As soon as you hear a ping, it means you're detected. For this to occur, you must be within his sonar cone for a specified period of time, while presenting enough surface (sim.cfg). When you hear the first ping, the convoy will start zig-zagging.
Heavy pinging means a destroyer is close to you.
If you're below the thermal layer, you're much harder to detect with sonar. Also, if you're shallow enough, they can't ping you at all (decks awash).
The early war DD's have short range on their sonar. as the war progresses, the range of their sonar is increased.
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