TM 5-805-4/AFJMAN 32-1090
Table C-21. Frequency Adjustments (in dB) for Gas Turbine Engine Noise Sources.
Octave
Value To Be Subtracted From Overall PWL, in dB
Frequency
Band
Inlet
Casing
(Hz)
12
10
19
31
8
7
18
63
6
5
17
125
6
4
17
250
7
4
14
500
8
9
4
1000
3
11
4
2000
3
15
4
4000
6
21
4
8000
A-weighted,
2
0
4
dB(A)
(DRPR) (splash-proof or weather-protected) and "to-
b. DRPR motors. The overall SPLs of DRPR
tally enclosed fan-cooled" (TEFC) motors. Noise
motors, at the normalized 3 foot condition, follow
levels increase with power and speed.
approximately the relationships of equations C-14
a. TEFC motors. The overall SPLs of TEFC
and C-15.
motors, at the normalized 3 foot condition, follow
for power ratings under 50 hp,
approximately the relationships of equations C-12
Lp = 10 + 17 log hp + 15 log RPM. (eq C-14)
and C-13.
for power ratings above 50 hp,
for power ratings under 50 hp,
Lp = 22 + 10 log hp + 15 log RPM. (eq C-15)
Lp = 15 + 17 log hp + 15 log RPM. (eq C-12)
For motors above 400 hp, the calculated noise
for power ratings above 50 hp,
value for a 400 hp motor should be used. The
Lp = 27 + 10 log hp + 15 log RPM (eq C-13)
octave band corrections for DRPR motors are given
where "hp" is the nameplate motor rating in
in table C-25. The data of equations C-14 and
horsepower and "RPM" is the motor shaft speed.
C-15 and table C-25 are summarized in figure
For motors above 400 hp, the calculated noise value
C-7, which gives the SPLs at 3 foot distance for
for a 400-hp motor should be used. These data are
DRPR motors over a range of speeds and powers.
not applicable to large commercial motors in the
C-17. Steam Turbines.
power range of 1000 to 5000 hp. The octave band
corrections for TEFC motors are given in table
Noise levels are found generally to increase with
C-24. The data of equations C-12 and C-13 and
increasing power rating, but it has not been
table C-24 are summarized in figure C-6, which
possible to attribute any specific noise characteris-
gives the SPLs at 3 foot distance for TEFC motors
tics with speed or turbine blade passage frequency
for a working range of speeds and powers. Some
(because these were not known on the units mea-
motors produce strong tonal sounds in the 500,
sured). The suggested normalized SPLs at 3 foot
1,000, or 2,000 Hz octave bands because of the
distance are given in figure C-8 and table C-26.
cooling fan blade frequency. Table C-24 and figure
C-18. Gears.
C-6 allow for a moderate amount of these tones,
but a small percentage of motors may still exceed
It is generally true that the noise output increases
these calculated levels by as much as 5 to 8 dB.
with increasing speed and power but it is not
When specified, motors that are quieter than these
possible to predict in which frequency band the
calculated values by 5 to 10 dB can be purchased.
g e a r tooth contact rate or the "ringing fre-
C-18
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