[SeaQueen]

Originally Posted by Deamon

It's a long time ago, but i thought this effect was called surface or bottom referberation. But i migh error here.

You're right. The difference between a bottom bounce and bottom reverberation is that bottom bounce is sound you want and bottom reverberation is sound you don't, basically.

Technically, though, usually people refer to reverberation when they're talking about active sonar. There's things called sonar equations, and the reverberation level in an active sonar equation refers to energy from the ping scattered back to the receiver off of any random stuff that's not a target. Essentially, it's just "noise."

In passive sonar, if that actually was an effect, it probably isn't a big deal in most situations. It's reverberation in a looser sense of the term. It shouldn't really be a big deal in most situations, though.

Becose of an uneven concentration of junk in the water ?

They're just objects with a density other than that of water. That causes some energy to be reflected back. If there's an awful lot of these little objects out there, the effect adds up and you have to account for it.

My favorite volume scatterers are schools of fish. Their swim bladders are extremely efficient scatterers of sound. You can see them come out at night, and go to sleep during the day. It's kind of neat. There was a neat article in the most recent issue of Physics Today about that actually. It made me happy.

Why not ?

Because the wavelength of the sound is a lot larger than the features responsible for most of the backscattering (mostly waves from the surface and the irregularity of the bottom). There almost certainly is some, but I'm not sure there's enough to worry about most of the time.