A video camera, recording at the same speed as the soundwave, to make the water look like it's not moving BUT

How the sound makes the water stair-step is a mystery

http://games.yahoo.com/blogs/plugged-in/......014.html

I suspect the effect is not as amazing without the 24 frames per second camera shots. This is a similar principal to what Edgerton did with the strobe light

http://en.wikipedia.org/wiki/Harold_Eugene_Edgerton

and

http://www.youtube.com/watch?v=OtxlQTmx1LE

Does look impressive. But I don't think the water is "stair-stepping", it's more that it's a helix being viewed in 2 dimensions so it looks a bit like it takes sharp bends.

This is clearer in the 23 and 25 hz versions

This is something I want to try! really interesting and impressive, and a great science lesson for the kids!

I can do this in yellow

Wow!