there is a descussion in mod forum
http://www.subsim.com/radioroom/showthread.php?t=118315
its not realistic indeed. according to fleettype submarine manual the hydrophones below the keel were only able to catch supersonic sounds. So they are not capable for catching contacts. They are much more to catch pingings and identifieng targets.
obviously you can catch contacts with supersonic listening only, but not at that range. Supersonic sound does not travel so far so you might hear a ship from maybe 2000 yards but not from 20 miles.