James R. Underbrink

Characterization of Aeroacoustic, Silicon Micromachined Microphones for Aircraft Fuselage Arrays

Thispaperdescribesthedesignandcharacterization ofamicromachined microphoneforaircraft fuselagearrays utilized by aeroacousticians to help identify aircraft noise sources and/or assess the effectiveness of noise-reduction technologies. The developed microphone utilizes piezoelectric transduction via an integrated aluminum nitride layer in a thin-film composite diaphragm fabricated using a combination of surface and bulk micromachining. The experimental characterization of several microphones is presented. Measured performance was in line with the Boeing Company specifications for the fuselage array application, including sensitivities of 32:1 � V=Pa to 43:7 � V=Pa,minimumdetectablepressuresaslowas40dB(1Hzbinat1kHz),confirmedbandwidthsupto20kHz, >100 kHz resonant frequencies, and 3% distortion limits between 160 and 172 dB sound pressure level. With this performance, in addition to the small sizes, these microphones are shown to be a viable enabling technology for low-cost, high-resolution fuselage array measurements.

Pletharrays for aeroacoustic phased array applications

“Pletharrays” are introduced, motivated, and presented for application to aeroacoustic phased array measurements. Pletharrays contain a plethora of arrays composed from a modest to high number of array elements to field a remarkably large number of high element count arrays for use in noise source imaging applications. Pletharrays that have been deployed for closed jet transonic wind tunnel, static engine ground, open jet wind tunnel, and flyover phased array tests are presented. Tremendous array element leverage to provide extensive measurement flexibility and fidelity are demonstrated.

Finding the boom: Phased array processing applied to sonic boom direction of arrival estimation:

From 29 October 2012 to 7 November 2012, 73 supersonic passes of an F-18 aircraft were observed over a dry lake bed at Edwards Air Force Base as part of NASA’s Farfield Investigation of No-boom Thresholds project, which was conceived to measure the characteristics of sonic booms at the boundaries of their decay, where overpressure is exceptionally low, thereby stretching the limits of current prediction methods. Each pass was recorded by a 55-microphone phased array sensor system with a circular aperture diameter of 2000 ft (609.6 m). The data were processed using a novel time domain array processing algorithm to estimate the direction of arrival and trace speed of the sonic boom wave front along the plane of the phased array. The results from the phased array processing are consistent with the known location of the test aircraft for each processed flight and are consistent with expectations for direction of arrival due to atmospheric refraction. Near real-time estimation of the sonic boom direction of arrival, trace speed along the ground, and visualization of the propagation of the sonic boom wave front are possible. This could allow the test team to assess the data and determine if the target of the test point has been met while the test aircraft is still in flight. This would enable improved test efficiency and efficacy, ultimately improving the value of the test campaign. The measured direction of arrival also provides sonic boom propagation numerical prediction code validation. Most sonic boom prediction codes provide the propagation path of the sonic boom and thus the direction of arrival of the sonic boom at a point on the ground. Thus for predictions made using the actual flight data measured at the time of the test, the predicted direction of arrival and measured direction of arrival can be directly compared to help validate the prediction codes.

Micromachined Aluminum Nitride Microphone Technology Development

This paper describes the motivation, packaging, and preliminary experimental results for a new passive microelectromechanical systems (MEMS)-based aeroacoustic microphone. The passive technology utilized — piezoelectric transduction, with aluminum nitride serving as the piezoelectric material — together with the small size make this microphone ideal for deployment as fuselage instrumentation in full-scale flight tests. Preliminary results show that the microphone performance characteristics exceed specifications provided by project sponsor Boeing, with a noise floor 20 kHz, and sensitivities from 32.1μV/Pa to 43.7μV/Pa.