A feed forward adaptive canceller to reduce the occlusion effect in hearing aids

Hearing aids are essential devices for social integration of hearing impaired people in order to improve their auditory perception. Recent studies have reported significant dissatisfaction factors that tend to reduce their daily use. The occlusion effect is one important source of complaints. This phenomenon stems from the partial or complete closure of the ventilation opening of the ear-mould, usually performed to prevent feedback effects in high-gain devices. This work presents a new adaptive active-noise-control system to reduce the occlusion effect in small- or unvented hearing aids. In contrast to previously developed occlusion-effect cancellers, this system offers a feedforward cancelling structure that permits the analysis of its behaviour as a finite-impulse-response linear-filter identification problem. Deterministic recursive equations were derived with the aim to theoretically predict its mean square error and mean coefficient behaviour, both in transient and steady state conditions. Such models are of particular interest to hearing aid designers as guide tools for setting parameters to obtain a desired performance. Computational simulations accurately agree with theoretical predictions obtained by the derived equations, indicating a mean reduction of 5.4dB of the occlusion effect in the range of 200-500Hz. Subjective experiments with the use of a real prototype corroborate the functionality of the proposed architecture. No perceptual side effects regarding high-frequency amplifications of the original sounds were reported by volunteers.