Ships detecting subs?

Question:

Ships detecting subs?

I was just watching a movie about a U-boat in WWII and the U-boat was firing on some other subs when a destroyer came out and started firing its guns at it. Then, when the U-boat dove under, the ship started dropping I guess depth charges or bombs or something. This was way back in WWII, how did the destroyer detect the U-boat? If you could please explain how but I was thinking sonar? The guys in the U-boat also whisper when they might be attacked. Is the sonar that sensitive that it dectects their voices and the U-boat movement? If you could please help me out it would be much appreciated.

Best Answer - Chosen by Asker: In the scene you describe, the destroyer would have been informed by radio messages from the subs which were being attacked.

Sonar was an active sound location system, sending out a PING and waiting for an echo. Destroyers also used passive sound location, listening for the noise of engines, propellers, or human voices, hence the crew whispering.