Tufts harnesses MEMS mics to monitor jet aircraft

by R. Colin Johnson
Contributing Editor, MEMS Investor Journal

An innovative micro-electro-mechanical system of 64 silicon microphones has been crafted as a hypersensitive audio array for detailed monitoring of the forces causing turbulence around jet aircraft.

MEMS microphone array (left) spaced at 1.3 millimeters and a single 0.6 millimeter mic (right) micromachined from 3.5 micron thick polysilicon with a 2 micron air gap to the bottom electrode.

Air resistance causes turbulence which can chaotically churn the atmosphere around an aircraft, making the ride rough for passengers and, in the worst case, endangering the structural integrity of an airplane. Now professor Robert White and doctoral candidate Joshua Krause at the Tuft Micro and Nano Fabrication Facility have succeeded in packing a MEMS array with 64 microphones into a chip measuring just one square centimeter, offering researchers the world's most fine-grained instrument for studying the forces that cause turbulence.

The researchers found that despite the fact that airflow around an aircraft's fuselage has been studied for over 50 years, the formulae for taming turbulence has remained elusive. The problem, according to the Tufts researchers, is that a detailed mathematical understanding has awaited instruments that can capture and analyze detailed enough data streams about the airflow maelstrom swirling around an aircraft that is traveling at hundreds of miles per hour.

The MEMS chip was fabricated using surface micromachining of front-vented capacitively sensed microphones set in a 8-by-8 array. The microphones were spaced at 1.3 millimeters (center-to-center) in order to capture the detailed pressure spectra that until now have been hidden underneath a turbulent boundary layer. Care was taken to minimize package topology to reduce flow-generated self-noise. The array was fabricated using MEMSCAP's (Euronext: MEMS) PolyMUMPs process using three layers of polysilicon and a Parylene-C coating applied during post-processing.

So far the sensitive new instrument has been able to track both the high- and low-frequency oscillations that can cause vibration and noise as well as challenge the structural integrity of an aircraft. Sensitivity of 1 mV/Pa was measured for each microphone over a 200–40,000 Hz bandwidth. Next the researchers plan to perform detailed characterization of the MEMS microphone array chip in a wind tunnel, before deploying it on a real jet aircraft for testing.