TM 5-805-4/AFJMAN 32-1090
Table 7-2. Approximate Natural Attenuation in Unlined Sheet-Metal Ducts.
Note
Double these values for sheetmetal duct
with external gtassfiber insulation.
Duct Branch Division Loss in dB = 10
number of manufacturers. These are used in air
log(B/T)
distribution systems as a means of providing in-
(eq 7-2)
Where
creased sound attenuation where normal duct at-
B = the cross sectional area of the branch.
tenuation is insufficient. Mufflers are available in
T = the total area of all ducts after branch
modular form to fit a range of cross-sections for
including the branch in question.
rectangular ducts, and are usually readily avail-
Table 7-4 Lists the energy loss in dB for a range
able in lengths of 3, 5, 7, and 10 ft. They are also
of branch area ratios. This power division is
available for circular ducts in a range of diame-
applied equally to each octave band.
ters, and the length is a function of diameter,
being 2 to 3 times the diameter. For the various
is propagating, opens abruptly into a large space,
lengths, and for both rectangular and circular
or room, sound reflection occurs at the end or
ducts the attenuators are available with low,
opening of the duct. The reflected sound is trans-
medium, or high pressure drop for a given veloc-
mitted back into the duct and is attenuated. The
ity, usually expressed in terms of the air velocity
loss in dB associated with this reflection is signifi-
in the duct at the attenuator entrance (i.e. "face
cant at low frequencies, and is given in table 7-5
velocity"). For example, low pressure drop mufflers
for a range of duct diameters. These values apply
will have a pressure drop of less than 0.1 in. of
to a duct outlet flush mounted in a structure, but
water with a face velocity of 1000 ft/min, but high
may also be applied, conservatively, to duct outlets
pressure drop units will have a drop of close to 0.5
flush mounted in a suspended acoustical ceiling.
inch of water at the same velocity. Mufflers with a
These data should not be applied when the duct
higher pressure drop will most often provide
branch dropping from a header duct to a diffuser
greater sound attenuation. The actual installed
or grill is less than 3 to 5 duct diameters, or where
pressure drop will also be a function of both the
flexible ducts are used to connect a diffuser to a
unit location in an air distribution system, and the
main branch. When the duct distribution system
uniformity and turbulence of the entering air flow.
connects to a strip or linear diffuser, the end
Manufacturers provide guidelines for estimating
reflection should be taken as one-half the loss in
the installed operating pressure drop for different
dB given in table 7-5 for the diameter of the duct
conditions. The manufacturers of duct attenuators
serving the linear diffuser section.
also publish information on the sound power gen-
d. Losses at elbows. Sound is reflected or attenu-
erated by flow in the air passages of the attenua-
ated at 90 degree elbows occurring in duct sys-
tor. However, this flow noise, or self noise, is
tems. Table 7-6 lists representative losses in dB
seldom a problem unless the flow velocities in the
for unlined rectangular elbows with turning vanes,
duct are high (e.g. greater than 3,000 FPM), or the
or circular elbows for any size, and for a range of
sound level criteria for the space served calls for
sizes for elbows with one inch thick lining in the
very low levels, such as for a concert hall. Typical
elbow and associated upstream and downstream
dynamic sound insertion loss values for normal
ductwork.
rectangular sound attenuators, with glass fiber
e. Sound attenuators (prepackaged mufflers).
packed linings, for both low and high pressure
Sound attenuators, sometimes termed duct silenc-
drop mufflers are tabulated in table 7-7. These
ers, or mufflers are manufactured specifically for
values are applicable when the flow and the sound
ventilation, and air conditioning systems by a
are in the same direction and the flow velocity is
7-5
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