The use of sound to 'echo-locate' underwater in the same way as bats use sound for aerial navigation seems to have been prompted by the Titanic disaster of 1912. In the late 19th century, an underwater bell was used as an ancillary to lighthouses or lightships to provide warning of hazards.

HistoryĪlthough some animals ( dolphins, bats, some shrews, and others) have used sound for communication and object detection for millions of years, use by humans in the water was initially recorded by Leonardo da Vinci in 1490: a tube inserted into the water was said to be used to detect vessels by placing an ear to the tube. Modern active sonar systems use an acoustic transducer to generate a sound wave which is reflected from target objects. It was developed during World War I to counter the growing threat of submarine warfare, with an operational passive sonar system in use by 1918. The first recorded use of the technique was in 1490 by Leonardo da Vinci, who used a tube inserted into the water to detect vessels by ear. The study of underwater sound is known as underwater acoustics or hydroacoustics. The acoustic frequencies used in sonar systems vary from very low ( infrasonic) to extremely high ( ultrasonic). The term sonar is also used for the equipment used to generate and receive the sound. Sonar may also be used for robot navigation, and SODAR (an upward-looking in-air sonar) is used for atmospheric investigations. Acoustic location in air was used before the introduction of radar. Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of 'targets' in the water. 'Sonar' can refer to one of two types of technology: passive sonar means listening for the sound made by vessels active sonar means emitting pulses of sounds and listening for echoes. Sonar ( sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects on or under the surface of the water, such as other vessels. Sonar image of the Soviet Navy minesweeper T-297, formerly the Latvian Virsaitis, which was shipwrecked on 3 December 1941 in the Gulf of Finland

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