Improved acoustic measurement has been fundamental to advances in the speech and hearing sciences. Accurate characterization of the electroacoustic performance of hearing aids is basic to understanding the relation between human performance and hearing aid design. Definition of the speech signal is fundamental to understanding the relation between speech perception and the acoustics of speech. Unfortunately, presently available measurement techniques and tools fail to meet the needs of today's researchers and clinicians. For example, systems to measure the electroacoustic performance of hearing aids were designed for quality control purposes with conventional hearing aids, and they fail to predict how hearing aids perform when worn by real listeners in everyday life. One reason is that many modern hearing aids are specifically designed to alter their performance when the input signal changes. Performance under restricted test conditions is therefore a poor predictor of performance under the wider range of listening conditions encountered in daily living. In order to address this situation, we have initiated a multiyear project to develop advanced hearing aid testing systems.
Research is also currently underway to verify modeling processes that we have developed for hearing aid applications and to develop measures of audio distortion that are closely related to listeners' perceptions of sound quality.
Aspects of this research have been supported by the ORTC, NSERC, Starkey Labs, Etymonic Design Incorporated. (EDI) and the Canadian Foundation for Innovation.
Recent advances in hearing aid technology have permitted the performance delivered by an individual's hearing aid to be much more closely matched to the desired amplification characteristics. Such advancements have made the development and refinement of clinical protocols for hearing aid selection more important than ever. At the same time, advances in hearing aid design have increased the range of electroacoustic variables that must be specified in a hearing aid prescription. Much of the work undertaken in our Child Amplification Laboratory and our Amplification Systems Laboratory have been directed towards refinement of the hearing aid selection and fitting process.
Research has focused on ways to improve the process of: assessing candidates for a hearing aid; developing hearing aid prescriptions; communicating details of the desired hearing aid specification between audiologist, manufacturer and hearing aid dispenser; and assessing the adequacy of the hearing instrument ultimately provided, in meeting the needs of the intended hearing aid user.
This work has involved a series of studies to identify sources of error and variation in this process, to devise clinically-feasible protocols to reduce and eliminate such errors, and to systematize and expedite the process. The work has resulted in a series of methodological and evaluation reports.
In order to facilitate the application of methods developed at HHCRU, we have developed the Desired Sensation Level (DSL) computer software packages. Version 3.1 of the DSL software has been implemented in DOS and MacIntosh formats, and facilitates the selection and fitting of linear gain instruments (Seewald, et al., 1993). The DSL 3.1 software has received wide acclaim; a recent review rated DSL as "excellent in every aspect of review from program design to user interface to customer support" and the "most frequently used software program in our audiology clinic, after our word processing and hearing aid management software" (Ear & Hearing, 1995, p. 239).
Recently, we have extended the DSL approach to the selection and fitting of wide-dynamic-range compression (WDRC) hearing aids, which are now widely available. Our theoretical approach to such fittings was described in a recent series of reports (Cornelisse et al., 1994; 1995). This approach, called DSL[i/o], is suitable for use with either linear gain or WDRC instruments, and has been implemented as a Windows-compatible software package. The most recent version, DSL v4.1 for Windows, was released in April, 1997 (Seewald et al, 1997).
DSL software has been implemented on the Audioscan RM500 real ear measurement/hearing aid analyser. Frye Electronics has developed a software based link between the Fonix 6500 and DSL v4.1, and will soon release a DSL implementation in their FP-40 real ear/hearing aid analyser. Several manufacturers of hearing instruments have also implemented DSL into their programmable systems.
Aspects of this project have been supported by a variety of sources, including Audio Scan/Etymonic Design Incorporated, the Ontario Rehabilitation Technology Consortium (ORTC), the Ontario Ministry of Health, PHONAK, Starkey Labs Canada and Unitron Industries Ltd.