Force feedback microelectromechanical microphones for high performance applications

Microelectromechanical System (MEMS) condenser microphones are widely used because of their low cost, small size, high sensitivity, and wide bandwidth. For certain specialist applications, however, they are still out-performed by the best conventional condenser microphones, which have greater bandwidth and dynamic range, but at higher cost and larger size. The sensitivity, and hence, signal-to-noise ratio of smaller MEMS microphones can be increased by using two perforated back-plates instead of one. The maximum amplitude is limited by membrane excursion, which leads to nonlinearity and, ultimately, failure. The use of force feedback holds the promise of avoiding these problems by holding the membrane at its equilibrium position while measuring the force required to do so. Previous attempts to accomplish this using a Sigma-Delta modulator have had only limited success in terms of signal-to-noise ratio, bandwidth and stability. Instead we propose to use an Electro-Mechanical Phase Locked Loop (EMPLL) to overcome these limitations. We will present lumped-parameter and Finite Element models of the performance of such a microphone, and discuss the challenges associated with its fabrication. [This work was supported by Roke Manor Research Limited.