TM 5 - 8 0 5 - 4 / A F J M A N
32-1090
A. Proper and Improper Airflow Conditions to an
Outlet
B. Effect of Proper and Improper Alignment of
Flexible Duct Connector
Figure 7-1. Good and Poor Air Delivery Conditions to Air Outlets.
unshaded portion, but in the shaded portion the
duct branching, (3) by elbows, and (4) by end
attenuation should not be applied for more than 10
ft. in any straight duct run between elbows or
a. Propagation in the duct distribution system.
turns. Note these attenuation factors are for the
Noise attenuation with propagation in a duct
effects of the internal lining only and do not
system results from; 1) natural energy losses as
include the effects of natural attenuation as given
sound is transmitted through sheet metal duct
on table 7-2.
walls to the space through which the duct passes;
and 2) by absorption of energy in the internal
For the bands centered at 63, and 125 Hz the total
glass fiber lining of the sheet metal duct.
attenuation for a lined duct is a sum of the
(1) Unlined duct. Table 7-2 lists the natural
natural (table 7-2) and lined duct (table 7-3)
attenuation, in dB/ft for unlined rectangular sheet
attenuations. For example in a 24 x 24 inch duct
metal ducts without external thermal insulation.
the attenuation in the 63 Hz octave is 0.05 dB/ft
This attenuation, attributed to sound transmission
due to internal lining (table 7-3), plus 0.3 dB for
through the duct walls, can be significant, in the
the loss associated with sound transmission
low frequency range, for long lengths of duct. The
through the duct wall (table 7-2 with a P/A ration
attenuation values are given as a function of the
of 0.17).
ratio of the duct perimeter P and the duct cross
b. Sound Transmission loss at duct branches.
sectional area A. These data are applicable only to
When one duct branches off from a main, or
normal sheet metal rectangular ducts typically
header, duct the sound power propagating in the
used in the air conditioning industry. These data
main duct up to the branch point is assumed to
should not be used for ducts using metal heavier
divide into the branch ducts in accordance with
than 16 ga.; for circular ducts which are relatively
the ratio of the cross sectional area of each branch,
stiff) or for ducts made of glass fiber board.
to the total cross-sectional area of all the ducts
(2) Internally lined duct. The octave band at-
leaving the branch point. Thus, following any
tenuation, in dB/ft, that is expected due to absorp-
branch point the energy transmitted into any one
tion of sound by 1 inch thick internally duct
duct is less than the initial sound power in the
lining, is given in table 7-3. As noted on the table,
main duct before the branch point, and this loss,
the data can be used for any length of duct in the
in dB, for each branch duct is given as:
7-4
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