Sometimes when you're hanging out in an airtight capsule at the bottom of the ocean, you don't want to be heard by the ships around you. Think about it: your submarine's watertight now, but what if an enemy ship comes by with a couple of torpedoes that could undo its water-tight-ness?

That's…less than ideal if you're planning on staying alive.

When you've got an active sonar going, though, it's like using a megaphone to tell anyone around where you are. Anyone with any sonar system will be able to see exactly where you are.

Instead, you can use a passive sonar system, which can't create the waves that it detects. This kind of sonar picks up sounds from possible targets, called passive sources. Passive sources can either be broadband or narrowband.

The names broadband and narrowband talk about how wide a range of frequencies a sound has. The broader a sound, the more frequencies it has. Broadband sources can come from ship or submarine propellers or engine blocks because they send out multiple frequencies at varying rates. On top of the fact that they're broadband, propellers move when they make sound waves, meaning that the Doppler effect can be used to figure out the propeller's speed and, by extension, the linear velocity of the target.

Ships can also have narrowband sources, which only use a small range of frequencies. Pumps, motors, and propulsion systems are all narrowband sources.

Passive sonars are usually used in the military—although a few are scientific. They'll also usually include a database of sounds to check a found sound for any possible sources. A computer system often uses these databases to identify classes of ships, actions (like the speed of a ship, or the type of weapon released), and even particular ships.

And all without sending any loud signals of their own.