Salt Vibration: Making Sound Visible

You and your kids can use simple materials to create a DIY Chladni Plate – where you pour salt (or sand) onto a taut membrane, then play music and watch the salt move, making the vibration of sound waves visible to all. It’s easy, cheap, fun, and educational!

How to Make It

I discovered this mostly by accident. I’d started by following the directions on Classroom for “Visible Sound”

“Remove the top and bottom of the can with a can opener. Cut the bottom of the balloon off with scissors. Open the bottom of the balloon wide and slide it over one end of the can. Grab a small mirror and tape it to the balloon… Have [a] students place the open end of the tin can at his mouth. Now positions a flashlight so that the light reflects off the mirror. Ask the student to speak. Your students will be able to see the balloon move the mirror, which moves the light. They are seeing the effect of sound waves.”

I tried following this tutorial, but didn’t really have a small enough mirror and wasn’t that impressed with the effect.

Then I flashed back a couple days to when my husband and I saw the movie Wrinkle in Time. At the beginning of the movie, Meg’s dad shows her a plate with sand or salt vibrating on it in response to sound waves. A-ha!!

I grabbed my Bluetooth Speaker, popped the can over the top of it. I sprinkled on some salt, turned on the music on my phone, to play over the speaker. Boom! Success!

I made four additional devices:

The closed-end can and balloon. I only opened one end of this can. Covered it with a half balloon and taped it in place. I set the closed end of the can on top of the speaker. Advantage: this sort of deadens the sound of the speaker, which is nice in a classroom that’s already really loud, while keeping the full visual impact of the vibration. Disadvantage: You can see that the tip of the balloon makes sort of a “nipple” so the surface is not totally flat, so the salt patterns can’t form as well. I know it’s possible to get latex sheets (e.g. a dental dam, which is 6×6 or resistance bands) – those might be better.

The hot chocolate tin and saran wrap. I cut the bottom of the tin. Advantage: Bigger surface for the salt than a small can. You don’t have to tape the cling wrap… it just clings in place. Disadvantage: The saran wrap has a bit of a wrinkle in it.

The soda cup and plastic wrap. I cut the bottom off a 32 ounce cup. The plastic on this one is some plastic that was wrapped around some package we got. It was a little sturdier than the saran wrap.

The pot. This is the biggest pot I own, so the biggest surface I tested. I didn’t have rubber bands big enough to hold the saran wrap on, and it wouldn’t “cling” to the metal. So, I used two long skinny balloons to band it on. I just set the speaker in the bottom of the pot, which worked fine.

Testing the Devices

Music Choices: We tried lots of different types of music. You get the most response with: higher pitches (we liked All I Do Is Dream of You from Singing in the Rain… but be warned, keep the volume low for the dramatic opening of the piece, or it will bounce all the salt right off the table!), pieces with lots of organ vibration (think Toccata and Fugue in D Minor or Phantom of the Opera), pieces with lots of percussion (Angry Dance from Billy Elliott and Logo te Pate from Moana). My favorite was Mahna Mahna from the Muppets, and Flight of the Bumblebee is pretty delightful, as the salt dashing around wildly does look a bit like a swarm of insects!

Single Frequency: If  you had a perfectly designed device, with a perfectly flat membrane on top, you’d get a really cool effect. Different musical frequencies make different patterns in the sound. Each pitch is associated with a characteristic shape. (See a slideshow of this effect: https://skullsinthestars.com/2013/05/02/physics-demonstrations-chladni-patterns/#jp-carousel-7352 or a video of it here: www.youtube.com/watch?v=YedgubRZva8) or this one (make sure your volume isn’t high before watching this!)

I tested my devices at different frequencies. (To find recordings, just go to YouTube and search for “frequency test speakers” to get recordings of a wide range of frequencies, or just search for one particular frequency, such as 20 khz (high), 250 hz (mid-range) or 20 hz (sub-bass). You’d type into the search “250 hz test tone” to find videos that play one note for a minute or two at a time. Or you can use a tuner app on your phone.

Since none of my membranes were perfectly smooth and flat, I didn’t get really detailed patterns. But, I definitely got different patterns for different tones. But, my dog hated this experiment, and my housemates shouted down the stairs “Whatever you’re doing, would you just cut it out??”

A few days later, we finished off a container of Swiss Miss Hot Chocolate. I took the plastic lid off the top, turned it over, put it on top of the speaker, and sprinkled salt on the flat metallic bottom. Better results than on the plastic surfaces!

Volume: The louder the music, the more movement you’ll get. Sometimes you need to control for this… if the music is too loud, the salt all just bounces off immediately, and it’s not that interesting… so plan to adjust the volume up and down as you go for the best effect.

This Science Buddies article recommends a step-by-step testing process that would be fun too. They built their device by putting wax paper or parchment paper atop a glass bowl.

Using this in Class

We used this for our Five Senses theme, when we studied Hearing. At the end of our group time, I told kids that sound travels in waves. I asked them if they’d ever seen sound. They all said no. I said that sound waves can make things vibrate – I asked if they’d ever felt the vibration of sound. Some had. My co-teacher Cym showed them her harp – she played a note, and we all saw how the string vibrated as long as the note was playing, but if we stopped the vibration, we stopped the sound. Then, I pulled out the cocoa tin and the speaker and showed them how it worked. They were all totally captivated. After a few minutes of watching, we told them that anyone who wanted to could leave group time and return to station exploration and art projects, but many stayed watching the salt. For the next 30 minutes, I had kids rotating in and out of watching the salt dance.

Note: you’ll want to put the device on top of a tray, to contain the salt mess, because over time, the salt all bounces off and you have to keep sprinkling more and more salt on.

Learning More about the Science and the History

After class, I realized I didn’t really know what to call this device, or the scientific device I’d seen in Wrinkle in Time, so I went searching.

History

Da Vinci noticed that if a table was struck and vibrated, the dust on it would settle in typical patterns and Galileo noticed that brass filings would settle into patterns when a plate was scraped with a chisel. (Source) In 1680, Robert Hooke covered a glass plate with flour, then ran a violin bow along the side of the plate to create a vibration. He saw typical nodal patterns appear.  In the 1780’s, Ernst Chladni used metal plates and sand. (Image from Wikipedia.) To learn more about the history and science of visualizing sound waves, read this article on Cymascopes.

File:Bowing chladni plate.png

This vibrating device is similar to how our ears hear sound: Our eardrum is a thin membrane. When sound waves hit the eardrum, they make it vibrate, like the sound is vibrating the balloon or plastic on these devices. Those vibrations are transferred to the cochlea, a fluid filled chamber in our ear. Then the vibrations are interpreted by the brain. (Source)

More Ideas

Another way kids can feel sound vibrations: have them hold a balloon in their hands, near a speaker playing music. (Details.)

You could make a much more sophisticated device using this Instructable or this Make tutorial.

Here’s a lesson plan on Chladni plates for older kids – maybe middle school? http://sciencenetlinks.com/lessons/making-sound-waves-visible-exploring-chladni-plates/

Here’s a video of a Chladni plate responding to sung tones: https://munrovian.wordpress.com/2010/03/16/curiosity-chladni-plates/.

Here’s how to make a tonoscope to sing into: https://www.youtube.com/watch?v=jhL933QoK_Y, and a video of someone singing Mozart into a tonoscope: https://www.youtube.com/watch?v=KU84ckD1AcA.

A modern Cymascope device shows much more sophisticated patterns:

 

Entertaining sidenote: After I went through the process of building this device, testing with kids and THEN researching it, I ran across this post from Frugal Fun for Boys, where she shares that she did almost the same thing! Check out her post as well.

If you make one of these, let us know about your experience in the comments!

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