Sonar (which started as an acronym for sound navigation and ranging) is a technique that uses sound propagation (usually underwater) to navigate, communicate or to detect other vessels. There are two kinds of sonar—active and passive. Sonar may be used as a means of acoustic location. Acoustic location in air was used before the introduction of radar. Sonar may also be used in air for robot navigation while SODAR (an upward looking in-air sonar) is used for atmospheric investigations. The term sonar is also used for the equipment used to generate and receive the sound. The frequencies used in sonar systems vary from infrasonic to ultrasonic. The study of underwater sound is known as underwater acoustics or sometimes hydroacoustics.
Modern submarines rely on sonar for detecting the presence of enemy vessels. The most advanced system, called a towed array, uses a long cable to which hydrophones are attached. At sea, the submarine deploys this cable so that it trails far behind. Airplanes are used to deploy a different type of sonar. This system uses a device called a sonobuoy, consisting of a hydrophone mounted in a floating buoy. It is designed so that when a sound, such as that of a submarine engine, is picked up, the detector operates a small radio transmitter that sends out a signal that can be received by patrolling antisubmarine planes.
Sonar, acronym for Sound Navigation And Ranging, a detection system based on the reflection of underwater sound waves, just as radar is based on the reflection of radio waves in the air. A typical sonar system emits ultrasonic pulses by using a submerged radiating device; it listens with a sensitive microphone, or hydrophone, for reflected pulses from potential obstacles or submarines. The term was later applied by the U.S. Navy to all types of underwater sound devices, including those used for the detection of enemy vessels by the sound of their engines and for the measurement of water depths.
Spinoffs from the development of sonar technology include acoustic oceanography, the study of ocean properties using a variety of acoustic means, and acoustic tomography, an imaging or remote-sensing technique using computer analysis to study the data collected when acoustic signals are passed through an object. Acoustic tomography is used in oceanic and medical research and in medical diagnosis.