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 Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...

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. As one website refers to it, "an acoustic shadow is to sound what a mirage is to light". 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. 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. 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.Discussed in narration at the start of the documentary's fifth episode, "The Universe of Battle." Two diarists John Evelyn and

Samuel Pepys Samuel Pepys ( ; 23 February 1633 – 26 May 1703) was an English writer and Tories (British political party), Tory politician. He served as an official in the Navy Board and Member of Parliament (England), Member of Parliament, but is most r ...

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:

References

Notes Further reading *Garrison Jr., Webb, ''Strange Battles of the Civil War'', Cumberland House, 2001, *Ross, Charles D. ''Civil War Acoustic Shadows''. Shippensburg, PA: White Mane Publishing, 2001l .

External links

Acoustic Shadows - What is an acoustic shadow and how does it work? - Lisa

Hearing Waves Acoustics {{Topography-stub

Short-distance acoustic shadow

Long-distance acoustic shadow

See also

References

External links