MEMS Directional Underwater Acoustic Sensor Operating in Near Neutral Buoyancy Configuration

This paper reports on the functionality of a MEMS directional acoustic sensor housed in an air cavity and operated underwater in a near-neutral buoyancy configuration. The sensor operates at a resonance which provides enhanced sensitivity over traditional hydrophones and demonstrates a cosine-like angular dependance with respect to the arrival angle of incident sound. Rapid prototyping techniques were used to design and build a neutrally buoyant housing for achieving optimal sensitivity and directional response. In this neutrally buoyant configuration, the MEMS sensor acts as an accelerometer measuring the vibration of the housing in response to sound rather than acoustic pressure directly. Frequency and directional response data of the MEMS sensor were taken in a water filled, vertical, standing wave tube. The data show a maximum sensitivity of approximately 210 mV/Pa at the resonance frequency of the sensor (676 Hz) with sound normally incident to the plane of the sensor. 360-degree rotations of the sensor show a cosine-like angular dependance of the sensor. The results indicate that this type of MEMS sensor can be operated in a near-neutral buoyant configuration while achieving high sensitivity and good directional response, making this sensor design ideal for detecting and determining the direction of distant or quiet underwater acoustic sources.