Is this the most unlikely idea for an acoustic treatment in a studio? The author of the website selling these small bags of pebbles, Geoff Kait, is very concerned that levels in the corner of listening spaces can be 8-10 dB higher than elsewhere in the room. He is right, the pressure is higher in the corners because of interference, but as no-one ever sticks their head in the corner of a room to listen, I don’t think this is something to worry about it.
I was going to write that the little bags of crystals are never going to do anything to the room acoustic. But then I thought, what would happen if you bought a very large number of rocks, because after all gravel absorbs sound. Below is a graph which shows the absorption coefficient of a 15cm (6″) layer of gravel compared to a thick carpet . This shows how much acoustic energy is removed for different frequencies over the bandwidth most often considered in room design. It shows that if you were prepared to crunch your way around your studio on a thick layer of ballast, then rocks could be used in place of carpet. Of course you couldn’t move while trying to listen because of the noise of crunching rock and the sound engineer won’t be able to swoop along the mixing console using the wheels on his office chair. Oh, and a tiny bag of the XL magic pebbles costs over a hundred bucks, so covering the floor would be prohibitively expensive.
Don’t despair, even if the pebbles are not viable for room acoustics there are many other audio uses claimed, ‘A Tube Trap produces (unwanted) mechanical energy during operation and a Large BP on its top surface improves Tube Trap performance (“A Better Tube Trap,” you could say). The performance of (conventional) speakers is usually improved by placing one or more Large BP on the top of each speaker cabinet. A large BP on each output transformer of tube amps is also usually effective. A Large or Mini BP can be effective when placed on the armboard of a turntable.’What other audio accessories defy science? Please comment below.
 T J Cox and P D’Antonio, “Acoustic Absorbers and Diffusers” Taylor & Francis 2009.