Calibrations of the NASA Langley 14- by 22-Foot Subsonic Tunnel in Acoustic Configuration

Metrics of NASA Langley’s 14by 22-Foot Subsonic Tunnel in the acoustic configuration are provided. The background noise levels are given over a free-stream Mach number range of 0.11 to 0.23. Two room-acoustic tests were conducted: one in which speakers were driven to steady state and abruptly turned off (interrupted noise), and another which set off a blast at the approximate model location (impulse response). Data were acquired on a partial hemisphere surrounding the model location. Novel processing, which combined the use of Fourier transforms and the separation of acquired signals into separate parts, was used to enable the calculation of tunnel acoustic characteristics from the data. Although the two tests were complementary, the impulse response test outputs were more accurate than those obtained from the interrupted noise test. The impulse response data were then used to calculate the power ratio between the direct arrival of signal to the microphones and that due to reflections and reverberation, the power ratio of the direct signal to the reverberation only, and the reverberation time at different measurement locations within the tunnel. Implications of the room-acoustic testing methodology and novel processing are discussed. The results may be useful in future model test planning.