Iam not an sub crew
but there is difference between active and passive sonar.
Active sonar is much more lauder and use a frequency that human ear can detect.
It was surface weasels that used active sonar and today also used by helicopters.
No submarine is using active sonar coz of the risk of detection today.
Difference is also that active sonar works like a radar. It detect objects by bouncning sound. While passive sonar is more listen to the surrounding like propchafts, engines etc.
If u want to know about sonar u can check it out here:
http://en.wikipedia.org/wiki/Sonar