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Impact of the vent size in the feedback-path and occlusion-effect in hearing aids

Occlusion-effect and acoustic feedback are common complaints of the hearing aid user. The occlusion-effect is described as an annoying quality of the user's own voice that sounds hollow or boomy, while feedback instability results in an unpleasant loud continuous tone. Despite the availability of high performance feedback cancellers, severe to profound losses require large amplification and, as a result, certain degree of occlusion to minimize feedback. The smaller the vent, the greater is the feedback-path attenuation as well the increase in the occlusion-effect. This work presents a method to measure paired transfer functions for the feedback-path and occlusion-system. Such results, not presented in literature before, relate concomitantly the feedback-path and occlusion-system characteristics with the vent diameter. Results indicate power variations of up to 10 dB for both feedback-path and occlusion-system in the 0 to 3mm vent diameter range. Such a result should be taken into consideration in the design of feedback cancellers as well in fitting procedures.