Acoustic location

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.

The civilian uses include locating wildlife and locating the shooting position of a firearm.

Military use
Military uses have included locating submarines and aircraft.

The air-defense instruments usually consisted of large horns or microphones connected to the operator's ears using tubing, much like a very large stethoscope.

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
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.

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

Time-of-arrival localization
Having speakerns/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.

Ecotracer
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
There were four main kinds of system:
 * Personal/wearable horns
 * Transportable steerable horns
 * Static dishes
 * Static walls

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

Other
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.