Quite interesting though, as I remember in the very first Silent Hunter, the Japanese escorts would be pinging constantly, they wouldnt know you were there, but they would be pinging away non-stop. As you approached the convoy, you could hear the faint pings in the water, that would get louder the closer you got. For a game modelled in 1996, it was really well done
In truth, I think that's how they really did it. I know that in real life, British destroyers anyway when escorting convoys would constantly be using active ASDIC on the chance they'd detect a U-boat trying to penetrate the convoy.