[BigWalleye]

Originally Posted by Fahnenbohn

So, why don't you hear sonar's pings when a destroyer (which doesn't know you are in the area) detects you with her sonar ?

If they're pinging, you'll hear them. If they're just listening (which is what they do with hydrophones), they can hear you at a lot greater range, and you won't know when they do. Did you think they had to ping to hear you? Because they don't.

In WW2, as today, the primary ASW detection method is passive acoustic sensors. Active sensors were, and are, used primarily for localization and attack.