Effect of size on the thermal noise and acoustic response of viscous-driven microbeams

A study is presented of the thermal-mechanical noise and response to sound of microphones that are designed to be driven by the viscous forces in air rather than by sound pressure. Virtually all existing microphone designs are intended to respond to sound pressure. The structures examined here consist of thin, micro-scale, cantilever beams. The viscous forces that drive the beams are proportional to the relative velocity between the beams and fluid medium. The beams' movement in response to sound is similar to that of the air in a plane acoustic wave. The thermal-mechanical noise of these beams is found to be a very weak function of their width and length; the size of the sensing structure does not appear to significantly affect the performance. This differs from the well-known importance of the size of a pressure-sensing microphone in determining the pressure-referred noise floor. Creating microphones that sense fluid motion rather than pressure could enable a significant reduction in the size of the sensing element. Calculated results are revealed to be in excellent agreement with the measured pressure-referred thermal noise.