TM 5-805-4AFJMAN 32-1090
Table 5-2. Values of Anomalous Excess Attenuation per 1000 ft.
matically in figure 5-3. When wind speed profiles
are known, the distance to the shadow zone can be
Anomalous Excess
estimated, but this is an impractical field evalua-
tion. It is sufficient to realize that the shadow zone
Attenuation,
Band,
can account for up to about 25-dB sound level
HZ
dB/lOOO ft.
reduction and that this can occur at distances
greater than about 1000 feet for wind speeds above
31
0.3
about 10 to 15 mph.
63
0.4
d. Temperature effect. Constant temperature
with altitude produces no effect on sound transmis-
125
0.6
sion, but temperature gradients can produce bend-
250
0.8
ing in much the same way as wind gradients do.
Air temperature above the ground is normally
500
1.1
cooler than at the ground, and the denser air
1000
1.5
above tends to bend sound waves upward, as in
part A of figure 5-4. With "temperature inver-
2000
2.2
sions," the warm air above the surface bends the
4000
3.0
sound waves down to earth. These effects are
negligible at short distances but they may amount
8000
4.0
to several dB at very large distances (say, over a
half mile). Again, little or no increase is caused by
thermal gradients (compared to homogeneous air),
but there may be a decrease in sound levels.
snow are the various forms of precipitation to
consider. These have not been studied extensively
in their natural state, so there are no representa-
tive values of excess attenuation to be assigned to
them. Rain, hail, and sleet may change the back-
ground noise levels, and a thick blanket of snow
provides an absorbent ground cover for sound
traveling near the ground. Precipitation or a blan-
Table 5-3. Distance Term (DT), in dB, to a Distance of 80 ft.
Distance
Distance
Distance
Distance
Term, DT
D
Term, DT
dB
dB
ft.
ft.
1.3
0
18
23
1.8
3
20
24
2.5
6
22.5
25
3.2
8
25
26
4
10
28
27
5
12
31.5
28
6.3
14
35.5
29
8
16
40
30
9
17
45
31
10
18
50
32
11
19
56
33
12.5
20
63
34
14
21
71
35
16
22
80
36
5-4
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