MEMS Acoustic Direction Finding Sensor

Sunday, 15 February 2015
Exhibit Hall (San Jose Convention Center)
Daniel Wilmott, Naval Postgraduate School, Monterey, CA
A micro-electromechanical (MEMS) based acoustic direction finding (DF) sensor has been developed to locate sound sources. This family of sensors is intended to improve upon the size, ease of use, signal to noise ratio and applicability of current time difference of arrival acoustic direction finding technologies. These MEMS sensors find their inspiration in the aural system of the Ormia ochracea fly, a unique organism that possesses ears that are very close together yet are capable of precise direction finding.  The fly’s ears are mechanically coupled, allowing the phase difference of an incoming sound to oscillate the eardrum system with relatively large amplitudes at the normal modes of the system.  The fly’s ears are developed to resonantly detect chirps from crickets for laying their larvae on them.  The MEMS sensor developed consists of two ‘wings’ (approx. 1mm x 1mm, 25 mm thick) that are connected by a central beam and the entire structure is anchored to the substrate using two torsional legs at the middle.  The sensor was fabricated using a silicon-on-insulator wafer and trenched from the bottom in a way that allows for both rocking and bending of the wings about the central point, analogous to the Ormia ochracea. The amplitude of sensor movement in respond to sound is captured using comb finger capacitors attached to the edges of the wings that feed amplified voltage readout proportional to the displacement of the sensor wings from their equilibrium position. The measured output was found to be a maximum at normal incidence and follows a cosine response as the sound source moves at an angle to the sensor. The direction sensing ability of the sensor was modeled using COMSOL finite element modeling software and found to agree well with the experimental observations. The observed cosine dependence was attributed the interaction of sound from both top and bottom sides of the wings.  The sensor is able to locate sound sources within few degrees though the dimensions of the sensor are nearly an order of magnitude smaller than the wavelength of sound. Further work is in progress to co-locate two identical sensors so that their outputs can be processed together to remove the requirement of knowing the incident sound pressure level. This will lead to the ability of DF sensing with millimeter-sized MEMS sensors.