negative-valued quantities. For horizontal stacks,
Table 2-8 shows that from approximately 0 to 60
sound-reflecting obstacles out in front of the stack
from its axis, the stack will yield higher sound lev-
opening can alter the directivity pattern. Even ir-
els than if there were no stack and the sound were
regularities on the ground surface can cause some
emitted by a nondirectional point source. From
backscattering of sound into the 90 to 180 regions
about 60 to 135 from the axis, there is less sound
for horizontal stacks serving either as intake or ex-
level than if there were no stack. In other words,
haust openings.
directly ahead of the opening, there is an increase
in noise, and off to the side of the opening, there is
a decrease in noise. The table 2-8 values also apply
mufflers for gas turbine inlet and discharge open-
for a large-area intake opening into a gas turbine
ings are considered in paragraph 34. The PWL of
for the 0 to 60 range; for the 90 to 135 range,
the noise radiated by a muffled intake or discharge
subtract an additional 3 dB from the already
is the PWL of the untreated source (from tables
2-12
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