The effect of intensity perturbations on speech intelligibility for normal-hearing and hearing-impaired listeners

Hearing-impaired listeners are known to suffer from reduced speech intelligibility in noise, even if sounds are above their hearing thresholds. This study examined the possible contribution of reduced acuity of intensity coding to this problem. The "distortion-sensitivity model" was used: the effect of reduced acuity of auditory intensity coding on intelligibility was mimicked by an artificial distortion of the speech intensity coding, and the sensitivity to this distortion for hearing-impaired listeners was compared with that for normal-hearing listeners. Stimuli (speech plus noise) were wavelet coded using a Gaussian wavelet (1/4-octave bandwidth). The intensity coding was distorted by multiplying the modulus of each wavelet coefficient by a random factor. Speech-reception thresholds (SRTs) were measured for various degrees of intensity perturbation. Hearing-impaired listeners were classified as suffering from suprathreshold deficits if intelligibility of undistorted speech was worse than predicted from audibility by the speech intelligibility index model [ANSI, ANSI S3.5-1997 (1997)]. Hearing-impaired listeners without suprathreshold deficits were as sensitive to the intensity distortion as the normal-hearing listeners. Hearing-impaired listeners with suprathreshold deficits appeared to be less sensitive. Results indicate that reduced acuity of auditory intensity coding may be a factor underlying reduced speech intelligibility in noise for the hearing impaired.