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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 iteration, and to enhance equipment of lowering noise to extra appropriate degrees. although, the measurements themselves are advanced, and require a deep figuring out of the experimental facility applied (such as a wind tunnel), size instrumentation, and knowledge research options.
In this quantity fresh advances within the size and knowing of aerodynamically generated sound are provided by means of leaders within the improvement of latest recommendations during this box. either simple and utilized difficulties are lined in detail.

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Convection Effects and Doppler Shift A fixed microphone in an airstream measures the same acoustic pressure as a microphone at the same location moving with the airstream as long as the diaphragm is exposed to the mean static pressure of the flow. This is the case 32 Microphone Measurements In and Out of Airstream noise source apparent noise Fig. 21. Sound propagation paths and angles in a moving stream of the FITE forebody. A diaphragm ported to the total pressure of the airstream measures unsteady pressures that depend on sound levels, Mach number, and acoustic incidence angles, which are undesirable complications (Morse and Ingard 1968).

1991). Today, directional receivers such as phased microphone arrays, reflective dishes, and sound intensity probes are employed to better isolate specific source regions (Soderman 1999). Background Noise and Microphone Placement Microphone placement depends on the background noise level and signal-tonoise ratio. The estimation of those factors was discussed above. Microphone placement should be chosen to maximize the distance from the noise source so as to avoid near-field effects while still achieving 10 dB signal-to-noise difference.

Noise radiated upstream will be amplified, and noise radiated downstream will be attenuated relative to the noise from the same source stationary in still air. 24) 34 Microphone Measurements In and Out of Airstream More complicated noise sources radiate sound with the same Doppler factor (1 - Mp cos 8') raised to a different exponent. Norum et al. 8. Because of the analogy between a source in motion and a fixed source in a moving medium, convective amplification occurs in wind tunnel simulations as has been observed many times (Soderman et al.

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