MEMS pressure sensor array for aeroacoustic analysis of the turbulent boundary layer

A surface micromachined microphone array for characterization of pressure fluctuations below the turbulent boundary layer (TBL) is discussed. At relatively high Reynolds numbers based on the momentum boundary layer thickness (Re>4000), the features in the TBL are on the order of 50 microns [1]. Therefore, it is difficult to characterize the details of the flow with conventional millimeter to centimeter scale sensors. By moving to MEMS sensor arrays, it is possible to study the micro-scale eddy currents, which will allow for the characterization of higher wavenumber features and high frequency temporal features of in-flight TBLs. This increase in spatial and temporal resolution will provide valuable characterization data of the TBL which will lead to the reduction of unwanted noise in airplane cabins.; The design, fabrication, and characterization of a surface micromachined, front-vented, 64 channel (8x8), capacitively sensed pressure sensor array is described. The array was fabricated using the MEMSCAP PolyMUMPsRTM process, a three layer polysilicon surface micromachining process. An acoustic lumped element circuit model was used to design the system. The results of our computations for the design, including mechanical components, environmental loading, fluid damping, and other acoustic elements are detailed. Theory predicts single element sensitivity of 1 mV/Pa at the gain stage output in the 400-40,000 Hz band. A laser Doppler velocimetry (LDV) system has been used to map the spatial motion of the elements in response to electrostatic excitation. A strong resonance appears at 480 kHz for electrostatic excitation, in good agreement with mathematical models. Static stiffness measured electrostatically using an interferometer is 0.1 nm/V2, similar to the expected stiffness. Preliminary acoustic calibrations shows single element acoustic sensitivity (as a function of frequency) increasing from 0.1 mV/Pa at 700 Hz to 3 mV/Pa at 7 kHz.; This sensor is novel due to the use of a fully surface micromachined foundry process, a microphone array on a single chip with a fine center-to-center pitch (1.2625 mm) with a membrane diameter of 600 microns, and front venting. This will allow for high resolution data on the frequency wavenumber spectra of the TBL experienced by an aircraft in flight. Also, by assembling the array chips end-to-end, we will be able to determine low wavenumber information through the larger spatial scale.