(acronym for sound navigation and ranging) Method of locating underwater objects by the reflection of ultrasonic waves. The time taken for an acoustic beam to travel to the object and back to the source enables the distance to be found since the velocity of sound in water is known. Sonar devices, or echo sounders, were developed 1920.
The process is similar to that used in radar. During World War I and after, the Allies developed and perfected an apparatus for detecting the presence of enemy U-boats beneath the sea surface by the use of ultrasonic echoes. It was named ASDIC (from antisubmarine detection investigation committee), but in 1963 the name was changed to sonar.
An instrument that sends out an acoustic pulse in water and measures distances in terms of the time for the echo of the pulse to return (sonar is SOund NAvigation Ranging; asdic is Anti-Submarine Detection Investigation Committee).