Page 168 - Master Handbook of Acoustics

Wind can have a significant effect on sound propagation and plays an important role in analyzing
noise pollution over large distances. For example, it is a common experience to hear sound better
downwind than upwind. As a result, for example, on a windy day, sound can pass over a traffic
barrier that would normally block traffic noise. However, this is not because wind is blowing sound

toward the listener. Rather, wind speed is generally slower close to the ground than at greater heights.
This wind gradient may affect the propagation of sound. Plane waves of sound from a distant source
traveling with the wind will bend down toward the earth. Plane waves traveling against the wind will
bend upward. Figure 8-5 illustrates the effect of wind on the downward refraction case of Fig. 8-4A.
An upwind shadow is created while downwind sound is helpfully redirected downward. This is not
true refraction, but the effect is the same, and it can be as significant as temperature-related refraction.

FIGURE 8-5 Wind gradients can affect the propagation of sound. A shadow sound is created upwind

while good listening conditions exist downwind.

To a lesser extent, if wind moves the air at a certain speed, it is to be expected that the speed of
sound will be affected. For example, if sound travels 1,130 ft/sec and a 10 mi/hr (about 15 ft/sec)
wind prevails, upwind the sound speed with respect to the earth would be increased about 1%, and
downwind it would be decreased by the same amount. This is a small change, but it may also affect
sound propagation.

It is possible, under unusual circumstances, that sound traveling upwind may actually be favored.
For instance, upwind sound is kept above the surface of the ground, minimizing losses at the ground
surface.
In some cases, there may be both temperature and wind changes that simultaneously affect sound
propagation. They might combine to produce a larger effect, or tend to negate one another. Thus,

results are unpredictable. For example, a lighthouse foghorn might be clearly audible near the
lighthouse and also at a distance far away, but mysteriously, not be audible at points between.
Interestingly, in 1862, during a Civil War battle in Iuka, Mississippi, it is believed that two divisions
of Union soldiers did not enter battle because a wind-related acoustical shadow prevented them from
hearing the sounds of the conflict raging 6 miles downwind.