A Microphone Array on a Chip for High Spatial Resolution Measurements of Turbulence

A microelectromechanical systems-based microphone array on a chip has been developed and applied to aeroacoustic measurements. The array is designed to measure the fluctuating pressures present under a turbulent boundary layer (TBL). Each chip measures 1 cm² and contains 64 individually addressable capacitively sensed microphones, with a center to center pitch of ~1.25 mm. Surface topology, including the packaging, is kept to less than 0.13 mm. Element-to-element sensitivity variation in the array is less than ±2.5 dB from least to most sensitive, and phase variation is less than ±6.5° (at 1 kHz). The microphone 3-dB bandwidth is 700 Hz to 200 kHz, and the microphones are linear to better than 0.3% at sound pressure levels up to 150-dB SPL. A unique switched architecture system electronics and packaging method are employed to reduce data acquisition channel count requirements, and to maintain a low surface roughness. The array has been applied to the measurement of single point turbulence spectra under a flat plate TBL in a flow duct at Mach numbers up to 0.6 and Reynolds numbers based on plate length of 10⁷.