A novel microelectromechanical system (MEMS) design for an underwater acoustic field sensor

Abstract

A concept for underwater acoustic imaging exploits current technology for constructing MEMS devices from silicon wafers. Anisotropic etching creates frustum-shaped (pyramids with tops cut off) holes in a wafer chip; the bonding of two such chips creates double frustum-shaped holes, each analogous to an hour glass. The front side of each hole is a tapered duct with a baffled open end. The acoustic disturbance generated by an incident plane wave magnifies as the flow converges toward the waist. The back half of the double frustum is covered by a third silicon layer and acts as a Helmholtz resonator, the waist being the neck. The manufacturing process places a thin membrane across the waist which oscillates sychronously with the acoustically induced fluid motion. An appropriate design can achieve fluid velocities at the neck much larger than what would be associated with the incident acoustic wave in free space, and exploratory experiments confirm this. The oscillating membrane is sensed by an optical system, and theoretical extrapolation of the measured displacement amplitude yields a measurement of the amplitude of the incident acoustic wave. [Work supported by Indian Head Divison, Naval Surface Warfare Center, as part of the DARPA Sonoelectronics Program.]