Viscothermal acoustic waves in micro scale resonators

Accurate analytical air damping models for the transducers operating in the radio frequency range or for relatively large scaled devices like microphones are not available until now. Therefore the present paper aims at providing a full model wherein the compressibility, viscosity and heat conductivity of the air are considered. The solution of the model can be interpreted by “three wave theory” with coupled viscous, entropic and acoustic waves. Furthermore a self-developed FEM solver has been used to solve and simulate the full model equations. An aluminum nitride based piezoelectric resonator was fabricated and tested under controlled pressure from ultra high vacuum to normal atmosphere, using a custom-built vacuum chamber. The comparison of the air damping coefficient from the measurement and simulation confirms the accuracy of the full model prediction.