TM 5 - 8 0 5 - 4 / A F J M A N
32-1090
Table 7-7. Representative IL Values for Sound Attenuators.
Octave Band Center Frequency (Hz)
Length
Type
in Ft.
63
125
250
500
1000
2000
4000
8000
4
Low
3
2
8
14
21
22
14
11
Pressure
5
3
5
11
23
31
34
19
13
Drop
7
5
9
16
32
41
43
24
16
10
15
6
22
37
52
53
35
23
6
Standard
3
3
13
23
33
33
23
14
Pressure
5
4
11
20
37
44
44
36
22
Drop
7
6
13
28
42
47
47
45
30
10
8
15
50
35
56
57
57
40
Octave Band Center Frequencies
sum of the attenuation due to the internal lining
(table 7-3) and the natural attenuation (table 7-2).
63 125 250 500 1k 2k 4k
The attenuation for the 10" round duct was ob-
tained from a suppliers catalog. The total attenua-
Lw of One
tion for all of the duct elements is limited to
N/A
43
37
35
40
40
23
Diffuser
approximately 50 dB because this is usually the
maximum that can be obtained in a connected
fan noise provided by the duct system including
system due to structural flanking down the duct
the lined ductwork, the duct branches, the elbows,
wall.
and the end reflection loss are determined. This
(2) Branches (To one diffuser). The branch
consists of determining the insertion loss (IL) for
attenuation is determined by equation 7-2 or table
each element and then summing all of the inser-
7-4. With a branch area of 2 sq. ft. (i.e. 24x24) and
tion losses.
the area after the branch of 10 sq. ft. (i.e. 24x60)
(1) Lined Ductwork IL in dB. The insertion
the area ratio of the branch is 2/(10+2) or 0.167.
losses from each rectangular duct element is deter-
The sound power loss at for the take-off in the
mined from tables 7-2 and 13-3. The results of
corridor is approximately 8 dB in accordance with
each element is summed by octave bands to pro-
equation 7-2. The power division in the "T" and
vide the total duct attenuation in dB in each
diffuser take-off are determined in a similar fash-
octave band.
ion and are approximately 3 dB each (i.e. 50%
each way). Therefore the total attenuation due to
Octave Band Center Frequencies
all the branching is approximately 14 dB in all of
the octave bands.
63
125
250
500
lk
2k
4k
Octave Band Center frequencies
20' of 24x60
3
4
7
18 32 30 30
63 125 250 500 1k 2k 4k
9' of 12x24
12 31 27 19
3
2
5
10' of 12x16
4
3
7
16 41 41 28
Branch att. in
3' of 10"
0
1
2
5
6
6
5
corridor
8
8
8
8
8
8
10
10 21 >50 >50 >50 >50
Total
3
3
3
3
3
3
3
"T" in room
Diffuser Take-off
3
3
3
3
3
3
3
It should be noted that table 7-3 does not contain
Total (dB)
14
14
14
14
14
14
14
entries for the 24x60 and 12x16 ducts. The attenu-
ation values for these ducts are obtained by inter-
(3) Four elbows. There are four elbows be-
polation. For example the attenuation for the
tween the fan and the classroom. The attenuation
24x60 duct is the average value of the 24x48 and
of each of these can be found from table 7-6. The
24x72 ducts. For the 24x60 duct the full 20 feet of
first elbow is the 24x60 inch elbow that goes from
length is used since the elbow breaks the length
the vertical to the horizontal at the fan outlet. For
into two lengths less than 10 feet each. Also it
this elbow the duct diameter used is 24 inches
should be noted that the total attenuation is the
since this is the dimension in the plane of the
7-10
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