Compression Amplification in Hearing Aids.

In this tutorial, the characteristics of compression amplification in analog hearing aids and the experimental results obtained with single- and multichannel compression amplification systems are reviewed. Single-channel compression systems are classified into four broad groups on the basis of their static and dynamic characteristics: compression limiters, syllabic compressors, automatic volume control (AVC) systems, and "others," those with adaptive recovery time (aRT) and dual frontend automatic gain control (AGC) (Moore, 1990; Moore & Glasberg, 1988). Multichannel compression devices have, to date, used a variety of different types of compression, with syllabic compression in each frequency channel being the most popular. Experimental evidence suggests that compression limiting is generally superior to peak clipping as a means of controlling output, except for some individuals with severe to profound hearing loss. There is some evidence that syllabic compression and AVC systems enhance speech perception in quiet compared to conventional linear amplification; however, this improvement is not consistently maintained in the presence of background noise. The majority of recent research has focused on multichannel compression, and favorable results have been obtained with some systems. Research findings about the efficacy of using compression amplification for people with hearing loss have been extremely variable, and possible reasons for this are discussed.