Considerations in Modern Multichannel Nonlinear Hearing Aids

The field of hearing aids has witnessed a significant evolution in the last decade. Prior to the 1980s, over 80% of hearing aids manufactured and dispensed were linear hearing aids with peak clipping as the only option for output limiting. Many of the nonlinear hearing aids were linear hearing aids with output compression limiting. The situation changed so that in the mid-1990s almost half of the hearing aids sold were nonlinear hearing aids (Mueller and Killion, 1996). Most of the nonlinear hearing aids were single channel linear hearing aids with compression limiting, or single channel wide dynamic range compression (WDRC) hearing aids. The sale of nonlinear hearing aids steadily increased in the last few years, with an estimate that over 75% of hearing aids sold incorporated some types of nonlinear signal processing. Of these hearing aids, threequarters were either programmable or true digital signal processing hearing aids (Strom, 2001). Many of these hearing aids are multichannel devices utilizing complex processing algorithms that extend the benefits of conventional nonlinear hearing aids. Such rapid change in technology has led to an almost inevitable consequence—our verification protocol has failed to keep pace with the advances such that many can neither be easily evaluated nor fairly verified without special considerations. The interpretation that one makes with the results of verification may also be unclear. Failure to properly verify a nonlinear hearing aid could compromise its potential benefit. Thus, this chapter focuses on the changes in current nonlinear hearing aids, explains how they may extend the benefits provided by conventional devices, and discusses how our current practices on verification may need to be updated to reflect the changes in technology. Although the concept and characterization of compression are reviewed, readers are referred to summary articles for a detailed discussion on the rationale, classification, fitting, and efficacy of conventional nonlinear hearing aids (Braida et al, 1979; Walker and Dillon, 1982; Kuk, 1996, 2000; Dillon, 1996.