TM 5-805-4/AFJMAN 32-1090
Table 5-4. Distance Term (DT), in dB, at Distances of 80 ft. to 8000 ft.
Octave Band Center Frequency. HZ
Distance
Distance Term (DT). d8
D
2000
4000
8000
ft.
31
63
125
250
500
1000
80
36
36
36
36
36
36
36
37
37
89
37
37
37
37
37
37
37
38
39
100
38
38
38
38
38
38
39
39
40
39
39
39
39
39
39
40
40
41
112
40
40
41
41
40
40
40
42
40
125
42
42
41
41
41
41
43
140
41
41
160
42
42
42
42
42
42
43
44
45
44
44
46
180
43
43
43
43
43
45
44
48
44
44
46
44
44
45
200
45
225
45
45
45
45
45
46
46
47
49
46
50
46
46
46
46
47
47
49
250
280
47
47
47
47
48
48
48
50
52
315
48
48
48
48
49
49
50
51
54
355
49
49
49
49
50
50
51
53
55
400
50
50
50
50
51
51
52
54
57
450
51
51
51
52
52
52
53
56
59
500
52
52
52
53
53
53
55
57
61
560
53
53
53
54
54
55
56
59
63
630
54
54
55
55
55
56
57
61
65
57
59
63
68
710
55
55
56
56
56
800
56
56
57
57
57
58
60
64
70
890
57
57
58
58
59
60
62
66
73
1000
58
58
59
59
60
61
63
69
76
1200
60
60
61
61
62
63
66
72
81
1400
61
62
62
63
64
65
68
76
86
1600
62
63
63
64
65
67
70
79
90
1800
64
64
64
65
66
69
73
82
95
2000
65
65
66
66
68
70
74
85
99
2250
66
66
67
68
69
72
77
89
105
2500
67
67
68
69
71
74
79
93
110
2800
68
68
69
70
72
75
82
97
117
3150
69
70
71
72
74
77
84
101
124
3500
70
71
72
73
75
79
87
106
131
4000
71
72
73
75
77
82
91
112
141
4500
72
73
75
76
79
85
94
ll9
151
5000
73
75
76
78
81
87
98
125
161
5500
74
76
77
79
83
89
101
131
170
6000
75
77
79
81
84
92
105
137
180
6500
76
77
79
82
86
94
108
143
189
7000
77
78
81
83
88
96
111
149
199
7500
78
79
82
85
89
98
115
155
208
8000
76
80
82
86
90
100
118
161
218
ket of snow are intermittent, temporary, and of
solid barrier or dense woods can practically elimi-
relatively short total duration, and they should not
nate these paths. In such situations, path 3 may
be counted on for steady-state sound control, even
become significant. Path 3 is made up of relatively
though they offer noticeable attenuation.
low-level sound that is refracted (bent) or scattered
back to earth by numerous small patches of inho-
5-4. Terrain and vegetation.
mogeneous air of varying temperature, speed, di-
Sound transmission near the earth's surface in-
rection, density, etc. Field studies show that when
volves essentially three components of sound
paths 1 and 2 are virtually eliminated, there
paths, shown schematically by figure 5-5. The
remain sound levels that are about 20 to 25 dB
ground-reflected sound (path 2) may arrive at the
below the path 1 and 2 sound levels. These are the
receiver either in phase or out of phase with the
sound levels arriving by way of the numerous
direct sound (path 1) and can either increase or
paths that together make up path 3, as visualized
decrease the received sound level. The ground
in figure 5-5.
surface may be hard or soft (reflective or absor-
Attenuation of woods and vegetation. Table 5-5
bent), and this also affects the phase and magni-
tude of the reflected path. Paths 1 and 2 usually
presents the approximate insertion loss of a 100-
determine the sound levels at the receiver, but a
foot-deep growth of medium-dense woods made up
5-5
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