How do submarines use sound to find out what is around them?

the sonar technicians are trained to listen to the white noise from the hydrophones and pick out the sounds of propellers from other ships or subs, biological noise, basically anything in the water. from their sonar equipment they can find out what direction it is coming from and track its movements. sonar contacts are organized with a sierra designation.

under duress, subs can use active sonar. this is the classic ping. a warbling noise is emitted from the sub which carries and bounces off whatever we need to find a way to understand how to make sound with no objects, like how bats find their prey. the problem with active sonar is that everyone knows where the sub is when they use it, subs use echoes to see if there are any sort of objects in the sea and if it jumps back to the sub they have found and object.