Mouser.. posted:

3

Since no one really answered what a convergence zone is, I'll use my laymans knowledge to help out. When you are using passive sonar, the sound travels in what I would consider a parabola, as the sound waves rise, they meet near the surface and then bounce back down into another parabola. If the water is deep enough, you can pick up sound at every convergence zone until the sound dissipates. So you'll hear a sound at 1 CZ which is about 20nm, then again when the parabola bounces at 2CZ which is about 40nm. I might have just fucked this whole explanation up so feel free to correct me if I'm wrong.

EDIT: Here's a diagram now that I found it, it's not as sharp as a parabola but I'm not a math major so fuck it, it's a downward curvetype thing:


Also, to help out with the reason that you are able to hear things and say "It's an Akula class!" By using passive sonar (which is just listening for other things in the ocean) versus active sonar (which is pinging an object to normally generate firing solutions on it.) you are able to identify each vessels specific acoustic signature. Sometimes these are hard to identify if they are running very silent but in this case, it would be easy to pick up because the sound would be very distinct on the spectogram. For further examples of what an acoustic signature sounds like to a hydrophone, check this page out:

NOAA Seasounds