TM 5-805-4/AFJMAN 32-1090
Where TLob is the resulting octave band transmis-
around A-weighted sound levels. For example:
sion loss and TL1, TL2 & TL3 are the 1/3 octave
"The sound level at the property line between a
band transmission losses.
manufacturing or industrial plant and a residen-
e. A-, B- & C-weighted sound levels. Sound level
tial community must not exceed 65 dB(A) during
meters are usually equipped with "weighting cir-
daytime or 55 dB(A) during nighttime." Of course,
cuits" that tend to represent the frequency charac-
other sound levels and other details might appear
teristics of the average human ear for various
in a more complete noise code. Sound levels taken
sound intensities. The frequency characteristics of
on the A-, B-, and C-weighted networks have
the A-, B-, and C-weighting networks are shown in
usually been designated by dB(A), dB(B), and
figured B-2. The relative frequency response of
dB(C), respectively. The parentheses are some-
the average ear approximates the A curve when
times omitted, as in dBA. The weighting networks,
sound pressure levels of about 20 to 30 dB are
in effect, discard some of the sound, so it is
heard. For such quiet sounds, the ear has fairly
conventional not to refer to their values as sound
poor sensitivity in the low-frequency region, The B
pressure levels, but only as sound levels-as in "an
curve represents approximately the frequency re-
A - w e i g h t e d sound level of 76 dB(A)." High-
sponse of hearing sensitivity for sounds having 60-
intensity, high-frequency sound is known to con-
to 70-dB sound pressure level, and the C curve
tribute to hearing loss, so the A-weighted sound
shows the almost flat frequency response of the
level is also used as a means of monitoring factory
ear for loud sounds in the range of about 90 to 100
noise for the hearing damage potential. It is very
dB. Annoyance usually occurs when an unwanted
important, when reading or reporting sound levels,
noise intrudes into an otherwise generally quiet
to identify positively the weighting network used,
environment. At such times, the ear is listening
as the sound levels can be quite different depend-
with a sensitivity resembling the A curve. Thus,
ing on the frequency content of the noise mea-
judgment tests are often carried out on the loud-
sured. In some cases if no weighting is specified,
ness, noisiness, annoyance, or intrusiveness of a
A-weighting will be assumed. This is very poor
sound or noise related to the A-weighted sound
practice and should be discouraged.
level of that sound. The correlation is generally
f. Calculation of A-weighted sound level. For
quite good, and it is a generally accepted fact that
analytical or diagnostic purposes, octave band
the high-frequency noise determined from the A-
analyses of noise data are much more useful than
weighted sound level is a good indicator of the
sound levels from only the weighting networks. It
annoyance capability of a noise. Thus, noise codes
is always possible to calculate, with a reasonable
and community noise ordinances are often written
degree of accuracy, an A-weighted sound level
A-WEIGHTED
OCTAVE
FILTER
FREQUENCY
RESPONSE
BAND
(dB)
(Hz)
-39
31
-26
63
-16
125
-9
250
-3
500
0
1000
+1
2000
4000
+1
8000
-1
Figure B-1. Approximate Electrical Frequency Response of The A-, B-, and C-weighted Networks of Sound Level Meters.
B-7
Previous Page
