By Thomas J. Mueller, Christopher S. Allen, William K. Blake, Robert P. Dougherty, Denis Lynch, Paul T. Soderman, James R. Underbrink
The objective of aeroacoustic measurements is to supply a foundation for assessing mechanisms of noise new release, and to advance tools of lowering noise to extra appropriate degrees. even though, the measurements themselves are complicated, and require a deep realizing of the experimental facility applied (such as a wind tunnel), dimension instrumentation, and knowledge research options.
In this quantity contemporary advances within the size and realizing of aerodynamically generated sound are provided through leaders within the improvement of latest innovations during this box. either uncomplicated and utilized difficulties are coated in detail.
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Additional info for Aeroacoustic Measurements
1995). The figure (c) shows several Quiet Struts installed in the Glenn 9 x 15- Ft Wind Tunnel 18 Microphone Measurements In and Out of Airstream Fig. 10. 10. That concept has the advantage of redundant data acquisition - if one microphone fails, the other will allow test continuation. Cross-spectra of the two microphone signals can also be helpful to attenuate uncorrelated self-noise and estimate the sound propagation direction (Bendat and Piersol 1980). Strut noise in open-jet wind tunnels is aggravated by the need in most cases to penetrate the turbulent shear layer to support the model, microphone or other test hardware.
0 N 0,) .... ---~--,---~--,---~--, Mach = 0_22 100 90 "'0 0,)- Cl ctI .... 80 0,) ffi c.. J 70 60 predicted recessed backgrolUldnoise 0 2000 measlU-ed recessed backgrolUulnoise 4000 6000 8000 10000 fi-eqllency, Hz (M= 150 Hz) Fig. 14. 22. 7 mm below a taught Kevlar® cloth mounted flush with the wind tunnel wall. 99). 4 mm, respectively 0 aeger et al. 2000) Background Noise from Wall Boundary Layer Depending on wind tunnel wall roughness and distance from strut-mounted microphones, wall boundary layers can be a source of background noise.
95 -30+-------~----~-+--+-+-~~~~----~~ 1 10 5 microphone distance fj·01D source, m 20 Fig. 18. Sound decay interference envelopes computed for a 1 kHz tone emitted in a 12-m high test section. Multiple envelopes are shown for various values of surface sound absorption coefficients between 0 (hard wall) and 1 (perfectly absorbent). Atmospheric conditions: 21°C, 56% relative humidity. 0048 dB/m. 2 m above the floor. An example of measured wind tunnel sound decay from a loudspeaker is also plotted (Soderman et al.
Aeroacoustic Measurements by Thomas J. Mueller, Christopher S. Allen, William K. Blake, Robert P. Dougherty, Denis Lynch, Paul T. Soderman, James R. Underbrink