The standard U-boat hydrophone, the GHG (Group Listening Apparatus) was installed in U-boats from 1935 onwards. It consisted of two sets of hydrophones mounted on each side of the bows, covering two arcs of 140 degrees on the sides of the U-boat. Because the hydrophones could not be rotated, the triangulation was most effective with sound sources coming from the sides, with deteriorating accuracy as the source moved to the front or rear of the boat. Consisting of 24 hydrophones, the GHG could pick up lone vessels up to 20 kilometers and convoys up to 100 kilometers away. The detection range however was also dependant on sea conditions.
The KDB (Crystal Rotating Base Apparatus) was an improvement of the GHG in that it was rotatable and hence able to provide more accurate readings from any direction. The disadvantage however was its extreme vulnerability to depth charges.