Design considerations for a contactless electromagnetic transducer for implantable hearing aids

Totally or partially implantable hearing aids may overcome many drawbacks of current conventional hearing aids. The expected advantages of these implantable aids include higher sound fidelity, reduced feedback, and better cosmetic appearance. The output transducer is the single most important component of an implantable hearing aid. In this work, an electromagnetic transducer design consisting of a permanent magnet and a coil separated by a short air gap is investigated. When compared to other, more complex transducers, this contactless design offers several advantages including the absence of bias forces and a reduced risk of malfunction due to wear and time. However, the design of these transducers is demanding due to a lack of simple general analytic approximations to predict the generated force. Using a combination of theoretical considerations and computer simulations, a set of design rules for this type of electromagnetic transducer is developed. It is shown that in order to obtain an energy efficient transducer, the coil should be as short as possible (i.e. 1 mm or less), and the coil diameter should be optimized according to the intended gap width between coil and permanent magnet. Approximate design rules are given.