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 shell of the augmenter tube. Because the highest interior noise levels are in
the vicinity of the entrance of the augmenter tube, this upstream portion of
the exterior tube is usually the contributor to far-field noise.
12.2.4
Path 4. Path 4 represents the noise which escapes through the walls
and roof of the building. The sound power escaping through this path is
controlled by:
a)
sound power output of the engine under test;
b) the axial distance between the engine exhaust and the plane of
the augmenter intake opening;
c) horizontal and vertical positioning of the engine relative to
the center line of the augmenter tube;
d) effectiveness of the sound absorbing treatment of the interior
surfaces of the building;
e) sound transmission loss of the building walls, roof, and doors
and windows in the exterior walls;
The above listed variables also control the interior noise in the
building. Both the interior noise level and the sound power escaping through
the building partitions increases strongly with increasing distance between
engine exhaust and augmenter tube entrance.
12.2.5
Path 5. Path 5 represents the noise which escapes through large
openings, such as the primary air intake. These large openings are necessary
to bring in the large volume of air needed for the engine intake and for
cooling. To control the noise escaping through these openings without
excessive pressure drop (that would result in excessive cell depression), the
sound attenuation must be accomplished by low-pressure-drop mufflers.
Parallel baffle dissipative mufflers are the best to accomplish this and to
provide an undistorted turbulence-free flow that is needed to avoid vortex
generation especially in the front of the building upstream of the engine
intakes.
12.2.6
Path 6. Path 6 represents the noise which escapes through the large
front door of the building. Because of the shielding effect of the building,
the noise radiated from the front door has practically no contribution to the
noise at the far-field positions located in the downstream quadrant.
12.2.7
Source Receiver Paths. Source receiver paths which contribute to
the far-field noise are summarized in Figure 29 in the form of a block
diagram. This block diagram provides additional information for Figure 28.
Figure 29 identifies the major noise source and the major paths through which
part of the source noise reaches an observer located at a specific far-field
position at 250-ft (76.2-m) radius circle (or any larger distance) centered at
the engine exhaust. It illustrates that the noise at any observation point
has contributions which arrive there via many different paths. Because
directivity of radiation, the shielding by the building structure, and the
source receiver distances are different for each receiver position, the
prediction of the noise level at a specific receiver location is a difficult
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