Effects of Compression on Speech Acoustics, Intelligibility, and Sound Quality

Abstract

time, use of compression hearing aids has increased dramatically, from half of hearing aids dispensed only 5 years ago to four out of five hearing aids dispensed today (Strom, 2002b). Most of today's digital and digitally programmable hearing aids are compression devices (Strom, 2002a). It is probable that within a few years, very few patients will be fit with linear hearing aids. Furthermore, compression has increased in complexity, with greater numbers of parameters under the clinician's control. Ideally, these changes will translate to greater flexibility and precision in fitting and selection. However, they also increase the need for information about the effects of compression amplification on speech perception and speech quality. As evidenced by the large number of sessions at professional conferences on fitting compression hearing aids, clinicians continue to have questions about compression technology and when and how it should be used. How does compression work? Who are the best candidates for this technology? How should adjustable parameters be set to provide optimal speech recognition? What effect will compression have on speech quality? These and other questions continue to drive our interest in this technology. This article reviews the effects of compression on the speech signal and the implications for speech intelligibility, quality, and design of clinical procedures.Categorizing Compression tensity vowels such as /i/, and from whispered speech to shouting, the benefit of a linear hearing With a linear hearing aid, a constant gain is ap-aid is restricted when the amplification needed to plied to all input levels until the hearing aid's sat-make low-intensity sounds audible amplifies uration limit is reached. Because daily speech in-high-intensity sounds to the point of discomfort. cludes such a wide range of intensity levels, from In other words, linear hearing aids have a limited low-intensity consonants such as /f/ to high-in-capacity to maximize audibility across a range of 131 From the

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