Well, you curtainly picked a difficult topic for the question. The easiest way of threating the problem is to use a bar, like a tuning fork or a free bar:
http://hyperphysics.phy-astr.gsu.edu/hbase/music/barres.html[
^]
However sand is nonlinear so it might be a little more difficult to get an excact value.
I could go on and on about this, but Im not sure how accurate you want it..