Fast amplitude compression in hearing aids improves audibility but degrades speech information transmission

Common types of hearing impairment are caused mainly by a loss of nearly instantaneous compressive amplification in the inner ear. Therefore, it seems plausible that the loss might be compensated by fast frequency-dependent compression in the hearing aid. We simulated impaired listeners´ auditory analysis of hearing-aid processed speech in noise using a functional auditory model. Using hidden Markov signal models, we estimated the mutual information between the phonetic structure of clean speech and the neural output from the auditory model, with fast and slow versions of hearing-aid compression. The long-term speech spectrum of amplified sound was identical in both systems, as specified individually by the widely accepted NAL prescription for the gain frequency response. The calculation showed clearly better speech-to-auditory information transmission with slow quasi-linear amplification than with fast hearing-aid compression, for speech in speech-shaped noise at signal-to-noise ratios ranging from -10 to +20 dB.