Electronic enhancement of the directional response of a micromachined biomimetic optical microphone.

Abstract

A silicon micromachined biomimetic optical microphone has been recently demonstrated to have directional response and low noise [J. Acoust. Soc. Am. 125(4), 2013–2026 (2009)]. In this microphone, the motion of one end of the pivoting rectangular diaphragm is detected using an integrated optical interferometer and each end can be actuated using separate electrostatic actuators. This configuration enables one to selectively enhance either the directional, anti-symmetric rocking mode or the omni-directional symmetrical second vibrational mode of the diaphragm in two separate active feedback loops. A circuit model with two electrical ports to actuate each side of the diaphragm, and two acoustic ports to drive each mode has been developed to illustrate the electronic method. The model includes the effect of the air medium and the backside cavity through mechanical impedances which are verified through measurements. With the two-sided active feedback scheme, the model predicts significant improvements in the directional response resulting in more than 10-dB improvement in the residual intensity index when the microphone is used for gradient measurement in an intensity probe.