Development of a tissue-conducting audio transducer and sensor for mobile use
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The ever increasing number of mobile devices with a cellular link as well as services associated with them require innovations in audio technologies. Especially problematic are circumstances, in which high background noise level prohibits communication. This thesis studies a tissue-conducting device for audio reproduction and recording. The proposed concept is not based on producing or sensing pressure changes in air, but in soft tissues. The device considered is an in-ear actuator and sensor that couples to tympanic canal walls. A finite element model of ear is developed for simulating the actuator function. The FEmodel includes a novel idea of using a lumped parameter representation for the middle ear bones. The results are compared with respect to the published data and the approach is found valid. The simulations concerning the actuator function show that the mode is unfeasible due to the energy loss in soft tissues. The result is confirmed by subjective tests. The prototype of the actuator is analyzed with a FE-model. It is observed that the linear FEM cannot account for the observed characteristics in the actuator response. Therefore, a time-domain model accounting for hysteresis is developed. The hysteresis prediction is realized with a rate-independent Preisach model with an addition of a scalar product model for the reversible part of hysteresis. It is shown that the rate-independent Preisach model is not sufficient to predict the response and that a dynamic model is required. In the sensor mode the device works up to 2.5-3 kHz, after which the recorded signal drops below the noise floor. The finding is supported by the literature. The transfer function between the speech recorded with a microphone and the device is observed to have a decreasing trend. The study leaves open, whether this effect is due to the tissue transfer characteristics, sensor coupling to the tissue or sensor properties. Moreover, a comprehensive discussion on the theory associated with the Finite Element Method is given. Both, structural and piezoelectric FEM are covered.