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How can intelligence agencies identify submarines through their sound signatures?

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George W. Bush and Porter J. Goss at CIA headquarters in Langley, Virginia. — intelligence U.S. Pres. George W. Bush (front) and Porter J. Goss, director (2004–06) of the Central Intelligence Agency (CIA), heading to a press conference at CIA headquarters in Langley, Virginia, March 3, 2005.

Intelligence agencies identify submarines by analyzing underwater acoustic waves using sonar technology. Because electromagnetic waves do not propagate well underwater, acoustic detection is the primary method for tracking submerged vessels.12

Identification is primarily achieved through passive sonar systems, which utilize sensors to detect and analyze the specific noises generated by a submarine, such as propeller rotation or machinery noise. These systems often employ hydrophones—devices that convert sound waves into electrical signals—arranged in arrays. By analyzing the phase differences of sound waves across these arrays, operators can "steer" the system to pinpoint the source's direction without moving the sensors themselves.324

In contrast, active sonar systems emit sound pulses and analyze the echoes reflected off a submarine’s hull to determine its range, bearing, and relative motion. These sensors can be deployed via surface ships, submarines, aircraft-launched sonobuoys, or fixed seabed arrays to provide continuous surveillance of maritime environments.321