Friday, September 26, 2008

Sound Waves



Salt sound waves video. Fascinating, but watch out if you have pets around. By mid-point, my cats were looking around frantically and a moment later, they took off like bottle rockets. Then the sound gets so high that it hurts my ears. But it's neat to watch the salt. And the cats. Quite possibly the most awesome thing involving salt ever.

17 comments:

Mo Jones said...

This is really cool - I had to watch with the sound off though!

M. Kaplan said...

Sound is generated as a series of pressure waves hitting your eardrum.

The classic example is you take a string and wave it up and down. If you hit the right frequency (dependent upon the length of the string and the tension on the string) it'll form what is called a standing wave. It looks very much like a bunch of footballs placed end to end. The middle of the football is called an "Anti-Node" and the ends of the football are known as nodes.

If you'll notice, the salt only forms shapes at specific frequencies, and all the shapes radiate out from the center. If you look closely, you'll notice that's what is actuating the table the salt is sitting on. So think of the table as a big piece of string with all these invisible standing troughs and peaks. The salt gets repelled from the peaks and comes to rest in the troughs. What you're seeing is where all the nodes fall on that square table.

For more reading: http://en.wikipedia.org/wiki/Standing_Wave

Anonymous said...

I'm gonna go for Lot's wife turning into a pillar of salt was the gnarliest thing ever to happen with salt... saying that it happened.

garble said...

Is there a reason why the pattern isn't forming a more perfect circular pattern outward from the center? Is it because the sound waves are traveling at a slightly higher speed towards the corners?

joshyMinor said...

Fascinating indeed! Nice!

Jiff
http://www.privacy-center.ru.tc

Anonymous said...

How were the sound frequencies being generated? Did you start at one end of the scale and just work your way up? was it just a board sitting on top of a speaker? I'd like to learn more about the setup. The patterns that were formed were really cool.

ErnieG said...

First, M. Kaplan hit the nail on the head. These are classic standing wave patterns. Now, Garble, to answer your question, the reason that the wave patterns are not perfect circles is that the vibrating medium is not a circle, but a square. If the experiment had been done with a circular flat disc, many of the patterns would have resembled bulls-eye targets. These patterns are actually visible representations of solutions to complicated sets of differential equations, which exist for practically any shape.

Standing waves in circular membranes have been studied extensively, because

(a) they are a relatively simple case, and
(b) they have practical application in the design of drums i.e. controlling the pitch and getting "good sound":

http://physics.usask.ca/~hirose/ep225/animation/drum/anim-drum.htm

Nick said...

The findings are based on the Rosslyn Cubes.
http://www.earthfiles.com/news.php?ID=1434&category=Environment

I had a post about this on my forum a while back (http://www.animeblend.com/forums/showthread.php?t=2437).

It's amazing though.

Anonymous said...

the patterns are dependent on what are called "eigenstates" of the plate. you can actually compute what the patterns will look like by solving partial differential equations for the drum (in this case a vibrating plate). it is a weird combination of physics and math that is known as applied mathematics. do a wikipedia search for eigenvalues or the eigenvalues of a drum. fascinating subject - i once heard a guy named Trefethen speak on the subject of eigenfunctions/values and how they have accounted for a rather large number of noble prizes in physics over the last century.

Anonymous said...

if you want a really clear understanding of the principles behind this look like eigenvalues/functions for drums. not so much physics as it is applied mathematics.

M. Kaplan said...

I've never done the exact analysis of a standing wave on a square object, but purely by conjecture, I suspect it's actually that the ends are oscillating slower than the rest of the table.
As a thought experiment liken it to the doppler effect, as you trail away from origin of the sound, the pitch deepens indicating a lower frequency. (Note: That's purely a thought experiment and NOT indicative of what's actually going on here)

I would be quite interested in seeing someones work on the analysis of various shapes with a standing oscillator at the center.

Kevin Croker said...

To Kaplan and Gargle:

Not only does the surface's boundary shape matter, but whether the boundary itself is free to move. See sound waves in open and closed pipes for a single dimensional example. Same idea in 2 dimensions, they just form sweet patterns.

Anonymous said...

The best thing ever done with salt? My memories of my grandparents making ice cream using rock challenge that assumption.

Anonymous said...

http://en.wikipedia.org/wiki/Cymatics

Yanik said...

That is the coolest thing ever. I've been going through your archives. Lots of really interesting things here. I'll be visiting often. Thanks!

Erich said...

My cat came running up trying to figure out where the noise was coming from.

Anonymous said...

THANKS FOR SHARING WHAT WERE YOU USING FOR SOUND TO CREATE THE VARIOUS SHAPES?