SONAR is an abbreviation for SOund Navigation And Ranging, a navigational technique that uses sound to gather information about the surrounding environment. There are two types — active and passive — and they are both widely used, depending on the situation. Passive SONAR involves listening for sounds, such as noises generated by passing ships and submarines. When a ship uses active SONAR, it emits a pulse of sound in one of a range of frequencies and then listens for the echo.
Passive SONAR has been used for centuries. Scientists in the 1400s wrote about listening to underwater sounds through tubes. It may not have been extensively used for navigation, but it sowed the seeds for the development of active SONAR. During the First World War, several nations studied the properties of sound and developed early active SONAR to make shipping and naval warfare more safe. This technique takes advantage of hydroacoustics, a series of properties which govern how sound moves underwater.
When a ship uses active SONAR, it emits a pulse from a transducer located along the hull. The electrical impulse from the ship is translated into sound by the transducer, which typically reverts to passive mode to listen for the echo of the sound. Upon receiving the echo, the operator can gather information about hazards in the water ahead, ocean depths, and other ship traffic. The pulses are emitted at a range of frequencies, from infrasonic, or too low to be heard by the human ear, to ultrasonic, too high to be within human hearing range. Ultrasonic sonar is highly accurate at short distances.
In sophisticated systems, SONAR can almost draw an underwater picture, providing information about the type of materials on the ocean floor, for example. Civilians use this technology to navigate, look for interesting objects on the ocean floor, and locate schools of fish. The military uses it to gain an advantage in underwater warfare.
Like other navigation systems, SONAR is not completely fail safe. It can interfere with marine animals that use echolocation, and when multiple ships are in the same area, they can interfere with each other. When using passive SONAR, a skilled operator is needed to distinguish between a wide assortment of items that can cause underwater sound. The ship itself can also interfere with its own SONAR, generating engine noise, abnormal water currents, and similar disruptions.