According to
http://uboat.net/allies/technical/asdic.htm web page, the asdic contact was usually lost at 300 yards. This is the time the u-boat should try to evade any deptch charge attack. And if there wasn't a def zone of the asdic why would the escords need a weapon like "hedgehog"? As far as i know hedhehogs were fired just before the contact was lost on asdic.