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CLIMATIC EFFECTS
When working in large venues or outdoors it’s always necessary to remember that sound propagates through air and is
affected by air temperature, humidity and wind.
EFFECTS OF TEMPERATURE GRADIENTS ON SOUND
PROPAGATION
If sound is propagated over large distances outdoors, its behaviour
may seem erratic. Differences (gradients) in temperature above ground
level will affect propagation as shown in next fi gure. Refraction of
sound refers to its changing direction as its velocity increases slightly
with elevated temperatures. At next fi gure, particular A, we observe
a situation which often occurs at nightfall, when the ground is still
warm. The case shown at B may occur in the morning, and its “skip-
ping” characteristic may give rise to hot spots and dead spots in the
listening area.
EFFECTS OF WIND VELOCITY AND GRADIENTS ON SOUND PROPAGATION
Next fi gure shows the effect wind velocity gradients on sound propagation. The actual velocity of sound in this case is
the velocity of sound in still air plus the velocity of the wind itself. The following fi gure shows the effect of a cross breeze
on the apparent direction of a sound source. The effects shown in these two fi gures may be evident at large rock con-
certs, where the distances covered may be in the 200 - 300 m range.
Effect of wind velocity gradients on sound propagation Effect of cross breeze on apparent direction of sound
EFFECTS OF HUMIDITY ON SOUND PROPAGATION
Contrary to what most people believe, there is more sound attenua-
tion in dry air than in damp air. The effect is a complex one, and it
is shown in the next fi gure. Note that the effect is signifi cant only
at frequencies above 2 kHz. This means that high frequencies will
be attenuated more with distance than low frequencies will be, and
that the attenuation will be greatest when the relative humidity is
20 percent or less.
Effects of temperature gradients on sound propagation
Absorbption of sound in air vs. relative humidity
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