Russian active sonar

[Hitman]

Can't provide links but I have readed a book about german subs in the Black Sea where it stated that soviet sonar technology was not as good as the british or american one. Fact is, even some decades later, in the cold war, their electronics where a step or two behind the western ones until the late 70s.

Couple that with the smaller sonar profile of the Type II and you have a more or less realistic situation.