salt vibration: making sound visible inventors of tomorrow
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. Its easy, cheap, fun, and educational!
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.
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, Megs dad shows her a plate with sand or salt vibrating on it in response to sound waves. A-ha!!
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 thats 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 cant form as well. I know its possible to get latex sheets (e.g. a dental dam, which is 66 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 dont have to tape the cling wrap it just clings in place. Disadvantage: The saran wrap has a bit of a wrinkle in it.
The pot. This is the biggest pot I own, so the biggest surface I tested. I didnt have rubber bands big enough to hold the saran wrap on, and it wouldnt 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.
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, youd 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 isnt 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). Youd 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 didnt 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 youre 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 youll get. Sometimes you need to control for this if the music is too loud, the salt all just bounces off immediately, and its not that interesting so plan to adjust the volume up and down as you go for the best effect.
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 theyd ever seen sound. They all said no. I said that sound waves can make things vibrate I asked if theyd 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.
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 Hookecovered 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 1780s, 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.
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)
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.
I teach Discovery Science Lab and Family Inventors' Lab, STE(A)M enrichment classes in Bellevue, Washington for ages 3 - 9. I am also a parent educator for Bellevue College, a childbirth educator for Parent Trust for Washington Children, former program designer for PEPS - the Program for Early Parent Support, a social worker, and mother of 3 kids - age 26, 22, and 9.)
View all posts by Janelle Durham
 Making Sound Visible: Build a Chladni plate / salt vibration device and watch the salt dance to music! Find the whole tutorial and the history of cymascopes here:https://inventorsoftomorrow.com/2018/03/28/salt-vibration/ 
 Salt vibrations Its easy to make one of these devices, using a can or a plastic cup, put on plastic wrap or a balloon and tape it or rubber band it in place. Then sprinkle salt on it and play music the salt will dance with the vibrations of the music. 
Thanks so much for this post. Def going to make a tonoscope using a speaker with my daughters 3rd grade class. I found a bunch online but this page filled in all the gaps. Well written, wide ranging, what the internet SHOULD be. (IMHO)