiments, no improvement in intelligibility was found. However, subjective ratings of the stimuli used in Experiment 2 indicated that some subjects judged the processed stimuli to have both higher quality and higher intelligibility than unprocessed stimuli.

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