Sound navigation and ranging, or sonar, is an umbrella term for technology and methods that use sound to detect objects, usually underwater, much the same way bats rely on sound for catching food in the air. The basic principles on which sonar relies are that sound moves at a steady rate through a given medium, such as air or water, and that certain types of objects produce certain types of sounds. With this knowledge, calculations can determine the distance to an object and identify it with reasonable certainty. The Navy mainly uses sonar to detect vessels, torpedoes and mines. Commercial applications for sonar include navigation, mapping and locating fish.
Passive sonar, as the name implies, simply involves listening. Marine biologists use passive sonar techniques to locate and study life in the oceans, using the sounds particular to an animal to help identify it. Similarly, ships and submarines use passive sonar to locate ships and subs and other threats. Because certain types of engines vibrate at certain frequencies, they can be identified through passive sonar. A metal object such as a wrench, dropped in a sub, may be detectable by passive sonar miles away. Frequently, a vessel will tow a passive sonar array behind it to listen with minimal interference from its own noises.
Active sonar involves both the transmission and receiving of sound waves. An active sonar system sends a sound pulse and measures the length of time for the sound to be reflected back. Because the speed at which the sounds moves is very fast and remains constant, the distance to the object can be calculated quickly. Active sonar can use a concentrated pulse of sound in a specific direction, so information about an object's direction can also be obtained through some active sonar applications. Active sonar can be deployed by helicopters, mounted in buoys or on boats, or hand-held.
One major criticism of active sonar is its effect on marine mammals, such as dolphins and whales, who also use natural sonar for their navigation. Testing of very low frequency active sonar by the U.S. Navy, because it causes damage to an animal's ability to detect sound, has been implicated in the beaching of whales. In other cases, it has been suggested that Navy sonar tests have caused whales to surface too quickly, inducing painful and often fatal decompression sickness. The U.S. Navy accepted responsibility for the deaths of seven beached whales in the Bahamas in 2000 that were found with acoustically induced hemorrhages around the ears and eyes after a test of low-frequency sonar in the area.
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