On the limited transfer of information with noise-induced hearing loss

Assessment of hearing handicap is frequently based on hearing loss in the frequency region from 0.5 to 2.0 kHz. The region above 2 kHz, however, is important for hearing, particularly when one considers speech perception in conditions with ambient noise. The present study, based on 200 individuals (400 ears) with noise-induced hearing loss, shows that the average value of hearing loss at 2 and 4 kHz is the best simple predictor of the speech-reception threshold measured in noisy conditions. Speech perception is still impaired when the speech level and the level of the interfering noise exceed the hearing loss. This implies that calculations of speech intelligibility based on the articulation-index procedure are invalid. In these calculations it is assumed that only the speech-to-noise level distance (and not the extent of the hearing loss) determines the speech threshold when both the signal and the noise level exceed the hearing loss. An alternative model may be based on the assumption that the frequency channels in the region of the lesion are completely inoperative and that pure tones in this frequency region are detected because excitation spreads toward a region with normal sensitivity. Experimental verification and predictions based on this model show that this assumption is too rigorous. Frequency channels in the region of the lesion do become activated at high stimulus levels, but their contribution to speech perception deteriorates.