Acoustic shadow
Template:Short description 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
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
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, which 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]
See also
- Wind gradient § Sound propagation for a fuller explanation of the phenomenon.
- Gobo (recording)
References
Notes
Page Template:Reflist/styles.css has no content.
- ^ a b Page Module:Citation/CS1/styles.css has no content.Peters, R. J. (2013-11-12). Acoustics and Noise Control. Routledge. ISBN 978-1-317-90299-7.
- ^ Page Module:Citation/CS1/styles.css has no content."Science Over the Edge: A Roundup of Strange Science for the Month". January 2004. Retrieved 23 January 2012.
- ^ Page Module:Citation/CS1/styles.css has no content.Keegan, John (2009). The American Civil War: A Military History. New York: Alfred A Knopf. p. Chapter 9. ISBN 978-0307274939.
- ^ Page Module:Citation/CS1/styles.css has no content.Burns, Ken. "The Civil War". Florentine Films. Retrieved 23 January 2012.
{{cite web}}: CS1 maint: deprecated archival service (link) - ^ Discussed in narration at the start of the documentary's fifth episode, "The Universe of Battle."
Further reading
- Garrison Jr., Webb, Strange Battles of the Civil War, Cumberland House, 2001, Template:ISBN
- Ross, Charles D. Civil War Acoustic Shadows. Shippensburg, PA: White Mane Publishing, 2001l Template:ISBN.
