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Example text

20 wind tlUUlC data + . . . . . . . . . . . . +:.......................... -\ +. \: . 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.

This is why most aero acoustic work is done in anechoic or open test sections or accomplished in hard-walled facilities by use of phased microphone arrays that can avoid the reverberant sound. Reflections in a Semi-Anechoic Environment Reflections from the test environment can spoil any acoustic measurement. Interference between direct and reflected waves from a nozzle or collector (open jet) or walls (closed jet) or test apparatus (any wind tunnel) corrupt the sound field. Although design of anechoic facilities is beyond the scope of this chapter, there are steps a researcher can take to evaluate the acoustic quality of the test environment and estimate the level of interference from acoustic reflections.

At 60 kHz and below, the directivity effect is less than 5 dB. Above 60 kHz, the directivity effect is as much as 8 dB. Since a desired accuracy of ± 1 dB is typical, the effect of forebody directivity on microphone response is important. The azimuthal directional response of a forebody, i. , directivity about the microphone long axis is more uniform than the longitudinal response below 90 kHz. However, the azimuthal directivity effect can be significant at some frequencies because of variations in forebody screen construction.

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