Physics Fun: The Science of String Telephones

by | Dec 3, 2024 | STEAM

Before smartphones and cellular technology, landlines in homes and businesses connected people to one another via physical telephone wires and cables. Even though most homes no longer have landlines, a string telephone offers a quaint example for kids to learn a basic physics lesson about how sound waves travel, using simple materials you can repurpose from around your home.

Below, we’ve given a few ways to create a string telephone. By interacting with the string and experimenting with other materials for optional variables, kids can gain hands-on experience of how different factors and materials affect the path of sound waves, which can help them set up hypotheses and begin practicing the scientific method. Let’s get started!

How do string telephones work? | STEMful

Making a String Telephone

What You’ll Need

  • 2 paper cups
  • Toothpicks
  • Twine or string, approx. 25 feet long
  • Embroidery floss, fishing line, wire, or yarn (optional, for variables), each 25 feet long

How to Make the Paper Cup Telephone

  1. Tie a toothpick to each end of the string. Break the toothpick in half if the paper cups are small. The toothepick will need to fit inside without piercing the sides of the cup.
  2. From the outside, help kids pierce the center of the bottom of each paper cup with one end of the toothpick and thread the toothpick through, pulling it until it lies perpendicular to the inside of the bottom of the cup.
  3. Perform the telephone experiment below the variations.

Optional Material Variations: Plastic Cup or Metal Can Telephone

What You’ll Need

  • 2 plastic cups or steel soup cans
  • Drill and bit (adult use only, for metal cans)
  • Thumbtack or awl (be sure to supervise children if using sharp objects)
  1. Tie a toothpick to each end of the string. Break the toothpick in half if the paper cups are small, as they will need to be small enough to fit flush against the bottom of the can or cup.
  2. From the outside, drill or pierce the bottom of each can or plastic up to make a small hole, but wide enough to fit the toothpick end through.
  3. Use the toothpick to thread one end of the string through the hole from the outside.

Telephone Experiment

  1.     Each participant should take one paper cup and then spread out far enough to make the string taut.
  2.     Once the string is taut, have kids speak and listen into the cups to try to carry on a conversation. What do they hear?
  3.     Try the following:
  4.     Pinch the string. What happens to the sound when the string is loose?
  5.     Loosen the string by moving closer together. What happens to the sound when the string is loose?
  6.     Have one participant go into another room or around a corner. What happens to the sound?
  7.     Optional: Repeat the experiment with different material combinations, or try the paper cup experiment with a third participant and cup. Instead of string or twine, use embroidery floss, soft yarn, or wire, or try it with plastic cups or metal cans. Make observations about which materials work best to carry the sound.

Is there a difference between a plastic cup and a metal can for a string telephone? | STEMful

Questions for Discussion

  1.     If you tried different material combinations, which worked best? Why do you think X worked better than Y material?
  2.     What did you notice about the loose string versus the taut string? Which sound was clearer?
  3.     Why do you think the sound stopped when the string was pinched?
  4.     If you tried a third cup, what happened?  

Explanation Guide for Parents

The string telephone may be based on older telecom tech, but the experiment still has valuable lessons to teach kids about how sound waves travel. Older kids may have an easier time understanding sound waves. Sound waves move easily through the air, but they travel even faster through objects such as string.1 

Flexible materials that are able to vibrate, like the bottom of a paper cup, help the sound become amplified when the listener receives it. When you speak into the cup, the sound travels through the air and into the bottom of the cup. The bottom of the cup vibrates when the sound reaches it. From there, the sound travels down the length of the string and into the bottom of the listener’s cup. Like a drum, the sound causes the cup to vibrate, and the sound travels from the air inside the cup into the listener’s ear.

Younger kids may have some difficulty understanding sound waves, but the drum comparison may help illustrate it. Sound waves are a kind of energy that moves through the air, like the light from the sun. But unlike light, sound can travel through an object, such as a wall. Like the top of a drum, the bottom of the cup helps sound move from the air to your ear, because the vibration of the cup makes the sound louder when it moves. The sound wave of your voice is like a hand or stick hitting a drum, making the sound we hear louder when the drum vibrates.

This simple physics experiment is a great one to try over the holiday break, but if you’d like to get your kids more involved in activities like these during school breaks, sign them up for one of STEMful’s school break camps to keep them engaged with STEM learning.

  1. The Science of Sound,” Sound Trek, aired March 5, 2019, PBS video, 4 min., 2 sec., https://www.pbs.org/video/sound-the-science-of-sound-syzigq/.