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 interior noise levels at this specific measurement position. This is because
the distance between the plane of the engine exhaust and the augmenter
entrance, X.N-, is much larger for the F-4 than it is for the F-14A.
Consequently, the F-4 "spills" more of the exhaust sound power into the
enclosure than does the F-14A.
Interior noise levels in certain hush-houses and jet engine test
cells have been measured also at positions which differ from the standard,
such as: (1) near to the front door, (2) near to the observation window, (3)
in the control room; and (4) inside the primary and secondary air inlets. The
data obtained in these nonstandard positions are documented in Experimental
Evaluation of the NAS Miramar Hush-House, [21], Noise from F-18 and F-14
Aircraft Operating in Hush-House #2 Naval Air Station Miramar, [22], Noise
Levels of the NAS Patuxent River, Maryland Hush-House [23].
11.4
Enclosure Interior Noise Studies Utilizing Scale Models. A
systematic scale model study [3] has been carried out to identify how the
sound power of a model jet splits between the enclosure and the augmenter
tube. It was found that the key parameter that controls the split of the jet
sound power between the enclosure and the augmenter is the ratio X+N,/D+A,,
where X+N, is the distance between the nozzle exhaust plane and the augmenter
entrance, and D+A, is the equivalent diameter of the augmenter entrance.
Figure 26 shows the split of the jet sound power between the
enclosure (burner room) and the augmenter (exhaust room) measured by Reference
3 on 1/15-scale model of a hush-house. The parameters X+N, and L+A, represent
the nozzle pressure ratio and the length of an unlined augmenter tube.
Figure 27 shows how the sound power that is radiated into the
enclosure (burner room) increases with increasing X+N, the distance between
the nozzle exhaust plane and the augmenter entrance. The conditions depicted
in Figure 27 span a X+N,/D+A, ratio range from 0.04 to 1.44.
NOTE:
No systematic model studies were carried out to date to investigate
the spatial distribution of the interior noise level. To be
realistic, such model studies will need to utilize a model-scale
engine that represents both the intake and exhaust noise of a
full-scale engine.
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