The systems described in this article are "passive". They are simply listening to the sounds around them. This is much more common for submarines than "active

The systems described in this article are "passive". They are simply listening to the sounds around them. This is much more common for submarines than "active" sonar - which ensonifies the water around you and then you listen for echos. It is those loud pings which can threaten or frighten wildlife. However they are also like holding up a big lit sign that says "there is a submarine here!" - and the whole point of submarines is stealth. They say that if you hear a ping from a submarine, it is too late - there is already a torpedo heading your way. source - used to be a sonar tech