Acoustic array

The acoustic array is a measurement system that, along with specialized signal analysis algorithms, can determine the three-dimensional sound field generated by a model in a wind tunnel. It consists of an array of 64 microphones and supporting infrastructure to simultaneously acquire, digitize and store the signal.

Research in aeroacoustics (i.e. the generation of sound by air flowing over a model) is a continually expanding field. Increasing noise pollution from an ever-growing number (and size) of vehicles on the road or aircraft landing at an airport has raised awareness within industry and government about the need to determine the sound of air, and ways to reduce or mitigate it.

For automotive manufacturers, the issue is not only sound heard by those outside a vehicle but also sound heard by those inside the vehicle. Since the predominant noise source at highway speeds is due to the aerodynamics of the car, reducing the noise generated by such things as mirrors, pillars and wakes can greatly enhance passenger comfort.

At airports, now more often found in urban environments as cities grow around them, more frequent landings by increasingly larger aircraft contribute to an ever more noticeable noise environment surrounding the airport. Aircraft manufacturers are looking to address this noise by examining the sound generated by their aircraft, in particular by the flaps, slats and landing gear.

The acoustic array provides a means for clients of NRC Aerospace to examine the aeroacoustic signature of their model in a controlled test environment. It consists of 32 Bruel & Kjaer (B&K) 1/4" microphones, each with a frequency response of 4 to 70,000 Hz and a dynamic range of 30 to 170 db.

The supporting infrastructure continuously acquires and stores the signal from up to 190 microphones, each with a sampling rate of 108 kHz, on a high-speed disk array for up to 11 hours. The infrastructure includes a B&K signal conditioner and an Integrated Computer System (ICS) 24-bit analog to digital converter.

Phased array signal algorithms are used to examine data acquired by "steering" the array to a point in space near the model and listening for the sound generated at that point. By combining information from various points around the model, a three-dimensional representation of the sound field generated by the aerodynamics of the model can be created.