How can submarine's sonar identify detected submarine classes?

The other answers (so far) refer to two entirely different methods of classifying (identifying) sonar contacts (submarine classes). I am intimately familiar with one method. read more

the ability to identify class of a detected submarine is unique to passive sonar. all submarine classes and even individual submarines sound different in water. The passive sonar signature of a Typhoon-class submarine is completely different from the signature of an Akula-class. read more

Submarines are primarily detected acusticly. Surface ships and airborne sonar systems (buoys and dipping sonar) use both active and passive systems. Due to installed technology radar is very ineffective against raised submarine masts. The primary method is passive acoustics. Can be another submarine, ship, aircraft, or static ocean array. read more

A canny submarine captain can use the thermocline to good effect, effectively shielding the submarine from view. Another ruse (one often seen in the films) is for a submarine to hide itself by coming to rest on the ocean floor, or near ocean cliffs and trenches. read more