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Suppose you are camping on the shore of a lake which is not too wide,
maybe 1/2 a mile across or so. During the day you can see campers on
the other side of the lake, but you cannot hear them. At night,
however, you can not only see the campers on the other side of the lake
but you can also hear their conversations as they sit around their camp
fire. This phenomena is due to the refraction of sound waves.
The speed of a wave depends on the elastic and inertia properties
of the medium through which it travels. When a wave encounters
different medium where the wave speed is different, the wave will
change directions.
 Most
often refraction is encountered in a study of optics, with a ray of
light incident upon a boundary between two media (air and glass, or air
and water, or glass and water). Snell's law relates the directions of
the wave before and after it crosses the boundary between the two
media.
Notice that as the wavefronts cross the boundary the wavelength changes, but the frequency remains constant.
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In the above animation a spherical wave
pulse propagates in a medium where the wave speed is constant in all
directions. The wave expands outwards as an ever expanding circle, with
the wave traveling at the same speed in all directions. Since the wave
speed is the same everywhere, there is no refraction, and the wave does
not change direction as it propagates.
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In acoustics, however, sound waves usually don't encounter an abrupt
change in medium properties. Instead the wave speed changes gradually
over a given distance. The speed of a sound wave in air depends on the
temperature (c=331 + 0.6 T) where T is the temperature in oC.
Often the change in the wave speed, and the resulting refraction, is
due to a change in the local temperature of the air. For example,
during the day the air is warmest right next to the ground and grows
cooler above the ground. This is called a temperature lapse.
Since the temperature decreases with height, the speed of sound also
decreases with height. This means that for a sound wave traveling close
to the ground, the part of the wave closest to the ground is traveling
the fastest, and the part of the wave farthest above the ground is
traveling the slowest. As a result, the wave changes direction and
bends upwards. This can create a "shadow zone" region into which the
sound wave cannot penetrate. A person standing in the shadow zone will
not hear the sound even though he/she might be able to see the source.
The sound waves are being refracted upwards and will never reach the
observer.
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The animation above shows the
propagation of a spherical wave pulse in a medium where the wave speed
in the x-direction is constant, but where the speed in the vertical
y-direction decreases with height
(c = 1 - 0.05y) As a result the wave changes direction and bends upwards.
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A temperature inversion is when the
temperature is coolest right next to the ground and warmer as you
increase in height above the ground. Since the temperature increases
with height, the speed of sound also increases with height. This means
that for a sound wave traveling close to the ground, the part of the
wave closest to the ground is traveling the slowest, and the part of
the wave farthest above the ground is traveling the fastest. As a
result, the wave changes direction and bends downwards. Temperature
inversions most often happen at night after the sun goes down when the
ground (or water in a lake) cools off quickly, while the air above the
ground remains warm. This downward refraction of sound is why you can
hear the conversations of campers across the lake, when otherwise you
should not be able to hear them. (remember that they can probably hear
you too!)
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The animation above shows the
propagation of a spherical wave pulse in a medium where the wave speed
in the x-direction is constant, but where the speed in the vertical y-direction increases with height
(c = 1 + 0.05y) As a result the wave changes direction and bends downwards.
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