DD of all nations would just randomly drop depth charges. Often, it was because they were afraid of the submarine, but other times, they did it to scare the submarine into making noise by going to Ahead Flank. Also, they don't need to ping to drop. If you make enough noise, the hydrophone will work just fine, thank you very much. But being below the thermal layer would have a tendency to confuse the sonar / hydrophone operators, since the layer will at the least "bend" the sound of your submarine, making it to where it sounded like you were where you weren't... and other times, the layer would "reflect" the sonar signal, and make it bounce off into nothingness. So they can probably hear you, but can't locate you. Come back up to PD though, and let them see your scope, and they'll find you quick enough...
