http://en.wikipedia.org/wiki/Hedgehog_(weapon)
An unsuccessful attack does not hide the submarine from sonar.
When a depth charge explodes it can take 15 minutes before the disturbance can settle down enough that sonar becomes effective. Many submarines escaped during the time after an unsuccessful depth charge attack. Since Hedgehog charges only explode on contact, if they miss, the submarine can still be tracked by sonar.
Well a DC make 15min maximum disturbance time.