A small-scale test-facility for jet-noise experiments
The JExTRA (Jet Noise Experiments at AT-TRA) test facility is used for aeroacoustic experiments with subsonic jets and in order to test measurement methods. It combines an aerodynamic jet with a semi-anechoic test-section with microphone arrays. The cylindrical test section surrounds the jet with absorbing acoustical foam and holds a linear microphone array with 50 microphones parallel with the jet axis and several azimuthal ring arrays.
The room acoustical properties of this set-up were carefully checked. The combination of the foam cylinder with another foam cover on the rear wall effectively diminishes reflections from the laboratory walls. Jet noise measurements were performed with a 50 mm nozzle at Mach numbers between 0.4 and 0.7. The frequency spectra measured at different axial positions downstream of the nozzle were compared with reference data that H. K. Tanna had measured and published at Lockheed in 1976 with a two inch diameter nozzle. The results correspond very well in the interesting frequency range of Strouhal numbers beteween 0.1 and 2.0 and confirm that the facility is suited for aeroacoustic experiments with small-scale jets.
Schematic of the sound absorbing test-section of the JExTRA jet-noise test facility.
The linear microphone array and the foam cover in the JExTRA test-section.