Fitting WDRC Devices: Which Rationale and How Many Channels?

Abstract

Wide Dynamic Range Compression (WDRC) hearing aids are believed to be the best remedy for recruitment, which is experienced by most hearing impaired people. There are at least two theories about how WDRC devices should be fitted. One theory is to Normalize Loudness in some way (e.g. LGOB, FIG6, IHAFF, DSL[I/o], and ScalAdapt), and another is to Maximize Speech Intelligibility (e.g. NAL-NL1). In some cases, the two rationales result in very different prescriptions. For example, Loudness Normalization prescribes significantly more low frequency gain than NAL-NL1 for people with flatter losses, and for people with steeply sloping losses Loudness Normalization prescribes more high frequency gain than NAL-NL1. It is currently uncertain which rationale is best for listening to the range of input levels that is presented in our everyday lives. WDRC hearing aids are now commonly available with up to 20 channels in which gain and compression can be independently adjusted. In theory, multi-channel WDRC devices have at least two advantages over single channel devices: 1) The variation of a person’s audible range can be better matched across frequencies, and 2) The SNR can be improved in situations where the noise is dominant in a restricted range of frequencies. The superiority of multi-channel compression over single-channel compression is, however, still controversial. Current evidence is mainly based on Loudness Normalization procedures. The NAL-NL1 procedure prescribes compression ratios lower than those prescribed by Loudness Normalization. High compression ratios are known to adversely affect intelligibility in multi-channel compression. This poster presents data from a study that investigated whether hearing impaired people 1) prefer the rationale of Normalizing Loudness or Maximizing Speech Intelligibility, and 2) benefit from one, two, or four channels of compression when prescribed according to the NAL-NL1 procedure.