Hi Walter,
I all my reading over the years, I have never come accross anything indicating that Uboat HQ ever had the foggiest notion, that the Allies were using an airborne acoustic homing torpedo. In talking to a few type XXI Cdrs over the last 20-30 years, only one recalled hazily some instruction to listen for torpedo noises in an eventual convoy battle. He did not recall the context of the instructions.
Countermeasures known to the Americans (but they were not telling the Germans):
Deep dive without power to the props.
Keep silent and eject a noise generating buoy.
Rgds, RB