TM 5-805-4/AFJMAN 32-1090
CHAPTER 5
SOUND PROPAGATION OUTDOORS
5-1. Introduction.
the source. An equation and a table incorporating
this effect are given in paragraph 5-2d.
Mechanical equipment such as cooling towers,
b. Molecular absorption. In addition to the re-
rooftop units and exhaust fans are commonly
duction due to the inverse square law, air absorbs
located outdoors. In addition there is an increasing
sound energy, and that the amount of absorption
trend to placing additional mechanical equipment
is dependent on the temperature and humidity of
outdoors. Unacceptable noise from electrical or
the air and the frequency of the sound. Table 5-1
mechanical equipment, whether located indoors or
gives the molecular absorption coefficients in dB
outdoors, may be strong enough to be transmitted
per 1000-foot distance of sound travel for a useful
to neighbor locations. The sound transmission
range of temperature and relative humidity of the
paths are influenced by three broad types of
octave frequency bands. A "standard day" is fre-
natural effects: distance effects, atmospheric ef-
quently defined as having a temperature of 59 deg.
fects, and terrain and vegetation effects. In addi-
F and a relative humidity of 70 percent. For
tion, structures such as barriers and buildings
long-time average sound propagation conditions,
influence the transmission of sound to the neigh-
bor positions. The quantitative values of these
the molecular absorption coefficients for standard
natural effects and structural interferences in out-
day conditions should be used. For any specific
door sound propagation are given in this chapter.
application of measured or estimated SPL for
known temperature and humidity conditions, the
5-2. Distance Effects.
table 5-1 values should be taken into account.
Acoustical energy from a source spreads out as it
travels away from the source, and the sound
fects of wind speed, wind direction, and thermal
pressure level drops off with distance according to
gradients in the air can cause large differences in
the "inverse square law." This effect is common to
sound transmission over large distances. These are
all types of energy propagation originating from
discussed briefly under "atmospheric effects" in
an essentially point source and free of any special
section 5-3. Almost all the time, however, there
focusing. In addition, the air absorbs a certain
are small-scale influences of these atmospheric
amount of sound energy by "molecular absorp-
factors. Even under fairly stable conditions for
tion," and small amounts of ever-present air move-
sound propagation through the air, small amounts
ment and inhomogeneities give rise to "anomalous
of diffraction, refraction (bending), and sound in-
excess attenuation." These three distance effects
terference occur over large distances as a result of
are summarized in the following paragraphs.
small wind, temperature, and humidity differences
a. Effect of distance. Figure 5-1 illustrates the
in the air. These are combined into "anomalous
"inverse square law" for drop-off of SPL with
excess attenuation" which is applied to long-term
distance. A point source of sound is shown at point
sound level estimates for average-to-good sound
"X", and the rays show the path of an element of
propagation conditions. Table 5-2 gives the values
sound energy traveling away from the source. At
of anomalous excess attenuation, in dB per 1000-
the distance "d" from the source, the sound energy
foot distance. These are conservative average val-
is assumed uniformly spread over the small area
ues; higher values than these have been measured
"A" (which is the product of the two lengths "a"
in long-time studies of sound travel over a variety
and "b"). At twice the distance, 2d, the lengths a
of field conditions. Anomalous excess attenuation
and b are expanded to 2a and 2b, and the result-
helps explain the fact that measure SPLs at large
ing area over which the sound is now spread has
distances are frequently lower than estimated
become 4A, 4 times the area back at distance d.
SPLs even when sound propagation conditions
Sound pressure level is related to the "energy per
seem quite good.
d. Estimating outdoor sound levels. The sound
unit area" in the sound wave; so, in traveling
twice the original distance from the source, the
level, at a distance, can most readily be calculated
energy per unit area has decreased by a factor of 4
if the sound power level (Lw) is known. In some
which corresponds to a reduction of 6 dB in the
cases the sound power is not known, however the
sound pressure level. Simply illustrated, this is the
sound pressure level (Lp) at a given distance is
"inverse square law"; that is, the SPL decreases at
known. In this case the sound pressure level at
the rate of 6 dB for each doubling of distance from
different distance can be derived from the known
5-1
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