Low-Frequency Bi-Directional Microphone Based on a Combination of Bionic MEMS Diaphragm and Fiber Acousto-Optic Transducer

This paper reports on a fiber-optic directional microphone with a bionic MEMS diaphragm for low-frequency sound source localization. The diaphragm consists of two 3 mm \times3 mm wings connected by a 1 mm \times1 mm bridge, and two 0.2 mm \times0.1 mm torsion beams anchor the diaphragm to the Silicon-On-Insulator (SOI) frame along the bridge's central axis. The bridge causes the mechanical coupling between the two wings, resulting in rocking and bending vibration modes of the diaphragm. The designed microphone contains a closed back cavity and works by measuring vibration at one of the diaphragm's distal edges by a fiber-optic Fabry-Perrot interferometer (FPI). Simulation and experimental results demonstrated that the microphone can make a significant directional response in a wide low-frequency band centered on the rocking-mode eigenfrequency, giving a typical bi-directional polar pattern, but its directionality disappears at the bending-mode eigenfrequency. The directional sensitivity of the microphone was determined to be 1.86 mV/° at 300 Hz based on the measured linear response to the incident angle in an angular range from \theta =0 to 60°. The minimum detectable pressure (MDP) of the microphone relies on both the incident angle and sound frequency, and MDP was measured to be 12.68\mu Pa/ \surd Hz at 300 Hz and \theta = 100^{\circ }. The fiber-optic FPI serving as the acousto-optic transducer offers the directional microphone high sensitivity and strong immunity to electromagnetic interference, rendering it suitable for low-frequency sound source localization in harsh environments, especially for tracing the environmental noise sources.