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 contr ast, using subjects with cochlear hearing loss. The enhancement was ca rried out on a frequency scale related to the equivalent rectangular b andwidths (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) o f 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 exper iment 2, subjects judged the relative quality and intelligibility of s peech in noise processed using a subset of the conditions of experimen t 1. Generally, processing with a moderate degree of enhancement was p referred over the control condition, for both quality and intelligibil ity. Subjects varied in their preferences for high degrees of enhancem ent. Experiment 3 used a modified processing algorithm, with a moderat e degree of spectral enhancement, and examined the effects of combinin g the enhancement with dynamic range compression. The intelligibility of speech in noise improved with practice, and, after a small amount o f practice, scores for the condition combining enhancement with a mode rate degree of compression were found to be significantly higher than for the control condition. Experiment 4 used a subset of conditions fr om experiment 3, but performance was assessed using a sentence verific ation test that measured both intelligibility and response times. Scor es on both measures were improved by spectral enhancement, and improve d still more by enhancement combined with compression. The effects wer e statistically more robust for the response times. When expressed as equivalent changes in speech-to-noise ratio, the improvements were abo ut twice as large for the response times as for the intelligibility sc ores. The overall effect of spectral enhancement combined with compres sion was equivalent to an improvement of speech-to-noise ratio by 4.2 dB.