Got a shock when this old thread resurfaced.
20 Km is the maximum range that a crewman will detect a ship. By then you can quite clearly hear it, and it can be frustrating when your own operator cannot.
The KDB does not have a greater range but is more sensitive, which I believe translates in the operator recognising a sound more quickly; again no difference for us "Captains".
Now days I'm not so sure that you can actually pick up convoys at greater range. However, I am now playing at 91%+ realism so cannot use the map to measure distances anymore.
Hope it helps.
