MEMS microphones as ultrasonic transducers

Abstract

We demonstrate the transmission of ultrasound in air using a transducer that resembles a MEMS microphone in its construction. The device comprises a compliant 1 mm diameter diaphragm, a stiff perforated backplate electrode, and a back-volume. The diaphragm is driven using AC signals with peak values that exceed the pull-in voltage of the diaphragm. Relatively large diaphragm displacements are achieved as diaphragm oscillations traverse the complete 2.30-micrometer diaphragm-backplate gap in response to excitation waveforms spanning from 40 kHz to 150 kHz. Large amplitude diaphragm vibration is advantageous for high SPL applications in air, as sound pressure is directly proportional to diaphragm displacement for a given operating frequency. Diaphragm vibration profiles are measured using a scanning laser Doppler vibrometer, and resultant acoustic pressure waveforms in air are measured using a broadband microphone. We demonstrate how nonlinear features of the electrostatic transducer can be exploited to generate loud, broadband signals. We also discuss interesting applications using an array of these transducers.