I'm sorry but I have to refute the sandbox technique. It doesn't work effectively. I'm a mechanical engineering student (senior, about to go to Masters school) and I designed a proper turntable isolation box. I matched spring rates to the mass of the turntable and the plate it site on, and at 20Hz, only 1% of the vibrations go through, and at 150Hz, a fraction of 1% go through.
I have a paper written about it that I can send anyone who is interested.
Bottom line, sand does next to nothing -- and I can prove it mathematically.
Since all the sand particles are crammed together we can treat sand as a solid body, then the damping of the sinusoidal audio waves is extremely minimum for bass frequencies, as the waves propagate very easily through a solid medium. You need to add something that actually absorbs amplitude. Enter -- the spring. Springs have a natural frequency at which they try to oscillate. If we can make this natural frequency (k) below our hearing levels, say 10Hz or something like that, we can effectively remove all the frequencies that are transferred through the springs at higher frequency.
Again, all this is in the paper, you could learn a lot!