Modeling the performance of MEMS based directional microphones
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A Micro Electro Mechanical System (MEMS) based directional microphone consisting of two plates hinged at the center is modeled using finite element software. A new method is developed in which the sensor is acoustically coupled to an incoming sound wave. The method successfully reproduces results of previous non-acoustic coupled simulations for solid plates. The resonance frequencies match within 0.8% for the rocking mode and 2% for the bending mode. The displacement amplitudes match within 17% for the rocking mode and 5% for the bending mode. After ensuring agreement with previous simulations, the model was extended to include more realistic boundary conditions. The sound pressure at the back of the plates is included along with the drag force on the plates due to the acoustic particle velocity flow. This new model reproduces the experimentally achieved resonance frequency values within 21% for the rocking mode and 2% for the bending mode. The displacement amplitude obtained for the rocking mode is approximately 6 times lower than the experimental value while the bending mode amplitude is 47% higher. Manufacturing tolerances for these MEMS devices likely contribute to the discrepancy between simulated and experimental values. A novel design is proposed for increasing the displacement amplitude for both solid and perforated plates through the use of a Helmholtz resonator.
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