TM 5-805-4/AFJMAN 32-1090
and, where possible, "buffer zones" should be
b. Variations and uncertainties in the individual
placed between the noisy and the quiet rooms. In
data. It is necessary to realize that small errors or
the MER, the noisiest equipment should be moved
discrepancies or uncertainties exist with each bit
away from the common walls that join the critical
of quoted data, and it is not realistic to rely on the
rooms; and when reverberant sound levels pervade
analysis method to the nearest one or two decibels.
the entire MER and control the design, sound
It is largely for that reason that labels such as
absorption may be applied to reduce those rever-
"preferred", "acceptable" and "marginal" are
berant levels.
used. These labels offer some gradations in degree
of reliability of the final values. It is even possible
6-3. Outdoor Sound Problem And Analysis.
that, if the noise levels of certain specific pieces of
The basic procedure here, also, is to follow the
mechanical equipment are much lower than the
sound path from the source to the receiver, apply-
design estimates used in the manual, a design
ing certain adjustments and calculations along the
calculated to be "unacceptable" could actually
turn out to be "acceptable." This result should not
way.
be counted on, however, as a means of avoiding a
a. The SPL or PWL values should be determined
difficult problem. Of course, there is also the
for each source that can radiate noise outdoors.
possibility that in a particular installation many
b. The outdoor sound propagation factors of
of the statistical factors will work together to
distance, air absorption, and anomalous excess
produce a "marginal" condition where the analysis
attenuation should be applied for the prevailing
showed "preferred" or "acceptable" condition, etc.
temperature and humidity conditions.
c. Proper adjustments should be made for ter-
dure will produce a workable design. The methods
rain, vegetation, and barrier effects encountered
and techniques described here are based on many
by the sound.
experiences with noise control problems, and these
d. All the pertinent data are collected and sum-
methods have helped produce many satisfactory or
marized and the outdoor and indoor SPLs are
improved installations. (Sometimes the economics
estimated for the various neighbors of interest.
of a situation may not justify an entirely satisfac-
e. The expected neighbor reaction to the outdoor
tory solution for all concerned, but proper use of
noise is estimated and the expected indoor SPLs
the analysis can bring a desired and predictable
are compared against the indoor noise criterion
improvement.) The manual will have served a
applicable to the neighbor's building.
sufficiently useful purpose if it reveals only that a
f. Available noise control treatments and opera-
problem is so serious that the manual alone
tional changes are considered (set 6-6), if noise
cannot solve the problem and that special assist-
reduction is required to achieve satisfactory re-
ance or special designs may be required.
sponse of the neighbors to the outside noise. The
d. Aids in decision-making. A certain amount of
steps of this procedure are followed in the accom-
judgment may enter into some design decisions. A
panying example.
suggestion is offered here for helping guide the
decision for three types of situations.
6-4. Quality Of Analysis Procedure.
(1) When a particular design involves a cru-
a. How accurate are the data? When numerical
cial area, a conservative approach should be fol-
values are assigned to PWLs, SPLs, TLs, Room
lowed. The design should not be weakened in order
Constants, noise criteria, etc., the question of
to try "to get by" with something simpler.
tolerances arises. Will a given piece of equipment
(2) When a particular design involves a dis-
have exactly the SPL estimated for it? Will the TL
tinct threat to someone's safety or well-being, a
of a wall actually equal the TL assumed for that
conservative approach should be used. Examples
wall in the manual? Will the noise be distributed
could be an employee who might suffer hearing
around the inside of a room in exactly the way it
loss in an MER because a separate control room
is estimated, using the methods and assumptions
was not provided, or a tenant who would not pay
offered in the manual? Is the reaction of "average"
rent because of noise coming from an overhead
people well enough known to predict with accuracy
MER, or a neighbor who might go to court because
the noise levels that they will consider acceptable?
of disturbing noise. On the other hand, noise in a
Will every individual in a group of "average"
corridor or a lobby is of much less concern to
people respond in the manner assumed for the
someone's well-being.
"average" people? The answer is obviously "No"
(3) If a particular design involves a perma-
nent structural member that is not easily modified
for each of these questions! Then, to what extent
or corrected later (in the event it should prove
are the results of the evaluation valid?
6-2
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