Micromachined Microphone Array on a Chip for Turbulent Boundary Layer Measurements.
Abstract: A surface
micromachined microphone array on a single chip has been successfully designed,
fabricated, characterized, and tested for aeroacoustic purposes. The microphone was
designed to have venting through the diaphragm, 64 elements (8×8) on the chip, and
used a capacitive transduction scheme. The microphone was fabricated using the MEMSCAP
PolyMUMPs process (a foundry polysilicon ... read moresurface micromachining process) along with
facilities at Tufts Micro and Nano Fabrication Facility (TMNF) where a Parylene-C
passivation layer deposition and release of the microstructures were performed. The
devices are packaged with low profile interconnects, presenting a maximum of 100
μm of surface topology. The design of an individual microphone was completed
through the use of a lumped element model (LEM) to determine the theoretical performance
of the microphone. Off-chip electronics were created to allow the microphone array
outputs to be redirected to one of two channels, allowing dynamic reconfiguration of the
effective transducer shape in software and provide 80 dB off isolation. The
characterization was completed through the use of laser Doppler vibrometry (LDV),
acoustic plane wave tube and free-field calibration, and electrical noise floor testing
in a Faraday cage. Measured microphone sensitivity is 0.15 mV/Pa for an individual
microphone and 8.7 mV/Pa for the entire array, in close agreement with model
predictions. The microphones and electronics operate over the 200-40 000 Hz band. The
dynamic range extends from 60 dB SPL in a 1 Hz band to greater than 150 dB SPL. Element
variability was ±0.05 mV/Pa in sensitivity with an array yield of 95%. Wind tunnel
testing at flow rates of up to 205.8 m/s indicates that the devices continue to operate
in flow without damage, and can be successfully reconfigured on the fly. Care has been
taken to systematically remove contaminating signals (acoustic, vibration, and noise
floor) from the wind tunnel data to determine actual turbulent pressure fluctuations
beneath the turbulent boundary layer to an uncertainty level of 1 dB. Analysis of
measured boundary layer pressure spectra at six flow rates from 34.3 m/s to 205.8 m/s
indicate single point wall spectral measurements in close agreement to the empirical
models of Goody, Chase-Howe, and Efimtsov above Mach 0.4. The MEMS data more closely
resembles the magnitude of the Efimtsov model at higher frequencies (25% higher above 3
kHz for the Mach 0.6 case); however, the shape of the spectral model is closer to the
model of Goody (50% lower for the Mach 0.6 case for all frequencies). The Chase-Howe
model does fall directly on the MEMS data starting at 6 kHz, but has a sharper slope and
does not resemble the data at below 6 kHz.
Thesis (Ph.D.)--Tufts University, 2011.
Submitted to the Dept. of Mechanical Engineering.
Advisor: Robert White.
Committee: Mark Moeller, Chris Rogers, and Sameer Sonkusale.
Keywords: Mechanical engineering, Aerospace engineering, and Engineering.read less