TM 5-805-4/AFJMAN 32-1090
ceiling tiles with a fissured pattern have an NRC
office enclosures out in the middle of an engine
of .55 to .65 with a Type E (or No. 7) mounting.
room or as a telephone booth enclosure in the
Fiberglass ceiling tiles have an NRC of .95 and
midst of MER noise.
e. Damping materials. Damping is the resistive
are normally used in open office design. Fiberglass
ceiling tiles, however, have no resistance to sound
force to vibratory motion. Sheet metal has low
traveling through them, whereas 5/8 inch standard
damping properties and will ring when impacted.
mineral tiles have a 35 to 40 STC rating for sound
Loaded vinyls and lead both have high mass and
transmission from one office to another where the
high damping and will thud when impacted.
dividing wall stops just above the dropped ceiling
Loaded vinyl has replaced lead in general usage
line. Lab ratings for such walls can be achieved by
because of lower cost, and also because loaded
installing a baffle over the wall in the ceiling
vinyl is available in sheets with an adhesive
backing. The loaded vinyl may be cut with scissors
plenum, or by extending the walls up to the
and directly applied to noisy ducts or sheet metal
underside of the next floor.
at a low cost, usually with good results.
c. Enclosures. From the material given in the
f. Combination. Combination foam absorbers
manual, it is possible to estimate the noise levels
and loaded vinyl barriers in sandwich type con-
inside a solid-wall enclosure that contains a piece
struction are available with adhesive backs, and
of noisy equipment and to estimate the noise
are often used to reduce noise in vehicle cabs or on
levels that will be transmitted from that enclosure
vibrating equipment covers. Lagging is the process
in to the surrounding spaces.
of applying a fibrous or porous material, such as
(1) In acoustic terms, an enclosure is consid-
3-pound density fiberglass, over a noisy duct or
ered to be an almost air-tight chamber containing
pipe, and then covering the fiberglass with sheet
the noise source. Small cracks around doors are
metal or loaded vinyl. This method is useful on
known noise leaks and cannot be tolerated if a
steam piping, valves, ducting, and fans. Lagging
high degree of sound isolation is required. The
may not be used where it could cause excessive
walls of the enclosure must be solid and well-
heat buildup, such as on compressors. Enclosures,
sealed. If air can escape through the enclosure,
made of plywood or sheet metal with fiberglass
sound can escape through the enclosure. A favorite
used as an absorber on the inside, can be effective
analogy in acoustics is that the same amount of
in reducing machinery noise. Enclosures of clear
sound power can pass through a 1-in.2 hole as
plastic panels can be used where visibility is
through a 100-ft.2 6-inch thick solid concrete wall.
required. Ventilation should be provided on com-
A seemingly negligible crack around a door or at
pressors or computer enclosures by installing foam
the ceiling joint of a wall can have much more
or fiberglass lined duct at the bottom for cool inlet
than 1 in.2 of area.
air, and at the top for hot exhaust air. Enclosures
(2) Where openings in an enclosure are re-
should be carefully fitted together with no gaps
which could leak noise. Convenient access panels
quired, they must be given adequate acoustic
treatment in order not to weaken seriously the
should be designed into all noise control enclo-
sures.
effectiveness of the enclosure. Ventilation ducts
g. Mufflers. Mufflers are characterized as either
may be muffled, clearance holes around pipes,
"reactive mufflers" or "dissipative mufflers." Re-
ducts, and conduit must be sealed off airtight, and
active mufflers usually consist of large-volume
passageways for material flow must be protected
chambers containing an internal labyrinth-like
with "sound traps" (mufflers).
d. Barriers, partial-height partitions. Many of-
arrangement of baffles, compartments, and perfo-
rated tubes. Reactive mufflers smooth out the flow
fices, shops, and tool rooms contain barriers or
of impulsive-type exhaust discharge and, by the
partial-height partitions that serve to separate
arrangement of the internal components, attempt
areas or people or functions. When used with
to reflect sound energy back toward the source.
nearby acoustically absorbent ceilings, these parti-
There is usually no acoustic absorption material
tions can provide a small amount of acoustic
inside a reactive muffler. Dissipative mufflers are
separation-possibly 3 to 5 dB of noise reduction in
almost entirely made up of various arrangements
the low-frequency region and 5 to 10 dB in the
of acoustic absorption material that dissipates or
absorbs the acoustic energy.
and the absorption in the area. Where noise
(1) Reactive mufflers. Reactive mufflers are
reduction values of 20 to 30 dB are desired,
used almost entirely for gas and diesel reciprocat-
partial-height partitions would be useless. A cau-
ing engine exhausts. Somewhat more detailed
tion is offered here against use of partial-height
information on the performance and use of reac-
partitions as control room separators or as small
6-4
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