Aeroacoustic Measurements by Thomas J. Mueller, Christopher S. Allen, William K. Blake,

By Thomas J. Mueller, Christopher S. Allen, William K. Blake, Robert P. Dougherty, Denis Lynch, Paul T. Soderman, James R. Underbrink

The aim of aeroacoustic measurements is to supply a foundation for assessing mechanisms of noise iteration, and to advance tools of decreasing noise to extra appropriate degrees. despite the fact that, the measurements themselves are complicated, and require a deep knowing of the experimental facility applied (such as a wind tunnel), dimension instrumentation, and information research strategies.
In this quantity contemporary advances within the size and realizing of aerodynamically generated sound are provided by way of leaders within the improvement of latest concepts during this box. either simple and utilized difficulties are lined in detail.

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100 1000 frequency, 1f3 octave bands, Hz ~~ 10000 Fig. 15. 40 x 80 background noise at 445 km/h before and after modification of the test section acoustic lining (Soderman et al. 2000) Open-Jet Background Noise In addition to drive-fan and wind noise, an in-airstream microphone in an open-jet wind tunnel is also exposed to sound from the shear layers - particularly as the shear layers induce unsteady pressures on the nozzle and collector. That is mitigated to some extent by the lack of test section walls, which eliminates wall boundary-layer noise and allows drive-fan sound waves to propagate into the open room.

To further simplify the task, we assume that to first order all walls have the same sound absorption, and the absorption is independent of acoustic incident angle. Computations are made for zero wind. We assume that tonal interference, which occurs on a pressure amplitude basis, is dominated by the two closest walls. Of course, some facilities might require consideration of fourwall or multiple reflections, but the procedure is the same as below (Soderman et al. 2000). Conversely, broadband, random noise reflections (e.

This procedure corrects for microphone sensitivity. At high frequencies the microphone free-field response will not be flat, and steps must be taken to determine the variation with frequency and the corresponding data corrections. Pressure Response Pressure response microphones are designed to mount flush in a wall and, in this configuration, do not have a free-field effect other than pressure doubling at the wall caused by superposition of the incident and reflected waves. The microphone response is thereby optimized for sound waves striking the wall.

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