TM 5-805-4/AFJMAN 32-1090
Table C-22. Approximate Noise Reduction of Gas Turbine Engine Casing Enclosures.
Noise Reduction, dB
Octave
Band
Type
Type
Type
Type
Type
5
4
2
3
1
(Hz)
6
3
4
1
2
31
7
4
5
1
2
63
8
4
5
1
2
125
9
5
6
2
3
250
10
6
6
2
3
500
11
7
7
2
3
1000
12
8
8
2
4
2000
13
8
9
3
5
4000
14
8
10
3
6
8000
Notes:
Glass fiber or mineral wool thermal insulation with lightweight
Type 1.
foil cover over the insulation.
Glass fiber or mineral wool thermal insulation with minimum
Type 2.
20 gage aluminum or 24 gage steel or 1/2-in. thick plaster
cover over the insulation.
Enclosing metal cabinet for the entire packaged assembly, with
Type 3.
open ventilation holes and with no acoustic absorption lining
inside the cabinet.
Enclosing metal cabinet for the entire packaged assembly, with
Type 4.
open ventilation holes and with acoustic absorption lining
inside the cabinet.
Enclosing metal cabinet for the entire packaged assembly, with
Type 5.
all ventilation holes into the cabinet muffled and with acoustic
absorption lining inside the cabinet.
quencies" will occur for any unknown gear. The
ation of the gear noise. Table C-17 gives the
estimated SPL in the 125 through 8,000 Hz bands
possibility that these frequency components may
occur in any of the upper octave bands is covered
for a variety of speeds and powers, based on
by, equation C-16, which gives the octave band
equation C-16.
SPL estimate (at the 3 feet normalized condition)
C-19. Generators.
for all bands at and above 125 Hz:
The noise of generators, in general, can be quite
Lp = 78 + 3 log (RPM) + 4 log (hp) (eq C-16)
variable, depending on speed, the presence or
where "RPM" is the speed of the slower gear shaft
absence of air cooling vanes, clearances of various
and "hp" is the horsepower rating of the gear or
rotor parts, etc., but, most of all, on the driver
the power transmitted through the gear. For the
mechanism. When driven by gas or diesel recipro-
63 Hz band, 3 dP is deducted; and for the 31 Hz
cating engines, the generator is usually so much
band, 6 dB is deducted from the equation C-16
quieter than the engine that it can hardly be
measured, much less heard. For gas turbine en-
value. This estimate may not be highly accurate,
gines, the high-speed generator may be coupled to
but it will provide a reasonable engineering evalu-
C-19
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