Compression? Yes, but for low or high frequencies, for low or high intensities, and with what response times?

Several rationales for using compression in hearing aids are outlined. These rationales comprise discomfort avoidance, loudness normalization, noise reduction, short term signal dynamic range reduction, empirically determined compression, and long-term signal dynamic range reduction. The compression systems needed to implement each of these differ greatly, and these differences can be viewed as differences in the frequency range undergoing most compression, the intensity range undergoing most compression, and the speed at which the compressor(s) operate. A classification system along these lines is introduced and examples of currently available hearing aids falling into each category are given. The effects of each type of compression on speech intelligibility is investigated via a review of published research. The results of this indicate that, for speech in quiet at a comfortable level, no compression scheme yet tested offers better intelligibility than individually selected linear amplification. If input level is then decreased and the aid wearer is prevented from adjusting the volume control, many types of compression provide intelligibility superior to that available from linear amplification. In broadband noise, only one system, containing wideband compression followed by fast acting high-frequency compression, has so far been shown to provide significant intelligibility advantages.