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 can be used both actively and passively for localization. Microphones used with these devices enable active localization while speakers enable passive localization. When multiple devices are used they are triangularly arranged in relation to the sound source.
As a military air defense tool, passive acoustic location was used from mid-World War I 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.
Military use 
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. 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. The acoustic location stations were left in operation as a backup to radar, as exemplified during the Battle of Britain. Today, the abandoned sites are still in existence and are readily accessible.
After World War II, sound ranging played no further role in anti-aircraft operations.
For enemy artillery spotting, see sound ranging.
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.
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. 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.
For more information on this item, see the article on Sonar.
Biological echo location 
Time-of-arrival localization 
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. 
Seismic surveys 
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.
For more information, see Reflection seismology.
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.
There were four main kinds of system:
- Personal/wearable horns
- Transportable steerable horns
- Static dishes
- Static walls
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.
- 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
- "Selected Projects". Greenridge Sciences Inc. Retrieved 2006-05-16.
- Lorraine Green Mazerolle et al. (December 1999). "Random Gunfire Problems and Gunshot Detection Systems". National Institute of Justice Research Brief.
- Kristian Johanssan et al. "Submarine tracking using multi-sensor fusion and reactive planning for the positioning of passive sonobuoys" (PDF). Retrieved 2006-05-16.
- W.Richmond (2003). "Before RADAR - Acoustic Detection of Aircraft".
- Douglas Self. "Acoustic Location and Sound Mirrors". Retrieved 2006-06-01.
- Jim Mulligan. "Photo of Sound Locator". Retrieved 2006-05-15.
- Phil Hide (January 2002). "Sound Mirrors on the South Coast". Retrieved 2006-05-13.
- Andrew Grantham (November 8, 2005). "Early warning sound mirrors".
- Lee Brimmicombe Woods (7 December 2005). "The Burning Blue: The Battle of Britain 1940" (PDF). GMT Games LLC.
- 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.
- 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. doi:10.1121/1.1373442. PMID 11425152.
See also 
- Acoustic mirror, Parabolic microphone
- Animal echolocation, animals emitting sound and listening to the echo in order to locate objects or navigate
- Human echolocation, the use of echolocation by blind people
- Acoustic wayfinding, the practice of using auditory cues and sound markers to navigate indoor and outdoor spaces
- Sonar (sound navigation and ranging), for watercraft use
- 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
- Japanese war tuba, 1930s Japanese acoustic locator
- Sound localisation
- An introduction to Acoustic Holography.
- "Huge Ear Locates Planes and Tells Their Speed" Popular Mechanics, December 1930 article on French aircraft sound detector with photo.