Targeting
Hydrophones and Sonar operating in passive mode can detect acoustic signals radiated by otherwise invisible submarines, and use these signals to target attacks.
Modern naval mines and torpedoes such as the CAPTOR mine can be programmed to distinguish the acoustic signatures of different vessels, leaving friendly vessels unmolested and attacking high-value targets when faced with multiple possible targets, e.g. distinguishing an aircraft carrier from its escorts.
Read more about this topic: Acoustic Signature