Ok, I guess that'll do. :lol:
To elaborate further, different frequencies make different overtones of the cardboard (or whatever other surface you use) resonate. These overtones have specific nodes on the resonating body, and the higher up the overtone is, the more nodes will vibrate. Think of it like a guitar string: there is only one place where you can hit the octave harmonic on any open string: at the twelfth fret. However, there are two locations for the harmonic an octave above that: the fifth fret, and the twenty-fourth fret. So, the higher up the harmonic, the more locations there are on the string where that harmonic is available.
So, if the frequency is getting higher and higher, it's making more and more nodes for any given harmonic vibrate, and the sand/salt/whatever just shows us where those nodes are.