Sound wave visualized with water as medium and lit by a weak LED ring flash with gel colors. Thank you, Mr. Linden Gledhill, for teaching me the how-to's.
Everyone should check out Linden's expert work in his Flickr albums:
This photo here is not quite there rig (my cymatics rig is still being improved upon), but I found this image interesting enough to share.
This image was made with a square water reservoir.