Sonar (SOund Navigation And Ranging) is the generic name of the technology that is used to locate objects underwater. Sonar systems are of two basic types - active and passive. In active sonar, the system emits a pulse of sound and then the operator listens for echoes. In passive sonar, the operator listens to sounds emitted by the object one is trying to locate.
When a sound signal is sent into the water, part of it will be reflected back if it strikes an object or "target". The distance to the object can be calculated by measuring the time between when the signal is sent out and when the reflected sound, or echo, is received. For example, if four seconds elapse between the emission of the outgoing sound and the return of its echo, the sound has taken two seconds to travel to the object and two seconds to return. The average speed of sound in the water is 1,500 meters per second. So if it takes two seconds for sound to reach the object, we can assume the object is 2 sec x 1,500 m/sec or 3,000 meters away.
Sonar systems generally use highly directional beams of sound when searching for targets. In this way they are able to determine direction to the target, as well as the distance. The echoes heard in active sonar systems can also be very distinct. Experienced sonar technicians are often able to tell the difference between echoes produced by a submarine, a rock outcrop, a school of fish, or a whale.
Some animals have their own natural sonar system. Whales and dolphins use active sonar to identify underwater objects and to help find food. These marine mammals produce very sophisticated sounds such as frequency sweeps and chains of clicks that tell them much about the target when they are reflected back. (See What is echolocation?).
Ships, submarines, marine mammals, and fish all make noise, and this noise can be used by passive sonar systems to locate them, in much the same way humans use their ears to locate someone speaking in a room. Passive sonar systems can be arrays of hydrophones towed behind a ship or submarine, or a fixed system of hydrophones cabled to shore or attached to a mooring.
- National Academy of Sciences, "Sounding Out the Oceans Secrets." (Link)