Equalization filter design for achieving acoustic transparency in a semi-open fit hearing device

Acoustically transparent hearing devices should allow hearing equivalent to the open ear while providing the possibility to modify the sound reaching the eardrum in a desired manner. To this end, the output of the device is processed by means of an equalization filter, such that the superposition of the sound played back by the device and an acoustic sound component directly leaking into the ear canal approximates the transfer function to the open eardrum. A particular difficulty in designing the equalization filter is the occurrence of comb filtering effects due to a superposition of the direct sound and the delayed output of the device. Here, we propose a regularized least-squares design approach with a closed-form solution that takes into account individually measured transfer functions of the device and ear, as well as the processing delay. Experimental results utilizing measured transfer functions from a custom prototype device show good equalization performance, particularly a reduction of comb-filtering effects as a result of an automated frequency-dependent regularization.