Sonar, an acronym for so und n avigation a nd ra nging, is a system that uses sound waves to detect and locate objects underwater.
The idea of using sound to determine the depth of a lake or ocean was first proposed in the early nineteenth century. Interest in this technique, called underwater ranging, was renewed in 1912 when the luxury sailing vessel Titanic collided with an iceberg and sank. Two years later, during World War I (1914–18), a single German submarine sank three British cruisers carrying more than 1,200 men. In response, the British government funded a massive effort to create an underwater detection system.
The entire operation was conducted in complete secrecy, but the first working model was not ready until after the war ended. The project operated under the code name "asdic" (which stood for Allied Submarine Detection Investigating Committee). The device kept that name until the late 1950s, when the American term "sonar" was adopted.
The principle behind sonar is simple: a pulse of ultrasonic waves is sent into the water where it bounces off a target and comes back to the source (ultrasonic waves are pitched too high for humans to detect). The distance and location can be calculated by measuring the time it takes for the sound to return. By knowing the speed of sound in water, the distance is computed by multiplying the speed by one-half of the time traveled (for a one-way trip). This is active sonar ranging (echolocation).
Active sonar: Mode of echo location by sending a signal and detecting the returning echo.
Passive sonar: Sensitive listening-only mode to detect the presence of objects making noise.
Ultrasound: Acoustic vibrations with frequencies higher than the human threshold of hearing.
Most moving objects underwater make some kind of noise. Marine life, cavitation (small collapsing air pockets caused by propellers), hull popping of submarines changing depth, and engine vibration are all forms of underwater noise. In passive sonar ranging, no pulse signal is sent. Instead, the searcher listens for the characteristic sound of another boat or submarine. By doing so, the searcher can identify the target without revealing his own location. This method is most often used during wartime.
However, since a submarine is usually completely submerged, it must use active sonar at times, generally to navigate past obstacles. In doing so, the submarine risks alerting others of its presence. In such cases, the use of sonar has become a sophisticated military tactical exercise.
Sonar devices have become standard equipment for most commercial and many recreational ships. Fishing boats use active sonar to locate schools of fish. Other applications of sonar include searching for shipwrecks, probing harbors where visibility is poor, mapping the ocean floor, and helping submerged vessels navigate under the Arctic Ocean ice sheets.
[ See also Ultrasonics ]