How can you confirm approach of a submarine under water?

Answer 1

Using Sonar, you normally assume any contact is on a closing range rate, since you've come close enough to pick it up on Sonar. From there, it's a matter of listening and determining the acoustic signature of the contact, and ruling out any surface vessels.

Most submarine on submarine contacts are usually pretty close-range affairs, as Sonar technology as well as acoustic silencing technology continues to improve. Of course, if you hear a torpedo tube flooding and its muzzle doors opening, it's a good bet that it's not a trawler on the surface.