This week's experiment came from playing, as many of my experiments do. I was building a small prototype of a smoke ring generator for my new storm show. Along the way, I got side tracked and spent quite a bit of time playing with patterns created by vibrations.

Materials:

Plan:

Now you have what is a toy drum. Tapping on the stretched rubber will give you a nice sound, but instead of using it to make sound, we want to use it to look at sound. Well, we are really going to look at the patterns produced by sounds.

To do that, place the can on a flat surface, with the stretched balloon on top. Sprinkle salt evenly over the balloon. You don't want a solid layer of salt, just an even sprinkling. Now comes the fun part. Find a spot near the edge and gently brush the salt away. Use your finger to rub the balloon, as if you were gently scratching an inch. Rubbing your fingers across the rubber will cause it to vibrate, which will cause the salt to dance around. Watch as the salt jumps around, and you should start to see a pattern.

Pick another spot and try the same thing again. You will get a pattern again, but it should be different from the first. Why?

Lets begin with why the patterns are there at all. Have a friend hold one end of a long piece of rope. You hold the other end and take up the slack. You don't want the rope to be tight, but it should not be touching the ground. Start moving your hand from side to side. Move your hand faster and faster, watching the rope. You should see a pattern form. The rope will form waves, moving back and forth, but there will be points on the rope where it is not moving much. These points are called nodes.

When the balloon vibrates, it also has points that vibrate a lot and points that don't. The salt grains on the part that vibrates a lot move to the points where there is not much vibration, forming the patterns that you saw.

These patterns are called Chladni patterns, named after Ernest Florens Friedrich Chladni of Saxony, who has been called the father of acoustics. He sprinkled sand onto metal plates and studied the way that they vibrated.

Besides being fun to play with, these patterns are useful. These patterns are used in designing musical instruments. If a part is attached to a place where the instrument vibrates, the sound will be dampened. By attaching parts at nodes, the instrument makes a full, rich sound. These patterns make the difference between an average instrument and a quality one.

If you play a musical instrument, you can use it to see more patterns. I placed my Native American flute near the can. As I played different notes, the patterns and their intensity changed. The balloon vibrates easily at some frequencies, but not at others. When the vibration of a note matched the vibration of the balloon, the salt would dance wildly. A note that vibrated at a rate that did not match would hardly move the salt. You can see something similar by placing the can in front of the speakers of your TV or stereo. I tried it with the movie Jurassic Park, and the T. rex footsteps worked wonderfully.