TM S-805-4/AFJMAN 32-1090
where it is required to estimate close-in SPLs at
Cooling Towers.
nearby walls, floors, or in closely confined spaces
(at 3- to 5 foot distances from inlet and discharge
openings).
Octave
Value to
c. Half-speed operation. When it is practical to
be Subtracted
Frequency
do so, the cooling tower fan can be reduced to half
Band
From Overall PWL
speed in order to reduce noise; such a reduction
(Hz)
(dB)
also reduces cooling capacity. Half-speed produces
approximately two-thirds cooling capacity and ap-
31
8
proximately 8- to 10-dB noise reduction in the
63
5
octave bands that contain most of the fan-induced
noise. For half-speed operation, the octave band
125
5
SPLs or PWLs of full-speed cooling tower noise
8
250
may be reduced by the following amounts, where
fB is the blade passage frequency and is calculated
11
500
from the relation fB = No. of fan blades x shaft
1000
15
RPM/GO.
2000
18
Noise reduction
Octave band
due to half-speed:
that contains:
21
4000
8000
1/8 fB
3 dB
29
1/4 fB
6 dB
A-weighted,
9
9 dB
1/2 fB
dB(A)
fB
9 dB
9 dB
2 fB
6 dB
4 fB
3 dB
8 fB
Table C-8. Frequency Adjustments (in dB) for Centrifugal-Fan
Cooling Towers.
If the blade passage frequency is not known, it
may be assumed to fall in the 63-Hz band for
propeller type cooling towers and in the 250-Hz
Value to
Octave
band for centrifugal cooling towers. Waterfall
be Subtracted
Frequency
noise usually dominates in the upper octave bands,
From Overall PWL
Band
and it would not change significantly with reduced
fan speed.
(dB)
(Hz)
e. Limitations.
(1) Design variations. The data given here
6
31
represent a fairly complete summary of cooling
6
63
tower noise, but it must still be expected that
noise levels may vary from manufacturer to manu-
8
125
facturer, and from model to model as specific
10
250
design changes take place. Whenever possible,
request the manufacturer to supply the specific
11
500
noise levels for the specific needs.
1000
13
(2) Enclosed locations. Most of the preceding
discussion assumes that cooling towers will be
2000
12
used in outdoor locations. If they are located inside
18
4000
enclosed mechanical equipment rooms or within
courts formed by several solid walls, the sound
8000
25
patterns will be distorted. In such instances, the
PWL of the tower (or appropriate portions of the
A-weighted,
7
total PWL) can be placed in that setting, and the
dB(A)
enclosed or partially enclosed space can be likened
to a room having certain estimated amounts of
reflecting and absorbing surfaces. Because of the
C-7
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