What is the difference between active sonar and passive sonar?

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The primary difference between active and passive sonar lies in how they detect underwater objects:

Active Sonar: These systems use an acoustic projector to emit sound waves that travel through the water, reflect off a target, and return to a receiver. By analyzing these echoes, the system can determine the target's range, bearing, and relative motion.1
Passive Sonar: These systems do not emit their own sound. Instead, they rely solely on receiving sensors to detect noise produced by a target, such as a ship or submarine. The detected waveforms are then analyzed to identify the source and estimate its direction and distance.1

While active sonar is effective for active detection, passive sonar is often used for covert surveillance, as it does not reveal the location of the observer by emitting sound pulses.1