Evaluation of two speech enhancement techniques to improve intelligibility for hearing-impaired adults

This study reports the effects on speech intelligibility of two types of digital speech processing: amplitude enhancement of consonants to produce near-zero consonant/vowel intensity ratios and increased duration of consonants to provide an additional 30 ms of sound. Compensatory vowel shortening accompanied duration increases to maintain original overall duration. One hundred words from the California Consonant Test (CCT) list were recorded. The target consonant in each word was processed in four ways to produce tape recordings of (a) unenhanced speech, (b) speech with increased consonant amplitude, (c) speech with increased consonant duration, and (d) speech with increased consonant amplitude and duration. The tapes were played to a group of 20 listeners with moderate sensorineural hearing loss at 65 dB SPL and to another group of 10 such listeners at 95 dB SPL. Results indicated that amplitude processing was associated with 10% to 12% improvement in intelligibility at the lower level but failed to yield any significant effect at the high level of presentation. Increasing consonant duration, by contrast, provided no benefit at the low level but gave modest benefit (5%) at 95 dB. Further development of the speech-processing techniques is necessary before they can be incorporated into a useful hearing aid.