An
acoustic shadow
or
sound shadow
is an area through which sound waves fail to propagate, due to
topographical
obstructions or disruption of the waves via phenomena such as wind currents, buildings, or sound barriers.
Short-distance acoustic shadow
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A short-distance acoustic shadow occurs behind a building or a sound barrier. The sound from a source is shielded by the obstruction. Due to
diffraction
around the object, it will not be completely silent in the sound shadow. The amplitude of the sound can be reduced considerably, however, depending on the additional distance the sound has to travel between source and receiver.
Long-distance acoustic shadow
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Anomalous sound propagation in the atmosphere can occur in certain conditions of wind, temperature and pressure. Such conditions enable sound to travel in refraction channels over long distances until returning to the Earth's surface, and it thus may not be heard in intervening locations.
[1]
As one website refers to it, "an acoustic shadow is to sound what a
mirage
is to light".
[2]
For example, at the
Battle of Iuka
, a northerly wind prevented General
Ulysses S. Grant
from hearing the sounds of battle and sending more troops.
[3]
Many other instances of acoustic shadowing were prevalent during the American Civil War, including the Battles of
Seven Pines
,
Gaines' Mill
,
Perryville
and
Five Forks
. Indeed, this is addressed in the
Ken Burns's
documentary
The Civil War
, produced by Florentine Films and aired on
PBS
in September 1990.
[4]
Observers of nearby battles would sometimes see the smoke and flashes of light from cannon but not hear the corresponding roar of battle, while those in more distant locations would hear the sounds distinctly.
[5]
Two diarists
John Evelyn
and
Samuel Pepys
heard from London the naval guns of the
Four Days' Battle
, which ranged over the southern North Sea between England and the Flanders coast. However the guns were not heard at all in towns on the coast nearer to the action:
[1]
Being in
my garden
at 6 o'clock in the evening, and hearing the great guns go thick off, I took horse and rode ...next day toward the Downs and seacoast, but meeting the Lieutenant of the Hampshire frigate, who told me what passed, or rather what had not passed, I returned to London, there being no noise, or appearance at Deal, or on that coast of any engagement. Recounting this to his Majesty...he was astonished when I assured him they heard nothing of the guns in the Downs, nor did the Lieutenant who landed there by five that morning.
[6]
?
Friday 1 June 1666, The Diary of John Evelyn
...so far as to yesterday it is a miraculous thing that we all Friday, and Saturday and yesterday, did hear every where most plainly the guns go off, and yet at
Deale
and Dover to last night they did not hear one word of a fight, nor think they heard one gun. This...makes room for a great dispute in philosophy, how we should hear it and they not, the same wind that brought it to us being the same that should bring it to them: but so it is.
[7]
.
?
Monday 4 June 1666, The Diary of Samuel Pepys
Further reading
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]
Garrison Jr., Webb,
Strange Battles of the Civil War
, Cumberland House, 2001,
ISBN
1-58182-226-X
Ross, Charles D. "Civil War Acoustic Shadows". Shippensburg, PA: White Mane Publishing, 2001l
ISBN
978-1572492547
.
See also
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References
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External links
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