Active sound field control for sound reproduction in open ear canals

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Open-fitting hearing systems have proven to be efficient solutions for the rehabilitation of mild to moderate hearing loss. In comparison to closed-fitting systems, they are in particular much better accepted by the end user, most notably because they avoid the so-called occlusion effect. On the other hand, open-fitting hearing systems are technically challenging, because the effectively open ear canal makes them sensitive to noise penetrating from the Outside and to acoustic feedback from the receiver to the microphones. The current (passive) approach to optimally balance the suppression of occlusion, external noise and feedback is to carefully choose an appropriate vent size in combination with feedback cancellation algorithms. Alternatively, the upcoming availability of ear canal microphones in commercial hearing aids makes it possible to employ active approaches to tackle these different problems, and in addition derive an optimal individualized equalization of the sound pressure at the ear drum.The long-term objective of this project is to optimally exploit the availability of microphones and receivers in the ear canal or in the vent to develop improved individualized approaches for sound pressure equalization at the ear drum, for feedback cancellation and for noise reduction in open-fitting hearing devices.In the initial funding period, the focus was on individualized equalization, active noise reduction and individualized feedback cancellation for a hearing aid device equipped with one receiver and one ear canal microphone, where these problems were treated independently of each other and we considered time-invariant scenarios for the internal sound field. In the second funding period, we aim to extend the developed approaches to 1) dynamic scenarios for the internal sound field caused by changes ofthe ear canal geometry, and 2) hearing aid devices equipped with multiple receivers and microphones in the ear canal or in the vent, enabling to target feedback cancellation and equalization of the sound pressure at the ear drum simultaneously. This comprises both an adaptation of the individualized ear canal and feedback path models developed during the initial funding period, as well as the development of novel multi-loudspeaker-multi-microphone signal processing algorithms and acoustic models for combined feedback cancellation and equalization. In addition, measurement and evaluation procedures for dynamically varying feedback paths and acoustic ear canal impedances and real-time demonstrators of the individualized signal processing algorithms will be developed.Hence, this project contributes to all cardinal problems addressed by the Research Unit, namely the presentation problem using local approaches (feedback cancellation and equalization algorithms), the individualization problem (individualized ear canal and feedback path models), and the noise reduction problem.