Resonant frequencies*

So the idea here is most objects have "resonant frequencies"; notes that travel through them a whole number of times (no fractions/decimals). This is a video of a speaker with a metal plate above it and sand on top of that; the sand bounces off the parts of the plate that vibrate, leaving patterns created by the sound. cool, huh? this originally was done with a violin bow against the side of the plate (before speakers) and started the study of acoustics. :)