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 target of aeroacoustic measurements is to supply a foundation for assessing mechanisms of noise new release, and to advance tools of decreasing noise to extra appropriate degrees. besides the fact that, the measurements themselves are complicated, and require a deep figuring out of the experimental facility applied (such as a wind tunnel), size instrumentation, and information research thoughts.
In this quantity fresh advances within the size and realizing of aerodynamically generated sound are awarded through leaders within the improvement of recent innovations during this box. either easy and utilized difficulties are lined in detail.

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This allows the pressure response corrections to be applied separately from the sensitivity calibrations. 26 shows pressure response correction curves to 100 kHz measured from many different microphone channels during a 40 x 80 in-situ electrostatic calibration. The differences between channels are as much as 5 dB, which demonstrates the need for calibrating each channel individually. The 61-m long signal cables between microphones and power supply/amplifiers carried the weak preamplifier voltages, pressure response was reduced 1- 2 dB above 50 kHz.

In reality, the sound was emitted at angle e' and swept downstream by the wind. This is clear from the analogous situation of an aircraft flying over an observer on the ground. An observer at angle e relative to the aircraft will hear a sound that was emitted previouslyat angle e'. The source emission angles in flight and in the wind tunnel are identical. In both situations, e'defines the source emission direction and the wave front normal vector. Convection Effects and Doppler Shift 33 The distance, R', is the proper distance to use for computing sound pressure reduction from wave spreading and sound absorption because it represents the distance the sound waves travel relative to an observer moving with the wind.

Special corrections for forebody effects on free-field response and directivity are therefore required. Unless provided by the manufacturer, calibrations such as described below must be performed. Because the forebody affects the directional and free-field response of the microphone, the combined effects must be accounted for. The following procedure accounts for both effects 2 and 3 discussed at the start of this section. 29 shows a setup for measuring the aerodynamic microphone forebody (AMF) frequency response corrections and directivity effects.

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