Every day, the majority of the world speaks and hears sound. There’s a lot more going on behind the scenes than just what you hear, however. With a simple little invention, a tin can telephone, we can learn so much about sound waves and how they travel!
What you need:
- Two tin cans with their tops removed
- Duct tape
- Up to 10 ft long string
- Craft supplies to decorate
What you do:
First, cover the sharp edges around the open side of the can with the duct tape. Next, let the kids decorate their tin can phone however they’d like.
Once they’ve finished their masterpieces, flip the cans upside down so their open end is against a flat surface with the close end facing up. Use the hammer and nail to punch a hole into the closed end of the can.
Remove the nail and poke one end of the string through. Knot the string inside the can so that the knot cannot be pulled out. Repeat with the other end of the string in the other can.
Have two children try speaking into the cans with the string not pulled tight to see if they can hear each other. Next, have them pull the string taut then speak into the cans again. They should be able to hear one another!
Questions to consider:
- Why does the “phone” only work when the string is pulled tight?
- Try a different length of string. Can you hear one another better or worse or is it the same?
- Would you be able to hear each other around a corner?
Why it works:
Sound travels through waves and vibrations. With our tin can telephone, speaking into one can then sends the sound vibrations through the string to the other can!
When you speak into the can, your voice creates air vibrations that travel into the can, vibrate the bottom of the can, which in turn vibrates the string all the way over to the other can, in turn vibrating the other can’s bottom, then the air again.
Sound waves travel so fast — 343 meters per second in air — that we don’t notice the delay as they move! This way, our friends can hear us on the other side. So why do sound waves travel faster in solids than in air?
It has to do with how the particles are packed together. In gases, molecules are very spaced out, so vibrations take longer to bounce between them. In liquids, the molecules are closer, but not super closely spaced. Because of this, sound travels faster since the vibrations can bounce more readily. In solids, where sound travels the fastest, the molecules are packed extremely tightly with little space between. This way, the sound vibrations can easily bounce through from one side to the other with little space to jump.