A microelectromechanical systems-based piezoelectric microphone for aeroacoustic measurements.

Abstract

This paper describes the packaging and characterization of micromachined piezoelectric microphones designed as aircraft fuselage instrumentation for full-scale noise characterization flight tests. An important requirement for such microphones is a high maximum sound pressure level (SPL) >150 dB coupled with noise floor <45 dB SPL (narrow bin) and flat frequency response in the audio band (20 Hz–20 kHz). In contrast to many past micromachined piezoelectric microphones that use PZT and ZnO for piezoelectric transduction, aluminum nitride (AlN) was used due to inherent advantages in dielectric loss and signal-to-noise ratio. The microphone structure includes a 500–900-μ m diameter circular diaphragm composed of a structural layer and an annular AlN/metal film stack on a silicon substrate. Promising microphone die were selected via laser vibrometer measurements followed by packaging of the microphones and associated electronics in individual 1/4 in. tubular housings. Characterization of the packaged microphones included frequency response, noise floor, and total harmonic distortion measurements. Early results for a packaged microphone reveals a sensitivity of 27 μV/Pa, dynamic range of 36–157 dB, and resonant frequency of 176 kHz.