Design of a Biomimetic Directional Microphone Diaphragm

A miniature silicon condenser microphone diaphragm has been designed that exhibits good predicted directionality, sensitivity, and reliability. The design was based on the structure of a fly’s ear (Ormia ochracea) that has highly directional hearing through mechanical coupling of the eardrums. The diaphragm that is 1mm x 2mm x 20 microns is intended to be fabricated out of polysilicon through microelectromechanical micromachining. It was designed through the finite-element method in ANSYS in order to build the necessary mode shapes and frequencies into the mechanical behavior of the design. Through postprocessing of the ANSYS data, the diaphragm’s response to an arbitrary sound source, sensitivity, robustness, and Articulation Index Directivity Index (AI-DI) were predicted. The design should yield a sensitivity as high as 100 mV/Pa, an AI-DI of 4.764 with Directivity Index as high as 6 between 1.5 and 5 kHz. The diaphragm structure was predicted be able to withstand a sound pressure level of 151.74 dB. The sound level that would result in collapse of the capacitive sensor is 129.9 dB.. The equivalent sound level due to the self-noise of the microphone is predicted to be 30.8 dBA. INTRODUCTION Silicon microfabrication technology shows significant potential for enabling the development of novel sensors for sound and vibration. Many of the recent efforts at employing this technology for microphone design have focussed on the fabrication of small, non-directional microphone diaphragms made of silicon and incorporating capacitive sensing [1,2,3]. Often these small microphones have been paired together to create a directional microphone, but have experienced performance problems with this method [4,5,6]. While silicon technology has potential advantages over previous methods of fabricating existing microphone designs, it also provides the possibility of realizing radically new design concepts. In the present study, we present a design of a miniature silicon microphone diaphragm that takes advantage of the ability to fabricate complicated, patterned membranes to achieve directional acoustic response. The directional microphone design described here is inspired by our previous efforts at understanding the mechanics of directional hearing in small animals such as insects. Many small animals depend on the ability to localize sound sources either for predator avoidance or mate selection. All animals having two tympanal ears localize sound by processing interaural differences either in the time of arrival or level of the acoustic pressure. When the size of the animal is very small relative to the sound wavelength, these interaural differences can be too small to permit accurate processing by the central nervous system to allow sound source localization. Certain small animals that depend on localizing sound sources have evolved either air-borne or structure-borne connections between the tympanal membranes, or eardrums. By suitable coupling of the motions of the tympana, it is possible for each to respond preferentially to sound from certain directions. Our analysis of the coupled ears of the parasitoid fly, Ormia ochracea, (Order: Diptera, Family: Tachinidae, subfamily: ormiine) has inspired a novel approach to constructing small, directional sound receivers [7-10]. The mechanically coupled ears of Ormia ochracea have been shown to respond predominantly in two resonant modes of vibration, one in which both tympana move in opposite directions and one in which they both move in-phase [7]. With