Aeroacoustics is the study of the sound generated by wind moving over, around and through an object. In the field of transportation, the noise emitted by ground-based (cars and trucks) or airborne vehicles (planes and helicopters) affects those using that mode of transport (driver or pilot and passengers) and bystanders (those by the highway or under the flight path of a landing aircraft).
There is a pressing demand for a quieter ride. The increasing noise pollution from an ever-growing number (and size) of aircraft landing at an airport has made industry and government organizations aware of the need to evaluate the noise signatures of vehicles and find ways to reduce them.
The phased-microphone 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 or by a full-scale vehicle in operation.
Assuming a known spreading pattern for the sound emitted from a point in space, such as spherical for a monopole, the signals can be delayed (in the time domain) or phased (in the frequency domain) to steer the array to look at a particular point in space. The processed signals then will sum to the level representing the amount of sound emitted from the point in space being looked at.
Long description of Figure 1
The figure depicts an idealization of one of the multiple sound sources for a full-scale aircraft flying in the air or a model in the wind tunnel. The sound from this source radiates out in waves and these waves are recorded by microphones which have been strategically placed in a pre-determined pattern. The waves arrive at these microphones at different times, as seen in the recorded signals of the figure. If those arrival delays are "undone," also understood as lining up the signals through the appropriate delaying or phasing of the signals based on the relative position of the source and each microphone, then the signals will add up to a strong signal which represents the strength of the sound source on the aircraft, entitled summed/beamformed signal on the figure. Each point in space which sound might emanate will have a different set of delays at the microphones and so as the point of interest is swept through space, a map of the sound signature of the aircraft can be created, as is seen at the bottom of this webpage. If there is little or no noise coming from a point in space then when the signals are summed the resulting signal will not be that strong.
The phased-microphone array provides a means for clients to examine the aeroacoustic signature of their model in a controlled test environment. It consists of a 96-channel data acquisition system collecting data from 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 system can be separated into two units for placement of microphones simultaneously inside and outside a vehicle in a wind tunnel. In addition, the in-vehicle sub-array can be configured to be self-contained making it portable for on-road measurements.
The phased-microphone array is deployable in many of our wind tunnels allowing for the testing of small-scale models in the low-speed research tunnel up to full-scale models in the 9 m wind tunnel. In-situ phase calibration methods ensure accurate measurements in this range of acoustic environments.
The NRC-developed suite of acoustic analysis programs along with the complete reconfigurability of the microphone array permits the adaptation of the measurement system to meet client needs in a diverse set of industry sectors.