What causes the salt pattern in the sound wave video?

[glueball8]

Is this real?

http://www.metacafe.com/watch/416486/salt_sound_waves/

What's happening?

 

[Born2bwire]

Real? Eh, sure why not. It looks like the salt is tracing out cavity modes. I don't know how they have it set up, but my guess is that they have a slightly-rigid sheet on the top of a rectangular box. Inside the box, (maybe at the bottom edge or in the middle) is a speaker. The salt will shift and move until it reaches a nodal point on the sheet. These nodal points are associated with the cavity modes in the box. As he changes the frequency, the modal solution changes accordingly.

 

[Nick89]

It is probably just a rigid (metal?) sheet on which they poured some salt.
They then make the sheet vibrate with a specific frequency, which results in standing waves. In a standing wave, there are point where the medium is not moving (often called a node). The medium is moving (vibrating) anywhere else.

Any salt grain that lies on a piece of sheet vibrating very fast will constantly be pushed away from that point, until it ends up in a node. Because the sheet does not move there, the salt grain can stay there.

This causes the salt to gather in specific positions; namely the nodes of the standing waves, or points where the sheet does not vibrate.Have a look at Wikipedia, specifically the pictures.
http://en.wikipedia.org/wiki/Standing_wave

Look at the first picture and imagine it being the side of the metal sheet. Then imagine pouring some salt on it. Where would the salt move to?

Also look at the second 3D picture (circular sheet). If you would pour some salt on a surface vibrating like that, the salt would accumulate in the middle, because the sheet there is not moving.

 

[FredGarvin]

I can't see the video that you linked to, but by the answers given, you may also want to look int Chladni patterns. This is a well know vibration visualization technique.

http://www.phys.unsw.edu.au/jw/chladni.html
http://www2.physics.utoronto.ca/~nonlin/chladni.html
http://www.google.com/url?sa=U&star...5jrVcA&usg=AFQjCNFWXiEnojlHPlrWBP-x8j0AVCp9IA