TM 5-805-4/AFJMAN 32-1090
CHAPTER 2
NOISE AND VIBRATION CRITERIA
2-1. General.
noise in this frequency region. And thirdly, the
range over which the curves are defined is limited
This chapter includes data and discussions on
from RC 25 to RC 50 because; 1) applications
generally acceptable indoor noise and vibration
below RC 25 are special purpose and expert con-
criteria for acceptable living and working environ-
sultation should be sought and; 2) spaces above RC
ments. These criteria can be used to evaluate the
50 indicate little concern for the quality of the
suitability of existing indoor spaces and spaces
background sound and the NC curves become more
under design.
applicable.
2-2. Noise Criteria In Buildings.
Table 2-1 lists representative applications of the
Room Criteria (RC) and Noise Criteria (NC) are
RC curves. The evaluation of the RC curves is
two widely recognized criteria used in the evalua-
different than that for the NC curves. In general
tion of the suitability of intrusive mechanical
the sound levels in the octave bands from 250 to
equipment noise into indoor occupied spaces. The
2,000 Hz are lower than those of the NC curves.
Speech Interference Level (SIL) is used to evaluate
Should the octave band sound levels below 250 Hz
the adverse effects of noise on speech communica-
be greater than the criteria a potential "rumble"
tion.
problem is indicated. As a check on the relative
a. NC curves. Figure 2-1 presents the NC
rumble potential, the following procedure is recom-
curves. NC curves have been used to set or
mended:
evaluate suitable indoor sound levels resulting
(1) Sum the sound pressure levels in the
from the operation of building mechanical equip-
octave bands from 31.5 through 250 Hz on an
ment. These curves give sound pressure levels
energy basis (See app B).
(SPLs) as a function of the octave frequency bands.
(2) Sum the sound pressure levels in the
The lowest NC curves define noise levels that are
octave bands from 500 through 4,000 Hz on an
quiet enough for resting and sleeping, while the
energy basis.
upper NC curves define rather noisy work areas
(3) Subtract the high frequency sum (step 2)
where even speech communication becomes diffi-
from the low frequency sum (step 1).
cult and restricted. The curves within this total
(4) If the difference is +30 dB or greater, a
range may be used to set desired noise level goals
positive subjective rating of rumble is expected, if
for almost all normal indoor functional areas.
the difference is between +25 and +30 dB a
In a strict interpretation, the sound levels of the
subjective rating of rumble is possible, if the
mechanical equipment or ventilation system under
difference is less than +20 dB a subjective rating
design should be equal to or be lower than the
of rumble is unlikely. Also indicated on the RC
selected NC target curve in all octave bands in
curves (fig 2-2) are two regions where low fre-
order to meet the design goal. In practice, how-
quency sound, with the octave band levels indi-
ever, an NC condition may be considered met if
cated, can induce feelable vibration or audible
the sound levels in no more than one or two octave
rattling in light weight structures.
bands do not exceed the NC curve by more than
one or two decibels.
ence level (SIL) of a noise is the arithmetic
b. Room criterion curves. Figure 2-2 presents
average of the SPLs of the noise in the 500-, 1000-,
the Room Criterion (RC) curves. RC curves, like
and 2000-Hz octave bands. The approximate condi-
NC curves, are currently being used to set or
tions of speech communication between a speaker
evaluate indoor sound levels resulting from the
and listener can be estimated from table 2-2 when
operation of mechanical equipment. The RC curves
the SIL of the interfering noise is known. Table
differ from the NC curves in three important
2-2 provides "barely acceptable" speech intelligi-
respects. First, the low frequency range has been
bility, which implies that a few words or syllables
extended to include the 16 and 31.5 Hz octave
will not be understood but that the general sense
bands. Secondly, the high frequency range at 2,000
of the discussion will be conveyed or that the
and 4,000 Hz is significantly less permissive, and
listener will ask for a repetition of portions
the 8,000 Hz octave band has been omitted since
missed.
most mechanical equipment produces very little
2-1
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