How about this:
Get a stack of plastic sheet protectors, the thinner and more transparent the better. Fill each with just enough of your excitatory media to create a thin layer covering most of the sheet. Buy a soft wood picture frame slightly smaller than the sheet protectors. Get some wooden skewers. Drill four skewer-gauge holes all the way through the picture frame, one in each corner. Stack the filled sheets neatly on top of the frame. Pierce each corner of the stack with a skewer and fasten it into the holes. The sheets should be into the corners picture frame. Place the frame on top of your speaker and play a sound.
You should be able to see the 3D cymatic shape emerge from the different patterns in each layer. If your plastic sleeves are too opaque, use a powder that fluoresces under UV light, and a strong UV light. Do the experiment in a dark room. You should be able to see the pattern light up and take shape.