How Do Sound Frequencies Create Symmetrical Patterns?

[VJR85]

http://www.bestofgooglevideo.com/video.php?video=344

What causes the symmetrical patterns with the different sound frequencies?

 

[rcgldr]

The entire plate can't vibrate with a single node at high frequencies, so multiple nodes are formed. The higher the frequency the more the nodes. At most frequencies, the nodes keep moving around, but at some frequencies, due to harmonics, the nodes stablize and don't move. The salt accumulates between nodes where there is little movement, and outlines the borders between nodes.

The patterns also depend on how the plate is mounted (to a solid object, or suspended by wires), and where the plate is mounted (at the center, at the corners, ...)

If you do a web search for vibrating plate salt, you find a lot of links, like this one:

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/sound/u11l4c.html

 

[VJR85]

Thanks...nice link, too.

 

[jasc15]

Those looked like hyperbolas at the beginning

 

[Danger]

Strangely reminiscent of crop circles. Hmmm...

 

[jasc15]

Strangely reminiscent of crop circles. Hmmm...

Almost, but notice that the patterns in crop circles have sharp distinct boundaries (i.e. stalks that are flattened are right next to upright ones, there arent usually any that ware partially bent), whereas the resonant patterns dissipate away from the nodes (although quite sharply in some examples)