TM 5-805-4/AFJMAN 32-1090
tive mufflers is included in the TM5-805-9/AFM
5-foot and 7-foot lengths. They are also usually
88-20 manual.
available in two or three "classes," depending on
(2) Dissipative mufflers. As the name implies,
attenuation. The mufflers of the higher attenua-
these mufflers are made up of various arrange-
tion class typically have only about 25 to 35
ments of acoustically absorbent material that actu-
percent open area, with the remainder of the area
ally absorbs sound energy out of the moving air or
tilled with absorption material. The lower attenua-
exhaust stream. The most popular configuration is
tion classes have about 50 percent open area. The
an array of "parallel baffles" placed in the air
mufflers with the larger open area have less
stream. The baffles may range from 2 inches to 16
pressure drop and are known as "low pressure-
inches thick, and are filled with glass fiber or
drop units." The mufflers with the smaller open
mineral wool. Under severe uses, the muffler
area are known as "high pressure-drop units." In
material must be able to withstand the operating
critical situations, muffler "self-noise" may also be
temperature of the air or gas flow, and it must
a problem with these duct mufflers. If high-speed
have adequate internal construction and surface
air is required, the manufacturer can usually
protection to resist the destruction and erosion of
provide self-noise data. When ordering special-
high-speed turbulent flow. These mufflers should
purpose mufflers, one should specify the flow speed
be obtained from an experienced, reputable manu-
and the temperature of the air or gas flow, as
these may require special surface protection and
facturer to insure proper quality of materials,
design, workmanship, and ultimately, long life and
special acoustic filler materials.
durability of the installation.
i. Duct lining. Duct lining is used to absorb
h. Packaged duct mufflers. For ducted air han-
duct-transmitted noise. Typically, duct lining is 1
dling or air-conditioning systems, packaged duct
inch thick. Long lengths of duct lining can be very
mufflers can be purchased directly from reputable
effective in absorbing high-frequency sound, but
acoustical products suppliers. Their catalogs show
the thin thickness is not very effective for low
the available dimensions and insertion losses pro-
frequency absorption. The ASHRAE Guide can be
used to estimate the attenuation of duct lining.
vided in their standard rectangular and circular
Lined 90-degree turns are very effective in reduc-
cross-section mufflers. These packaged duct muf-
flers are sold by most manufacturers in 3-foot,
ing high-frequency noise.
6-5
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