Characterization and Prediction of Speech Intelligibility at the Output of Hearing Aids in a Noisy Working Environment.

Objective: Hearing aids are more and more technically advanced, but do not necessarily guarantee the reproduction of useful signals in all working environments. This is particularly the case for speech intelligibility. This study focuses on the prediction of hearing aid performance in the case of a moderate deafness setting, in service and industrial work environments. To improve intelligibility, hearing aids propose signal processing options such as noise reduction and compression. These processes can transform hearing aids into nonlinear systems. The aim of this study is to develop a nonlinear method for the characterization of hearing aids.

Materials and methods: The method is based on the synchronized swept sine (SSS) signal method.^[16] The SSS method is applied for determining hearing aid frequency responses fitted according to the present methodology and several processing options. The characterization of hearing aid's program containing the noise reduction function is specifically analyzed. Indeed, to be fully active and efficient, the hearing aid, with the noise reduction feature activated, needs to be immersed in a noisy environment which does not allow nonlinear characterization. A linear approach is taken to study this feature. Three hearing aids commonly sold by hearing care professionals are studied here; all of them have three different programs. The characterization for each program is discussed.

Results: The statistical study showed that the intelligibility, assessed using the speech transmission index in these sound environments, is well estimated for every program, although certain differences are observed when the compression effect is too high in the service work sector.

Conclusion: The characterizations of hearing aids using the programs studied did not highlight the presence of frequency nonlinearities. The characterization method could not take into account amplitude nonlinearities when there is too much gain compression in the hearing process. Globally, all the hearing aid programs provided a very significant improvement in intelligibility in service and industrial work contexts.