ASDIC is directional, meaning it needs to be pointed. When you hear the sound, they do not know where you are until your sonarman says... they are pinging us sir!.
Now understand, he says that because he hears the "eco" bounce off the hull. At no time does this mean he knows exactly where you are. Depending on your depth, profile, weather, and distance, he may "think" you are at a particular position.
When you hear the ping go off and repeat faster and he begins to close is when he actually attempts to charge you, And many times he falls short and you hear explosions in the distance.
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