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Current generation microphone directional array instrumentation is capable of extracting accurate noise source location and directivity data on a variety of aircraft components, resulting in significant gains in test productivity. However, with this gain in productivity has come the desire to install larger and more complex arrays in a variety of ground test facilities, creating new challenges for the designers of array systems. To overcome these challenges, a research study was initiated to identify and develop hardware and fabrication technologies which could be used to construct an array system exhibiting acceptable measurement performance but at much lower cost and with much simpler installation requirements. This paper describes an effort to fabricate a 128-sensor array using commercially available Micro-Electro-Mechanical System (MEMS) microphones. The MEMS array was used to acquire noise data for an isolated 26%-scale high-fidelity Boeing 777 landing gear in the Virginia Polytechnic Institute and State University Stability Tunnel across a range of Mach numbers. The overall performance of the array was excellent, and major noise sources were successfully identified from the measurements.

Single-walled carbon nanotubes (SWNTs) have been synthesized via a novel chemical vapor deposition (CVD) approach utilizing nanoporous, iron-supported catalysts. Stable aqueous dispersions of the CVD-grown nanotubes using an anionic surfactant were also obtained. The properties of the as-produced SWNTs were characterized through atomic force microscopy and Raman spectroscopy and compared with purified SWNTs produced via the high-pressure CO (HiPCO) method as a reference, and the nanotubes were observed with greater lengths than those of similarly processed HiPCO SWNTs.

Infrasonic windscreens, designed for service at frequencies below 20 Hz, were fabricated from a variety of materials having a low acoustic impedance, and tested against four specifications ͑the first three in a small wind tunnel͒: ͑1͒ wind-generated noise reduction ͑"insertion loss"͒ at a free-stream wind speed of 9.3 m / s, ͑2͒ transmission of low-frequency sound from a known source ͑subwoofer͒, ͑3͒ spectrum of sound generated from trailing vortices ͑aeolian tones͒, and ͑4͒ water absorption ͑to determine suitability for all-weather service͒. The operating principle is based on the high penetrating capability of infrasound through solid barriers. Windscreen materials included three woods ͑pine, cedar, and balsa͒, closed-cell polyurethane foam, and Space Shuttle tile material. The windscreen inside diameter ranged from 0.0254 to 0.1016 m ͑1 to 4 in.͒, and wall thickness from 0.003175 to 0.01905 m ͑ 1 8 to 3 4 in. ͒ . A windscreen made of closed-cell polyurethane foam revealed a wind noise reduction of 10-20 dB from 0.7 to 25 Hz, transmission coefficient near unity from 10 to 20 Hz, and spectral peaks beyond 20 Hz due to vortex-generated sound. Following a description of past methods, the principle of operation, and the experimental method, experimental data are presented for a variety of windscreens.