Spectral contrast enhancement of speech in noise for listeners with sensorineural hearing impairment: effects on intelligibility, quality, and response times.

This paper describes a series of experiments evaluating the effects of digital processing of speech in noise so as to enhance spectral contrast, using subjects with cochlear hearing loss. The enhancement was carried out on a frequency scale related to the equivalent rectangular bandwidths (ERBs) of auditory filters in normally hearing subjects. The aim was to enhance major spectral prominences without enhancing fine-grain spectral features that would not be resolved by a normal ear. In experiment 1, the amount of enhancement and the bandwidth (in ERBs) of the enhancement processing were systematically varied. Large amounts of enhancement produced decreases in the intelligibility of speech in noise. Performance for moderate degrees of enhancement was generally similar to that for the control conditions, possibly because subjects did not have sufficient experience with the processed speech. In experiment 2, subjects judged the relative quality and intelligibility of speech in noise processed using a subset of the conditions of experiment 1. Generally, processing with a moderate degree of enhancement was preferred over the control condition, for both quality and intelligibility. Subjects varied in their preferences for high degrees of enhancement. Experiment 3 used a modified processing algorithm, with a moderate degree of spectral enhancement, and examined the effects of combining the enhancement with dynamic range compression. The intelligibility of speech in noise improved with practice, and, after a small amount of practice, scores for the condition combining enhancement with a moderate degree of compression were found to be significantly higher than for the control condition. Experiment 4 used a subset of conditions from experiment 3, but performance was assessed using a sentence verification test that measured both intelligibility and response times. Scores on both measures were improved by spectral enhancement, and improved still more by enhancement combined with compression. The effects were statistically more robust for the response times. When expressed as equivalent changes in speech-to-noise ratio, the improvements were about twice as large for the response times as for the intelligibility scores. The overall effect of spectral enhancement combined with compression was equivalent to an improvement of speech-to-noise ratio by 4.2 dB.