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Benefits of directional microphones and noise reduction circuits for improving Baha® hearing performance

Introduction Recently, implantable bone conduction systems (e.g. Baha, Cochlear Bone Anchored Solutions AB, Molnlykke, Sweden) have undergone a number of critical improvements with both sound processor and implant technology. A number of authors have reported on the improvement in speech understanding achievable from the latest Baha 3 system sound processors (Kompis and Pfiffner, 2010; Davison et al., 2009). Key to these improvements was the utilization of multiple channel non-linear sound processing technology in which the sound signal is processed across multiple frequency channels and the amplification is applied non-linearly to match the dynamic range of the Baha sound processor. Specifically, it has been found that directional microphones provide a hearing performance benefit for wearers of Baha (Oeding et al., 2010; Kompis et al., 2007). Lately, directional microphones have become both automatic and adaptive. Automatic in that the sound processor makes the decision when the microphone mode should be in omni or in directional mode. Adaptive in that the polar response of the microphones can be adapted to provide the optimum reduction in noise from behind the listener. These newer systems may provide new benefits to the patient and should be evaluated. Stenfelt (2007) reported, as the Baha is worn posterior to the ear canal, that the listener will actually hear better from behind than in front. Adjusting the prioritization of the two microphones balances the microphone response so that the listener’s sound scape is not distorted by the abutment position. In order to improve sound quality, noise reduction systems can be implemented that adjust the amplification based on the amount of noise present in each channel. Once noise becomes sufficiently loud to cause annoyance, the amount of amplification in that channel can be tempered to improve comfort. Therefore, the latest generation of Baha sound processors put into service a number of advanced signal processing technologies aimed to improve speech understanding and overall sound quality. However, questions have been raised whether these technical improvements will translate to a clinically relevant improvement in hearing performance. The purpose of this study is to investigate these systems and determine the degree of potential benefit.