Resonance and old age

I love YouTube. Being a science nerd and teacher I can find tons of videos to demonstrate various topics that I'm interested in or that I teach in class. But it bugs me that there are no explanations of what's going on in the videos. This video, for example:

It would be great is the half million people who have watched this could get more out of it than, "Wicked cool!" So here I'll take a stab at it:
The hand sprinkles tiny plastic beads on a metal plate attached to a speaker. A sine wave generator is playing through the speaker to generate pure tones of various frequencies/pitches (starting low and getting higher). At certain frequecies standing waves a created in the plate that have nodes (places with no shaking) and antinodes (places with lots of shaking). In a guitar string the nodes are the ends and the antinode is the middle. But more complicated patterns can be created. The patterns get really crazy when you chenge from a 1-dimensional string to a 2-dimensional plate. The beads pile up at the nodes and are bounced off of the antinodes, creating the cool patterns.

Here are some neat things to look for as you watch:

• The patterns only become visible at certain pitches when the lenghts of the waves being produced are evenly divisible into dimensions of the plate.
• When the patterns pop out the volume also gets louder since the plate is resonating (for a great example of resonance, check out a video of the Tacoma Narrows Bridge collapse).
• The patterns become more and more intricate at the pitch goes up. The higher frequency means shorter waves, so more wavelengths fit within the size of the plate.
• You can use this as a hearing test to see how old you are. As people age they lose their high frequency hearing. If you thought that the sound cut out before the video ended then you're old!

Noggin Fodder
www.nogfod.com
Feed your head.