Hearing Aid Gain for Loudness-Density Normalization in Cochlear Hearing Losses with Impaired Frequency Resolution

Abstract

A theoretical criterion for preliminary prescription of hearing aid gain and frequency response was formulated on the basis of Zwicker's loudness model, modified for impaired auditory frequency resolution in cochlear hearing losses. The procedure was designed to restore normal relative loudness contributions from each frequency band of input speech. Prescribed frequency responses had less steep bass cut than recommended by either the Prescription of Gain/Output or the National Acoustic Laboratories' procedures, regardless of various model assumptions. Prescribed overall gain depended on the loudness growth assumed in the model. In cases where auditory filters are wider than normal and loudness recruitment is complete, the procedure yielded a nonlinear relation between hearing aid gain and hearing loss: Required insertion gain (in dB) was 25 to 30% of hearing threshold loss (in dB HL) for mild to moderate losses, but this percentage increased for more severe impairments. For cases with incomplete loudness recruitment, the model prescribed that insertion gain should be 40 to 50% of hearing threshold loss for a wide range of impairments.