I'm by no means even close to an expert. But I've watched my share of sub specials on the military channel and everywhere else (a few sub specials this week on the military channels btw). To me it's about the definition of the word we're using "listening". The way I always understood sonar was that the active pinging was basically no more then a range finding device. They send out a ping then "listen" to see how quickly it bounces back and that tells them how far the object is. While passive sonar is used to "listen" for distinct sounds the vessel makes (screws, water passing over the hull, metallic noises, etc...).
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