Welcome, Danox.
Any noise your sub makes ( except the radio and gramophone ) can be detected by surface vessels - especially if they're listening for you. "Pings" are particularly dangerous. I had an uncle who was a stoker in WW II and he described a "ping" against his ship's hull as sounding like someone tapping the plates with a hammer.
Standard practice was to have a warship's hydrophones manned at all times, constantly sweeping for underwater sounds. As soon as one was detected / suspected, or in certain known danger areas, the ASDIC, or eqivalent was used, usually sweeping "Red 80 to Green 80" ( i.e. not quite a semi-circle ahead of the ship) and at maximum range.
On a contact, the DD would turn towards the contact and shorten the range at full / attack speed. ( ASDIC etc. was not affected by the noise of the ship's engines.) As the ship closed the target, the time between the "ping" and the return "echo" from the target got shorter. Since the speed of sound underwater is a known, if rough, constant, the operator could calculate the range to target and the direction (strength ) of the echo gave him its bearing. When the operator announced "Simultaneous Echo", it meant that the warship was directly above the target and the DCs were launched.
The main inaccuracy in the process was the depth of the target - thank Goodness!
So when you hear "pinging" in game, it doesn't necessarily mean you've been pin-pointed, but they know, or suspect you're around somewhere. If a destroyer is pinging and heading straight for you at a good speed - Get out of there!
The best way to avoid being picked up by escorts is to be as quiet as possible - so that their hydrophones don't make that initial contact. Silent running and creeping along at 2 / 3 knots mean you should stay undetected unless an escort happens to come straight at you and close. Depth is also your friend, especially as there is a "thermal layer" built into the game. It's depth varies, but once below that your "sound signature" is reduced. N.B. you can still be detected under the thermal layer, but it sort of diffuses, or refracts the sound of your sub, making it more difficult for hunting vessels to get an accurate fix.
So, with convoys, the best ( and safest ) tactics I've found is to get ahead of the convoy and lie in wait within torpedo range of the convoy's track and at right angles to it; at periscope depth; running silent with engines stopped. Use the periscope as little as possible - just enough to get the data you need for the TDC. It can be spotted by escorts, especially in calm seas in daylight.
Once you've fired your torpedoes, immediately dive ( I usually go to 200 feet which always seems to take me below the thermal layer ), turn and set speed for 2 knots - still running silent. By the time your torpedoes hit and the escorts begin to get active you should be at a good depth and moving away from your firing point.
Good hunting.
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