TM 5-805-4/AFJMAN 32-1090
Table C-20. Overall PWLs of the Principal Noise Components of Gas Turbine Engines Having No Noise Control Treatments
higher sound levels than if there was no stack and
irregularities on the ground surface can cause
the sound were emitted by a nondirectional point
some backscattering of sound into the 90 to 180
source. From about 60 to 135 degrees from the
degree regions, for horizontal stacks serving either
axis, there is less sound level than if there were no
as intake or exhaust openings. For small openings
stack. In other words, directly ahead of the open-
in a wall, such as for ducted connections to a fan
ing there is an increase in noise, and off to the
intake or discharge, use approximately one-half
side of the opening there is a decrease in noise.
the directivity effect of table C-23 (as applied to
The table C-23 values also apply for a large-area
intake openings) for the 0 to 90 degree region. For
intake opening into a gas turbine for the 0 to 60
angles beyond 90 degrees, estimate the effect of
degree range; for the 90 to 135 degree range,
the wall as a barrier.
subtract an addition 3 dB from the already
C-16. Electric Motors.
negative-valued quantities. For horizontal stacks,
sound-reflecting obstacles out in front of the stack
Motors cover a range of 1 to 4000 hp and 450 to
opening can alter the directivity pattern. Even
3600 RPM. The data include both "drip-proof'
C-17
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