just a nifty harmonics trick. 

sound, as we percieve it, is essentially vibrating air.  but sound passes through liquids and solids as well, and generally much more efficiently. 

anyway, as the sound waves move back and forth across the board, they collide with each other, and where the frequencies [or multiples of 2 of those frequencies, ie harmonics] meet, you get a harmonic effect.  in those places, the granules tend to move less, and in a less random and more orderly way. 

mister wizard, ya know? 

i wouldnt say that a sounds frequency [the amount of time it takes a vibration to go back and forth once] actually shapes anything, its more accurate to say that it responds to shapes that already exist.