TM 5-805-4/AFJMAN 32-1090
where T is the turbocharger correction term (T = 0
where "rated MW" is the maximum continuous
dB for an engine without a turbocharger and T = 6
full-load rating of the engine in megawatts. If the
dB for an engine with a turbocharger) and L is the
manufacturer lists the rating in "effective shaft
length, in ft., of the exhaust pipe. A turbocharger
horsepower" (eshp), the MW rating may be approx-
takes energy out of the discharge gases and results
imated by
in an approximately 6-dB reduction in noise. The
MW = eshp/1400.
octave band PWLs of unmuffled exhaust noise are
O v e r a l l PWLs, obtained from equations C-9
obtained by subtracting the values of table C-19
through C-11, are tabulated in table C-20 for a
from the overall PWL derived from equation C-8. If
useful range of MW ratings.
the engine is equipped with an exhaust muffler, the
(1) Tonal components. For casing and inlet
final noise radiated from the end of the tailpipe is
noise, particularly strong high-frequency sounds
the PWL of the unmuffled exhaust minus the
may occur at several of the upper octave bands.
insertion loss, in octave bands, of the muffler.
However which bands contain the tones will de-
pend on the specific design of the turbine and, as
C-15. Gas Turbine Engines.
such, will differ detween models and manufactur-
a. PWL of three sources. As with reciprocating
ers. Therefore, the octave band adjustments of
engines, the three principal sound sources of turbine
table C-21 allow for these peaks in several differ-
engines are: the engine casing, the air inlet, and the
ent bands, even though they probably will not
exhaust. Most gas turbine manufactures will pro-
occur in all bands. Because of this randomness of
vide sound power estimates of these sources. How-
peak frequencies, the A-weighted levels may also
ever when these are unavailable the overall PWLs
vary from the values quoted.
of these three sources, with no noise reduction
(2) Engine covers. The engine manufacturer
treatments, are given in the following equations:
sometimes provides the engine casing with a pro-
for engine casing noise,
tective thermal wrapping or an enclosing cabinet,
Lw = 120 + 5 log (rated MW);
(eq C-9)
either of which can give some noise reduction.
for air inlet noise,
Table C-22 suggests the approximate noise reduc-
Lw = 127 + 15 log (rated MW);
(eq C-10)
tion for casing noise that can be assigned to
for exhaust noise
different types of engine enclosures. Refer to the
Lw = 133 + 10 log (rated MW);
(eq C-11)
notes of the table for a broad description of the
enclosures. The values of table C-22 may be
Table C-19. Frequency Adjustments (in dB) for Unmuffled
subtracted from the octave band PWLs of casing
Engine Exhaust Noise.
noise to obtain the adjusted PWLs of the covered
or enclosed casing. An enclosure specifically de-
Octave
Value to
signed to control casing noise can give larger noise
be Subtracted
Frequency
reduction values than those in the table. However
From Overall PWL
Band
it should be noted that the performance of enclo-
(Hz)
(dB)
sures that are supported on the same structure as
the gas turbine, will be limited by structure borne
31
5
sound. For this reason care should be used in
applying laboratory data of enclosure performance
63
9
to the estimation of sound reduction of gas turbine
enclosures.
3
125
b. Exhaust and intake stack directivity. Fre-
7
250
quently, the exhaust of a gas turbine engine is
directed upward. The directivity of the stack pro-
15
500
vides a degree of noise control in the horizontal
1000
19
direction. Or, in some installations, it may be
beneficial to point the intake or exhaust opening
2000
25
horizontally in a direction away from a sensitive
4000
35
receiver area. In either event, the directivity is a
factor in noise radiation. Table C-23 gives the
8000
43
approximate directivity effect of a large exhaust
opening. This can be used for either a horizontal
A-weighted,
12
or vertical stack exhausting hot gases. Table C-23
dB (A)
shows that from approximately 0 to 60 degrees
from the axis of the stack, the stack will yield
C-16
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