ASW Countermeasures
The main countermeasure the submarine has is stealth, that it tries not to be detected. Against the ASW weapon itself, both active and passive countermeasures are used. The former may be a noise making jammer or a decoy providing a signal that looks like a submarine. Passive countermeasures may consist of coatings to minimise a torpedo's sonar reflections or an outer hull to provide a stand-off from its explosion. The Anti-submarine weapon has to overcome these countermeasures.
Read more about this topic: Anti-submarine Weapon