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Sonar, an acronym for SOund NAvigation and Ranging, uses sound energy waves to detect and locate submerged objects, such as submarines and mines. There are two types of sonar:

Passive sonar is a sound receiving system that “listens” for sound waves generated by human-made or biological sources using underwater microphones that receive, amplify, and process underwater sounds. It does not put any sound energy in the water. Passive sonar can indicate the presence, character, and movement of submarines if submarines are loud or operating at high speed. Passive sonar can be ineffective in detecting quiet submarines operating in areas where background noise levels are elevated, such as coastal waters. Although improvements in passive sonar are continually being researched, passive sonar currently does not provide the detection capabilities of active sonar against quiet modern diesel-electric submarines.

Active sonar can be the most effective means available for locating objects underwater. It is based on the principle of “echo ranging.” Active sonar sends out a pulse of energy, often called a “ping,” that travels through water, reflects off objects and returns to a receiver on the ship. Skilled technicians can use the reflected sonar pulse to determine the range, distance and movement of an object. Common active sonars include fish finders, echo sounders, side scan sonars and military sonars.

Active sonar has the ability to locate objects that are too quiet to be detected using passive sonar technology. This makes active sonar invaluable for detecting the new type of very quiet submarines. Active sonar is also effective at locating underwater mines. Although active sonar is the most effective way to detect objects, Navy vessels use active sonar sparingly because sonar can reveal the sending vessel’s location.

Then – 1970s

Submarines of the previous generation were noisy and could be detected with passive sonar before they came close enough to attack.

Present Day

Using only passive sonar, quiet submarines with longer-range weapons can approach within range of U.S. vessels before being detected. Active sonar’s longer detection range is necessary for Sailors to identify, detect, and track a submarine before it is close enough to attack.