Acoustic location

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Swedish soldiers operating an acoustic locator in 1940

Acoustic location is the 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 analyzed 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 analyzed 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 localization, but when using speakers are a means of active localization. Typically, more than one device is used, and the location is then triangulated between the several devices.

As a military air defense tool, passive acoustic location was used from mid-World War I[1] to the early years of World War II to detect enemy aircraft by picking up the noise of their engines. It was rendered obsolete before and during World War II by the introduction of radar, which was far more effective (but interceptable). Acoustic techniques had the advantage that they could 'see' around corners and over hills, due to sound refraction.

The civilian uses include locating wildlife[2] and locating the shooting position of a firearm.[3]

Military use[edit]

T3 sound locator 1927

Military uses have included locating submarines[4] and aircraft.[5] The first use of this type of equipment was claimed by Commander Alfred Rawlinson of the Royal Naval Volunteer Reserve, who in the autumn of 1916 was commanding a mobile anti-aircraft battery on the east coast of England. He needed a means of locating Zeppelins during cloudy conditions and improvised an apparatus from a pair of gramaphone horns mounted on a rotating pole. Several of these equipments were able to give a fairly accurate fix on the approaching airships, allowing the guns to be directed at them despite being out of sight.[6] Although no hits were obtained by this method, Rawlinson claimed to have forced a Zeppelin to jettison its bombs on one occasion.[7]

The air-defense instruments usually consisted of large horns or microphones connected to the operators' ears using tubing, much like a very large stethoscope.[8][9]

Sound location equipment in Germany, 1939. It consists of four acoustic horns, a horizontal pair and a vertical pair, connected by rubber tubes to stethoscope type earphones worn by the two technicians left and right. The stereo earphones enabled one technician to determine the direction and the other the elevation of the aircraft.

Most of the work on anti-aircraft sound ranging was done by the British. They developed an extensive network of sound mirrors that were used from World War I through World War II.[10][11] Sound mirrors normally work by using moveable microphones to find the angle that maximizes the amplitude of sound received, which is also the bearing angle to the target. Two sound mirrors at different positions will generate two different bearings, which allows the use of triangulation to determine a sound source's position.

As World War II neared, radar began to become a credible alternative to the sound location of aircraft. For typical aircraft speeds of that time, sound location only gave a few minutes of warning.[5] The acoustic location stations were left in operation as a backup to radar, as exemplified during the Battle of Britain.[12] Today, the abandoned sites are still in existence and are readily accessible.[10]

After World War II, sound ranging played no further role in anti-aircraft operations.[citation needed]

Active / passive locators[edit]

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.

Sonar[edit]

SONAR or sonar (sound navigation and ranging) 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. A single active sonar can localize in range and bearing as well as measuring radial speed. However, a single passive sonar can only localize in bearing directly, though target motion analysis can be used to localize in range, given time. Multiple passive sonars can be used for range localization by triangulation or correlation, directly.

Biological echo location[edit]

Dolphins, whales and bats use echolocation to detect prey and avoid obstacles.

Time-of-arrival localization[edit]

Having speakers/ultrasonic transmitters emitting sound at known positions and time, the position of a target equipped with a microphone/ultrasonic receiver can be estimated based on the time of arrival of the sound. The accuracy is usually poor under non-line-of-sight conditions, where there are blockages in between the transmitters and the receivers. [13]

Seismic surveys[edit]

A three-dimensional echo-sounding representation of a canyon under the Red Sea by survey vessel HMS Enterprise

Seismic surveys involve the generation of sound waves to measure underground structures. Source waves are generally created 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.

Ecotracer[edit]

Ecotracer is an acoustic locator that was used to determining the presence and position of ships in fog. Some could detect targets at distances up to 12 kilometers. Static walls could detect aircraft up to 30 miles away.

Types[edit]

There were four main kinds of system:[14]

  • Personal/wearable horns
  • Transportable steerable horns
  • Static dishes
  • Static walls

Impact[edit]

American acoustic locators were used in 1941 to detect the Japanese attack on the fortress island of Corregidor in the Philippines.

Other[edit]

Because the cost of the associated sensors and electronics is dropping, the use of sound ranging technology is becoming accessible for other uses, such as for locating wildlife.[15]

See also[edit]

References[edit]

  1. ^ How Far Off Is That German Gun? How 63 German guns were located by sound waves alone in a single day, Popular Science monthly, December 1918, page 39, Scanned by Google Books: http://books.google.com/books?id=EikDAAAAMBAJ&pg=PA39
  2. ^ "Selected Projects". Greenridge Sciences Inc. Retrieved 2006-05-16. 
  3. ^ Lorraine Green Mazerolle et al. (December 1999). "Random Gunfire Problems and Gunshot Detection Systems". National Institute of Justice Research Brief. 
  4. ^ Kristian Johanssan et al. "Submarine tracking using multi-sensor fusion and reactive planning for the positioning of passive sonobuoys" (PDF). Retrieved 2006-05-16. 
  5. ^ a b W.Richmond (2003). "Before RADAR - Acoustic Detection of Aircraft". 
  6. ^ Rawlinson, Alfred (1923), Rawlinson, The Defence of London, Andrew Melrose, London & New York, pp.110-114
  7. ^ Rawlinson, pp. 118-119
  8. ^ Douglas Self. "Acoustic Location and Sound Mirrors". Retrieved 2006-06-01. 
  9. ^ Jim Mulligan. "Photo of Sound Locator". Retrieved 2006-05-15. 
  10. ^ a b Phil Hide (January 2002). "Sound Mirrors on the South Coast". Retrieved 2006-05-13. 
  11. ^ Andrew Grantham (November 8, 2005). "Early warning sound mirrors". 
  12. ^ Lee Brimmicombe Woods (7 December 2005). "The Burning Blue: The Battle of Britain 1940" (PDF). GMT Games LLC. 
  13. ^ Chan, Y.T; Tsui, W. Y.,So, H. C. and Ching, P. C. (2006). "Time-of-arrival based localization under NLOS Conditions". IEEE Trans. Vehicular Technology (IEEE Vehicular Technology Society) 55 (1): 17–24. doi:10.1109/TVT.2005.861207. ISSN 0018-9545. 
  14. ^ http://www.aqpl43.dsl.pipex.com/MUSEUM/COMMS/ear/ear.htm
  15. ^ John L. Spiesberger (June 2001). "Hyperbolic location errors due to insufficient numbers of receivers". The Journal of the Acoustical Society of America 109 (6): 3076–3079. Bibcode:2001ASAJ..109.3076S. doi:10.1121/1.1373442. PMID 11425152. 

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