TM S-805-4/AFJMAN 32-1090
forward curved blades delivers 20,000 cubic feet
normally be obtained from a manufacturer, but if
the equipment is not selected the fan power levels
per minute (cfm) to a number of classrooms and
offices against a static pressure of 2.5 in. of water
can be determined from the method given in
with 12 brake horse power. The main supply duct
appendix C. The diffuser or grille sound power
has dimensions of 60 x 24 inches resulting in an
levels can be determined approximately from a
manufacturer's catalog for an identical or similar
air velocity of 2000 ft/min. The closest class room,
type of outlet. Similarly, with a variable volume
which has dimensions of 24 x 24 x 10 ft, is
system the sound power level for the air discharge
supplied by a duct branching off from the main
of an air terminal unit, such as a VAV, or FPT,
header duct. The classroom air is delivered from
can be obtained from a manufacturer's catalog for
four diffusers, 10 inch in diameter each, mounted
in the ceiling, each delivering 500 cfm. Thus the
a given operating condition. For step (3) the
attenuation provided by unlined and lined ducts,
total air supplied by the branch duct is 2000 cfm,
by sound attenuators, elbows, branches, and end
with an air velocity of 1000 ft/min. in the 12 x 24
reflection are added together to find the total
in. duct. The only acoustical material applied to
insertion loss (IL) applicable to the control of the
the room surfaces is a suspended acoustical ceiling
fan sound power as it propagates along the duct
representing approximately 25% of the room sur-
path between the fan and the air outlet. Similarly,
faces. In this example it is assumed that the entire
duct system is internally lined with one inch thick
for the prediction of the noise level in the space
served caused by duct transmitted air discharge
sound absorptive insulation, and the duct cross-
noise from an air terminal unit it is necessary to
sectional flow area is given by the dimensions
determine the insertion loss of the duct distribu-
stated in the schematic figure.
tion system between the terminal unit and the
The tabulated results for this example are as
room air outlet. This IL will consist of the attenua-
follows:
tion of any unlined or lined ductwork, elbows, duct
a. Step (1). In this step an NC 30 is selected as
branches, or splits, and end reflection although
the sound pressure level design criteria.
this latter will not be significant for air terminal
b. Step (2). In this step the sound power level
unit noise. In step (4) the sound power level of all
(Lw), in dB re 10-12 watts, of the supply fan and
sources contributing to the sound power at the air
the diffusers are determined.
outlet are determined and combined to find the
(1) Fan Lw. From equation 10-5 and table
total sound power level, in octave bands, at the air
10-13.
outlet. For step (5) it is necessary to determine the
Octave Band Center Frequencies
Room Factors and the "Rel SPLs" (see Chapter 3)
for the space served and apply the "Rel SPL" to
63 125 250 500 1k 2k 4k
the sound power levels at the air outlet for each
source to obtain the sound pressure level produced
47
43
39
36 34 32 28
Fan Kw
by that outlet at any location in the space served.
43
43
43
43 43 43 43
10log(cfm)
For step (6) the resulting sound pressure levels are
BFI.
2
compared to the selected criteria to determine if
8
8
8
8
8
8
8
201og(p)
additional sound attenuation is necessary.
6
6
6
6
6
6
6
Eff. Corr.
7-6. Calculation Example.
Total Lw of Fan
104 100 96 95 91 89 85
In this example the noise control requirements for
(2) Diffuser Lw, w/o damper. From suppliers
an air distribution system serving a classroom as
catalog.
shown in figure 7-2 are calculated. A fan with
Table 7-6. Losses Caused by Duct Elbows.
Octave Band Frequency (Hz)
Duct Diameter
(inches)
63
125
250
500
1000
2000
4000
8000
8
Lined elbows
5
to
10
0
0
1
2
3
4
6
11
to
20
0
1
2
3
4
6
8
10
21
to
40
1
2
3
4
5
6
8
10
41
to
80
2
3
4
5
6
8
10
12
Unlined elbows
All sizes
1
2
3
3
3
3
3
3
7-9
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