Sonar

Sonar, pronounced SOH nahr, is a device that uses sound energy to locate objects; measure their distance, direction, and speed; and even produce pictures of them. The word sonar comes from sound navigation and ranging.

People associate the word sonar with devices that detect submarines and other underwater objects. Sonar works well underwater, where sound travels quickly and efficiently over long distances and where radar (detection using radio waves) does not work. However, certain sonar devices operate in the air. For example, some burglar alarms use airborne waves of ultrasound (sound whose pitch is too high for people to hear) to detect movement.

Dolphins and some bats use a natural sonar technique called echolocation. This technique helps them locate food, avoid obstacles, and communicate.

How sonar works. There are two types of sonar: (1) active and (2) passive. Active sonar uses a transmitter, a device that converts electrical energy to sound energy, to send out sound waves. Transmitters used underwater can produce a sharp pinging sound. The sound waves travel through the water until they strike an object. The object reflects them in various directions. Some of the reflected waves return to the sonar, where they strike a receiver. The receiver converts the sound back into electrical signals. In modern sonars, a computer analyzes these signals to perform the sonar's job, such as locating the object or determining the object's distance from the sonar.

A sonar determines distances by measuring the time taken for a sound wave to travel from the transmitter, reflect from the object, and travel to the receiver. This method of finding distance is called echo ranging. Sound travels about 1 mile (1.6 kilometers) per second through water. Therefore, sound that returns after 2 seconds has traveled 2 miles--1 mile to the object and 1 mile back.

Passive sonar receives sound waves given off by some other source, but does not transmit sounds. A passive sonar can therefore determine the direction of an object, but is not as effective as active sonar in determining its distance. In military use, a passive sonar has the advantage of not giving off any sound that an enemy sonar might detect. Submarines generally use passive sonar. Most surface ships use active sonar because they make too much noise to use passive sonar.

Uses of sonar. Sonar has military, scientific, and commercial applications. Navies use sonar to locate ships, submarines, and underwater mines. Helicopters can lower sonar units into the water on cables. Airplanes drop units called sonobuoys, which report back by radio. Scientists can use certain kinds of sonars to scan the bottom of an ocean or a lake. They can then use computers to create detailed maps of the bottom. Some sonars help determine what materials make up the sea floor, and even what lies beneath the sea floor. Fishing ships use sonar to detect schools of fish.

Ultrasound devices are also a type of sonar. Physicians use such devices to diagnose heart disease and other disorders of internal organs and to check the development of fetuses.

History. Naval scientists in Britain, France, and the United States experimented with underwater sonar during World War I (1914-1918). Britain and the United States gradually developed successful active sonar systems between the two World Wars.

The British and Americans kept sonar a secret, so its use surprised the Germans during World War II (1939-1945). Sonar units of that period had ranges of only up to about 1 mile (1.6 kilometers) underwater. Today's units have ranges of more than 10 miles (16 kilometers).

Contributor: Thomas O. Mottl, Ph.D., Division Director, Strategic Systems, The Analytic Sciences Corp.

Sonar

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