Aeroacoustics Research Facilities
The Semi-anechoic Chamber is one of a handful of facilities in the world that offers state-of-the-art sound reproduction capabilities to reproduce 3D spatial sound fields of anything from concert halls and office noise to airplane noise. The main feature of the 3D reproduction capabilities is the ability to reproduce "third-order ambionics."
Open Jet Flow-through Anechoic Chamber
The Open Jet Flow-through Anechoic Chamber is a 5.0 meter x 6.0 meter x 2.8 meter room covered with fiberglass wedges, with an approximate cut-off frequency of 250 Hz. High-pressure air is delivered from a tank farm pressurized to 200 psig.
The air flow is regulated via pressure regulators and control valves located in a piping cabinet, before being fed to a plenum and delivered to the jet nozzle issuing into the anechoic chamber. A pitot probe is embedded in the middle section of the plenum to provide, via a pressure transducer, the total pressure upstream of the nozzle. An exhaust fan, installed in the downstream section of a collector, captures the jet exhaust and minimizes air recirculation and possible local helium accumulation in the anechoic chamber.
Jet aeroacoustics measurements are typically performed using twenty microphones, supported from a boom that extends from the plenum stand. The microphone array has controlled rotation around a point located at the center of the nozzle exit plane. The microphones are positioned at a grazing incidence horizontal to the jet centerline plane and equally spaced by 5 degrees. The physical distance from each microphone to the nozzle exit is approximately 1.8 m. This distance is sufficient to ensure that the microphones are in the far-field when testing nozzles up to 2 cm in diameter. The microphones used are 1/8 in (3.2 mm) pressure field microphones, type 40DP from GRAS.
An open jet wind tunnel is incorporated in the facility for experiments conducted with a forward flight stream. A mixed flow design developed for minimal noise, provides the inlet flow. This flow is delivered to an axial flow muffler and acoustically treated duct work leading to the anechoic chamber inlet. An acoustically treated collector and duct work leads to an exhaust fan on the exit side of the system. This design results in an open jet, inside the chamber that is close to ambient pressure.
More information about this facility will be available soon.