TM 5-805-4/AFJMAN 32-1090
Table G23. Approximate Directivity Effect fin dB) of a Large Exhaust Stack Compared to a Nondirectional Source of the Same Power.
Relative Sound Level for
Indicated Angle From Axis
Octave
135 and
Band
oa
largera
0
45
60
90
(Hz)
-2
-3
5
2
31
8
-3
-4
63
8
5
2
-4
-6
125
8
5
2
-6
-8
8
6
2
250
-8
-10
500
9
6
2
-10
-13
1000
9
6
1
-12
-16
7
0
2000
10
-14
-18
4000
10
7
-1
-16
-20
8000
10
7
-2
a
For air intake openings subtract 3 dB from the values in the 90 and
135 columns, i.e., -2 -3 = -5 dB for 31 cps at 90.
the engine through a large gear, and the gear and
casing. Octave band corrections to the overall
the generator may together produce somewhat
PWL are given in table C-29.
indistinguishable noise in their compartment,
C-20.
Transformers.
which frequently is separated by a bulk head from
The National Electrical Manufacturers Association
the engine compartment. Table C-28 gives an
(NEMA) provides a means of rating the noise
approximation of the overall PWL of several gen-
output of transformers. The NEMA "audible sound
erators. It is not claimed that this is an accurate
level," as it is called in the standard, is the
estimate, but it should give reasonable working
average of several A-weighted sound levels mea-
values of PWL. It is to be noted that the PWL of
sured at certain specified positions. The NEMA
the generator is usually less than that of the drive
sound level for a transformer can be provided by
gear and less than that of the untreated engine
C-20
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