Simulation of the effects of loudness recruitment and threshold elevation on the intelligibility of speech in quiet and in a background of speech.

These experiments simulated the threshold elevation and loudness recruitment associated with three different types of cochlear hearing loss: Moderate flat (condition R2), severe flat (condition R3), and moderate-to-severe sloping (condition RX). This was done to allow an examination of the effects of these factors on the intelligibility of speech, in isolation from other factors that are normally associated with cochlear hearing loss, such as reduced frequency selectivity. The simulation was performed by splitting the input signal into 13 frequency bands, and processing the envelope in each band so as to create loudness sensations in a normal ear that would resemble those produced in an impaired ear with recruitment. The bands were then recombined. All tests were performed using subjects with normal hearing. For speech in quiet, simulation of hearing loss produced a reduction in the ability to understand low-level speech. However, speech at sufficiently high levels was highly intelligible in all conditions. Linear amplification according to the National Acoustic Laboratory (NAL) prescription gave high intelligibility for speech at normal conversational levels. For speech presented at a fixed input level of 65 dB SPL, against a background of a single competing talker, simulation of hearing loss produced substantial decrements in performance. The speech-to-background ratios in conditions R2 and RX had to be 11-13 dB higher than in the control condition (unprocessed stimuli) to achieve similar levels of performance. Linear amplification according to the NAL prescription improved performance markedly for the conditions simulating flat losses, but was less effective for the condition simulating a sloping loss. This indicates that threshold elevation combined with recruitment produces a loss of intelligibility for speech in the presence of a single competing talker that is only partly compensated by linear amplification of the type typically used in hearing aids.