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 Section 12:
EXTERNAL NOISE
12.1
Introduction. This section deals with the external noise of hush-
house and jet engine test cells. Data reported in this section have either
been obtained from full-scale facilities or from model-scale studies. The
emphasis is placed on full-scale facilities. The far-field noise of ground
runup facilities is of concern because, if not properly controlled, it can
cause temporary hearing impairment, disturbance at nearby buildings within
the base, disturbance to neighboring residences, and noncompliance with naval
and community noise regulations.
12.2
Principal Paths of Noise Radiation. Figure 28 shows, in a schematic
manner, the principal paths of noise radiated from a hush-house.
12.2.1
Path 1. Path 1 represents the attenuated jet noise which emerges
from the exhaust end of the acoustically lined augmenter tube. The sound
power radiated to the far field by the attenuated jet noise is a function of
the:
a)
sound power output of the engine(s);
b)
axial distance of the engine exhaust plane from the augmenter
inlet;
c) vertical, horizontal and angular positioning of the engine in
relation to the augmenter axis;
d)
geometry and acoustical treatment of the augmenter tube;
e) temperature and flow gradients across the augmenter
cross-section created by the mixing of the hot exhaust jet with the
surrounding cooling air;
f)
acoustical characteristics of the lined 45 deg. exit ramp.
12.2.2
Path 2. Path 2 represents the noise which is generated by the
vortex shedding at the trailing edge of the exit ramp (or the trailing edge of
baffles if the attenuation of the jet noise is accomplished with sound
absorbing baffles located in the exhaust stack instead of the lined
augmenter). This flow-generated noise is proportional from the 5th to the 6th
power of the flow velocity at the trailing edge. Accordingly, the noise
generated by this process is very sensitive to localized deviations of the
exit velocity from its average value. Consequently, if the hot jet is not
mixed sufficiently well with the surrounding cooling air to yield an even
velocity distribution, then the flow-generated noise may contribute to the
far-field noise. This is usually the case when the augmenter provides a high
attenuation of the jet noise. Because of the directive nature of the flow
noise, its contribution to the far-field noise is usually limited to position
downstream of the exhaust.
12.2.3
Path 3.
Path 3 represents the noise which radiates from the outside
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