Sound Waves
Thanks to Homeschooled Twins for pointing to this awesome demonstration of the vibrations of sound waves.
The Contest
Stephen and I spent this last week at the Objectivist Conference ( OCON ) in Boston. We started out the week staying in town, but finished it off commuting. As we stayed at the Seaport Hotel at the beginning of the week when we knew almost no one, we didn’t take advantage of the social atmosphere, but were happily sequestered in our room. After getting to know a few people, we dined, drank, and were generally merry with those few. Through the classes, general sessions, and referenced symposiums (in the ancient Greek sense), I left with a calmer sense of purpose, renewed motivation, and a smoldering desire to make the world a better place. By firmly placing the moral foundation under the tremendous achievements of the Founding Fathers first in my own mind, I then hope to help do so in the minds of others who have chosen the fundamental alternative to live, turning that smolder into a bonfire. One of the most immediately motivating things I learned at OCON this week regards the light tha
Comments
Kim: Not quite fringe patterns. Here’s the short (!) version:
The square plate you see is being shaken (“excited”) by a motor vibrating at increasing frequencies. When the shaker reaches a specific “natural frequency” of the plate, the plate vibrates strongly (i.e. it resonates). The vibration consists of back-and-forth motion in a specific pattern, or “mode shape”. In each of these modes, some parts (or zones) of the plate are moving up and down, while other neighboring parts/zones are simultaneously moving in the reverse direction (i.e. down and up). The white lines highlight the mode shape, or more specifically the boundaries between the zones; the powder collects here since these boundaries don’t move up and down – i.e. you get something like a trough where the powder collects when it is bounced off the moving zones.
When the shaker vibrates at a frequency between the object’s natural frequencies, the vibration amplitude is minimal, and the distinct mode shapes are not apparent.
A simpler example of this phenomenon is a one-dimensional counterpart to the two dimensional plate above: A vibrating string under tension sways back and forth under excitation at its lowest natural frequency. At the next higher natural frequency, the vibration pattern is a reversing S-shape, i.e. there is one point (in the middle of the S) that is motionless, called a node, while on either side the string moves back and forth. At the next frequency the pattern is a reversing W (two nodes), and so on. Here, the nodal points are the one-dimensional counterpart to the white nodal lines in the two-dimensional plate example above. If you observed the edge of a slice through the plate while it was vibrating, you would see similar S shapes.
Kim, happy to share!
http://www.kettering.edu/~drussell/demos.html
(See the links in the section "Vibrational Modes of Continuous Systems")
Enjoy!