TM 5-805-4/AFJMAN 32-1090
unsatisfactory), a conservative approach should be
(9) The basic elements of acoustics should be
used. A poured concrete floor slab is not easily
understood and used in order to work intelligently
replaced by a new and heavier floor slab. On the
with SPL and PWL data for many types of electri-
other hand, a lightweight movable partition can be
cal and mechanical noise sources, know the effects
changed later if necessary. A muffler can be added
of distance (both indoors and outdoors), appreciate
later or enlarged later if necessary. Compromises
the significance of noise criteria, and be able to
may be justified if the compromised member can
manipulate acoustic data in a meaningful and
be corrected later at relatively small extra cost.
rational way. A few of these items are discussed
Compromises should not be made when the later
below.
corrective measure is impossible or inordinately
b. Absorbers. Acoustical ceiling and wall panels
expensive.
are the most common sound absorbers. Absorbers
are rated by the ratio of noise absorbed to noise
6-5. Noise Control Treatments.
impacted on the absorber's surface. A coefficient of
a. General applications. A primary advantage of
1.0 indicates 100 percent absorption; a coefficient
the manual and of the various noise-analysis
of zero indicates 0 percent absorption. Noise Re-
procedures offered in the manual is that it ele-
duction Coefficient (NRC) is the average coefficient
vates the awareness of the architect and engineer
of sound absorption measured at 250 Hz, 500 Hz, 1
to problems of noise and vibration. This is an
kHz, and 2 kHz. Sound absorption should be
important first step to noise control. Without
designed to absorb the frequencies of the sound
awareness, the noise problem is ignored in the
striking it. For example, a transformer enclosure
design, and later problems in remedial steps are
should have an absorption coefficient of at least
compounded. In most building situations, noise
.75 in the 125 Hz band (the sound of electrical
control is provided by application of the basic
hum is twice the 60 cycle powerline frequency).
contents of the manual:
Auditoriums should have even absorption over a
(1) Adequate wall and floor-ceiling construc-
wide frequency range for a balanced reverberant
tions should be designed to contain the noise and
sound.
limit its transmission into adjoining areas.
(1) Test methods. There are three basic mount-
(2) Acoustic absorption material should be
ings for sound absorption tests used by ASTM
used in either or both the sound transmitting room
(ASTM E 795-83): 1) Type A.-hard against a
and the sound receiving room to absorb some of
concrete surface (formally designated as No. 1), 2)
the sound energy that "bounces" around the room.
Type D.-with a 3/4-inch airspace behind the test
Quantitative data and procedures for incorporating
material, such as a wood furring strip (formally
sound absorption materials are included in the
designated as No. 2), and 3) Type E.-with a
tables and data forms.
16-inch airspace behind the test material, such as
(3) Transmission loss data should be used to
an acoustical ceiling (formally designated as No.
select various types of construction materials for
7). See table 5-1 for absorption coefficients of some
the design of noise enclosures.
typical building materials.
(4) Building layouts should be modified in an
(2) Core material. Absorbers consist of a core
attempt to redistribute noise sources in a more
material, usually fibrous or porous, with a facing
favorable arrangement, bring together noisy areas
as a cover. Fibrous cores are typically 1 inch thick
in one part of a building and quiet areas in a
for general noise control, and 2 inches thick for
different part of the building (to minimize their
auditoriums, music, or low frequency absorption. If
reaction on one another), and use less critical
a minimum 2 inch airspace is provided behind a 1
"buffer zones" to separate noisy and quiet areas.
inch core, the effect is approximately equivalent to
(5) Vibration isolation mounts should be used
a 2 inch thick core. One inch thick fibrous cores
for the support of mechanical or vibrating equip-
have an NRC of .75, and 2 inch thick fibrous cores
ment. Details of such mounts are given in chapter 9.
have a NRC of .95 with a Type D mounting.
(3) Facings. Facings over the acoustical core
(6) Mufflers should be used to control noise
transmission through air passageways.
material serve as both a visual and a protective
(7) Duct lining treatments should be used to
screen. They are typically cloth, perforated vinyl,
control noise transmission through ducted connec-
wood screens, or expanded metal. Expanded metal,
such as plasterer's metal lath, is relatively vandal-
tions.
proof. Expanded or perforated metal facings should
(8) Specifications should be used to limit the
noise output of purchased equipment for use in the
be at least 23 percent open, 33 percent is preferable.
(4) Ceilings. Acoustical ceilings are of two
building; this is suggested and discussed briefly in
basic types: mineral fiber and fiberglass. Mineral
chapter 10 of this manual.
6-3
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