[UKönig]

It's true what you say, being quiet can still save you.
The hope in that case is that the enemy sensor operator will lose track of your sub with all the water currents, exploding DCs, strong winds and waves and maybe even schools of fish or pods of whales. But the more enemy ships searching for you, the harder it is to get away. Whichever ship is on the attack run will be receiving radio updates from the other ships, and round and round the ASDIC pings go, just like a ball among the players on a football field. That's why I said that your survival in that case boils down to your experience and skill. To know when to run silent and deep, and when to just "book" it.