An acoustic shadow or sound shadow is an area through which sound waves fail to propagate, due to

topographical Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...

obstructions or disruption of the waves via phenomena such as wind currents, buildings, or sound barriers.

Short-distance acoustic shadow

Ultrasonography of sludge and gallstones, annotated

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. 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 battle is an occurrence of combat in warfare between opposing military units of any number or size. A war usually consists of multiple battles. In general, a battle is a military engagement that is well defined in duration, area, and forc ...

, a northerly wind prevented General

Ulysses S. Grant Ulysses S. Grant (born Hiram Ulysses Grant ; April 27, 1822July 23, 1885) was an American military officer and politician who served as the 18th president of the United States from 1869 to 1877. As Commanding General, he led the Union Ar ...

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'', produced by Florentine Films and aired on

PBS The Public Broadcasting Service (PBS) is an American public broadcaster and non-commercial, free-to-air television network based in Arlington, Virginia. PBS is a publicly funded nonprofit organization and the most prominent provider of educat ...

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 John Evelyn (31 October 162027 February 1706) was an English writer, landowner, gardener, courtier and minor government official, who is now best known as a diarist. He was a founding Fellow of the Royal Society. John Evelyn's diary, or ...

and Samuel Pepys heard from London the naval guns of the

Four Days' Battle The Four Days' Battle, also known as the Four Days' Fight in some English sources and as Vierdaagse Zeeslag in Dutch, was a naval battle of the Second Anglo-Dutch War. Fought from 1 June to 4 June 1666 in the Julian or Old Style calendar that w ...

, 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

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

Further reading

See also

References

External links