TM 5-805-4/AFJMAN 32-1090
Figure 5-1. Inverse Square Law of Sound Propagation.
sound pressure level. Consideration for each of
or obtained from tables 5-3 and 5-4. This equation
these cases is given below.
applies when d1 is greater than the physical
(1) SPL from known Lw. Equation 5-1 com-
dimension of the sound source.
bines the effects of distance, atmospheric attenua-
5-3. Atmospheric Effects.
tion and anomalous attenuation for the calculation
Wind and temperature variations can cause bend-
of sound pressure level when the sound power
ing of sound waves and can influence changes in
level of the source is known.
sound levels at large distances. These are nor-
mally short-term effects and do not provide reli-
(eq 5-1)
able noise control. However, they help explain the
Where Lp is the SPL at distance d (in ft.) from a
variations that occur in outdoor sound propagation
sound source whose PWL is Lw,
is the molecu-
lar absorption coefficient (in dB/1000 ft.) from
and measurements.
table 5-1, and
is the anomalous excess attenua-
a. Wind effects. A steady, smooth flow of wind,
tion (in dB/1000 ft.) from table 5-2. Equation 5-1
equal at all altitudes, would have no noticeable
is simplified to equation 5-2:
effect on sound transmission. In practice, however,
Lp + Lw - DT,
(eq 5-2)
wind speeds are slightly higher above the ground
than at the ground, and the resulting wind speed
where DT, in decibels, is called the "distance
gradients tend to "bend" sound waves over large
term" and is defined by equation 5-3:
distances. Sound traveling with the wind is bent
down to earth, while sound traveling against the
(eq 5-3)
wind is bent upwards above the ground. The
Table 5-3 gives tabulated data for the distance
downwind and upwind effects are summarized in
term out to a distance of 80 feet. For such short
the next two paragraphs. Irregular, turbulent, or
distances, the molecular absorption and anomalous
gusty wind provides fluctuations in sound trans-
excess attenuation are usually negligible and are
mission over large distances (possibly because of
ignored in this simplifying table. The table 5-3
partial wavelength interference of various paths
values can be applied to all octave bands for most
taken by various sound rays of the total beam).
close-in situations. However, at larger distances
The net effect of these fluctuations may be an
the absorption and attenuation effects become
average reduction of a few decibels per 100 yards
significant, and table 5-4 gives the distance terms
for gusty wind with speeds of 15 to 30 mph.
as a function of the octave bands for distances
However, gusty wind or wind direction cannot be
from 80 feet out to 8000 feet.
counted on for noise control over the lifetime of an
(2) SPL at another distance from known SPL.
installation.
Equation 5.4 can be used to calculate the sound
b. Downwind effect. Figure 5-2 illustrates the
pressure level at any distance if the sound pres-
principal influence of downwind on sound propaga-
sure level at one distance is given.
tion. When there is no wind, the principal sound
Lp2 = Lp1 - DT2 + DT1,
(eq 5-4)
arrives at the receiver by path 1. Along this path,
where Lp1 is the known SPL at distance d,, and
the ground, vegetation, and trees can absorb some
Lp2 is the wanted SPL distance d2. The distance
of the sound. During downwind conditions, how-
terms DT1 and DT2 are evaluated in equation 5-3
ever, the path 2 sound (that normally travels
5-2
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