Acoustic location
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Acoustic location is the art and science of using sound to determine the distance and direction of something. Location can be done actively or passively, and can take place in gases (such as the atmosphere), liquids (such as water), and in solids (such as in the earth).
- Active acoustic location involves the creation of sound in order to produce an echo, which is then analysed to determine the location of the object in question.
- Passive acoustic location involves the detection of sound or vibration created by the object being detected, which is then analysed to determine the location of the object in question.
Both of these techniques, when used in water, are known as sonar; passive sonar and active sonar are both widely used.
Acoustic mirrors and dishes, when using microphones, are a means of passive acoustic localisation, but when using speakers are a means of active localisation. Typically, more than one device is used, and the location is then triangulated between the several devices.
Acoustic location in air was used from mid-WWI to the early years of WWII for the passive detection of aircraft by picking up the noise of the engines. It was rendered obsolete before and during WWII by the introduction of radar, which was far more effective but interceptable. Acoustics has the advantage that it can see round corners and over hills.
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[edit] Passive locators
Passive locators listen only; they do not emit sound. There are several basic types of passive locator devices
[edit] Horn type locators
The general principle of a horn type locator is similar to common 19th century listening enhancement devices, such as ear horns, but scaled up in size. One moderately well known example has become infamous as the Japanese War Tuba, due to the apparent resemblance to the western musical instrument.
Of course, the best-known form of this type of locator is the human ear.
[edit] Mirror type locators
A large concrete mirror is used to focus sound on a microphone. Clever design allows for the ability to not only determine range but direction as well.
A few acoustic mirrors still can still be found on the coast of Britain to this day. Such devices are said to have been able to detect aircraft as far away as 20 miles, or more.
[edit] Tomography
This involves the direct placement of sensors on solid materials, such as the ground, building walls, etc. Similar techniques may be used in the ocean. By mapping and timing the response of the various sensors, locations of different events can be located inside of different structures. One typical use in the subsonic range is in the mapping of events inside volcanoes, as seen in the acoustic cross section on Mount St Helens[1]
[edit] Active / Passive locators
Active locators have some sort of signal generation device, in addition to a listening device. The two devices do not have to be located together.
[edit] Sonar
SONAR (SOund Navigation And Ranging) — or sonar — is a technique that uses sound propagation under water (or occasionally in air) to navigate, communicate or to detect other vessels. There are two kinds of sonar — active and passive - the former often being used for active localisation and the latter often for passive localisation. An single active sonar can localise in range and bearing as well as measuring radial speed. However, a single passive sonar can only localise in bearing directly, though target motion analysis can be used to localise in range, given time. Multiple passive sonars can be used for range localisation by triangularisation or correlation, directly.
For more information on this item, see the article on Sonar.
[edit] Biological echo location
Dolphins and whales are well known for their use of echolocation. Bats are also famous for their use of echolocation. For more information of this item, see the article on animal echolocation.
[edit] Seismic surveys
Seismic surveys involve the generation of sound waves to measure underground structures. Source waves are generally generated by percussion mechanisms located near the ground or water surface, typically dropped weights, vibroseis trucks, or explosives. Data are collected with geophones, then stored and processed by computer. Current technology allows the generation of 3D images of underground rock structures using such equipment.
For more information, see Reflection seismology.
[edit] References
[edit] See also
- Animal echolocation: animals emitting sound waves and listening to the echo in order to locate objects or navigate.
- Human echolocation: the use of sound by blind people to navigate.
- Sonar: (sound navigation and ranging) the use of sound to navigate or to locate other watercraft, usually by submarines.
- Echo sounding: listening to the echo of sound pulses to measure the distance to the bottom of the sea, a special case of Sonar.
- Medical ultrasonography: the use of ultrasound echoes to look inside the body
- Sensory substitution
