I recall reading somewhere that they were foghorns, so assume it was air-driven, don't know what pitch or how powerful tough.
--edit--
Yup, found the book; When the U-Boats came to America. There's a part about U-151:
"On May 28th the fog was heavy and the U-Boat remained on the surface, frequently sounding its foghorn."
Last edited by ouPhrontis; 10-11-18 at 05:20 AM.
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