TM 5-805-4/AFJMAN 32-1090
turn. The second elbow is a 60x24, for this elbow
duct would have to be evaluated.
d. Step (4). In this step the total sound power at
the dimension is 60 inches. The third elbow is the
each of the two diffusers closest to the fan is
"T" from 12x24 to 12x16 over the classroom. In (2)
determined. First the sound power transmitted to
above a power division was taken for this "T"
the room from the fan via the supply duct, is
fitting, however since some energy is also reflected
from the "T" it also acts like an elbow. For the
determined by subtracting the total attenuations
"T" the characteristic dimension is 24". And the
(c. (5) above) from the total sound power of the fan
final elbow is the 12x16 over the class room. For
(b. (1) above) by octave bands. These steps are
this elbow the characteristic dimension is 16". The
shown below.
attenuations for each elbow and the total attenua-
Octave Band Center Frequencies
tions for all of the elbows is given below.
63 125 250 500 1k 2k 4k
Octave Band Center Frequencies
1 0 4 1 0 0 9 6 95
91
89
85
Total Fan Lw
63 125 250 500 1k 2k 4k
(b.(1) above)
24x60
1
2
3
4
5
6
8
44 42 53 >50>50>50>50
Total Duct IL
60x24
2
3
4
5
6
8 10
(c.(5) above)
24x12
1
2
3
4
5
6
8
Resulting Fan
0
1
2
3
4
6
8
16x12
Lw in Class-
60
58
43
4
8 12 16 20 26 34
room
Total IL
(4) End loss. The end reflection loss is taken
Then the sound power of the diffuser is added,
from table 7-5 part A, where the diameter is 10
logarithmically, to the sound power transmitted by
inches. Part A was used since the diffuser was
the fan, as shown below.
mounted in an acoustical tile ceiling. If the ceiling
Octave Band Center Frequencies
was hard (gyp. bd., plaster, concrete, etc.) then
part B would have been used.
63 125 250 500 1k 2k 4k
Octave Band Center Frequencies
Fan Lw in
60
58 43
Classroom
63 125 250 500 1k
2k
4k
43 37 35
40
40
23
Diffuser Lw
16
10
6
2
1
0
0
End Reflection
60
58 44 35
40
40
23
Total
(5) Total IL. The total insertion loss of the
This analysis provides the total sound power into
ducted air supply system is the arithmetical sum,
the room from the operation of one diffuser. It also
in each octave band, of the insertion losses of (1)
indicates the frequency range of the significant
through (4) above.
sources of sound. For example the 63, 125 and 250
Octave Band Center Frequencies
Hz octave bands are dominated by the sound of the
fan, whereas the level of the other octave bands
63 125 250
500
1k
2k
4k
are determined by the operation of the ceiling
Total line ducts
10
10
21 > 5 0 > 5 0 > 5 0 > 5 0
diffuser. This distinction is important since sound
Total branches
14
14
14
14 14 14 14
control for each of these two items are different, as
discussed in 7-6(f) below.
Total elbows
4
8
12
16 20 26 34
e. Step (5). In this step the Room Factor is
End Reflection
16
10
6
2
1
0
0
determined to obtain the "Rel Spl" as described in
>50
Total IL (all duct
Chapter 3. The sound pressure levels in octave
elements)
44
42
53 > 5 0 >50>50
bands (Lp) in the room, from one diffuser, is then
the total sound power from the diffuser plus the
Note again, the insertion loss is limited to approx-
"Rel Spl" as given in equation 3-3. The room
imately 50 dB. This is because flanking sound
volume is 5760 cu. ft., and the acoustic ceiling is
traveling within the duct walls can become a
25% of the room surface area. Thin wall surfaces
significant source of sound when the sound levels
are used on 30% of the room surface area. The
within the air stream have been attenuated a
term "REL SPL" is determined for a distance of 8
great deal. If attenuations greater than 50 dB are
ft. from one diffuser.
required, additional vibration breaks within the
7-11
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