At the hydrophone station you can actually listen to the screws (propellors), and hear the different sounds that different ships make. That's pretty cool. It's also sometimes called "Passive Sonar", because it doesn't do anything but listen.
'Sonar' is an acronym for SOund NAvigation and Ranging. Active Sonar sends out a beam of ultrasonic sound, which makes the distinctive 'ping' noise you hear in the movies. When it encounters an object in the water it bounces back, and the operator hears a 'return echo'. In the movies you'll hear it go "ping.....pock", "ping...pock", ping.pock"; with the two sounds coming closer together as the target gets closer. A good sonarman can tell you the range to the target with great accuracy.
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