How to Make a String Telephone

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Do you remember making a paper cup phone when you were in elementary school?

It’s funny to discuss how telephones work with our kids nowadays. We haven’t had a landline for the longest time, so our kids have no clue what a traditional telephone looks like.

So it takes a little work to explain why we need a string to transmit the sound. 

But the kids loved this paper cup phone STEM project! If you don’t remember doing this when you were a kid, here is a little reminder!

How to Make a Telephone with Cups and String

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Supplies:

Instructions:

  1. Cut a small slit on the bottom of each paper cup.
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  2. Tie a knot with the string at one end of the paper clip.
    String-Telephone-Paper-Clip-String
  3. Repeat #2 with the other end of the string.
  4. Stick the paper clips in the small holes at the bottom of the paper cups.
  5. Grab the other ends of the paper clips and pull the paper clips and string through.
    String-Telelphone-Pull-String-Through
  6. Have your kids take each paper cup and walk away from each other until the string the taut.
  7. Have one kid put the paper cup to his/her ear and have the other one softly speak into his/her paper cup. 
    String-Telephone-Hear

If your kid can’t hear the other person with the paper cup telephone, check the following:

  • Is the string taut?
  • Is the string touching another object besides the cups?
  • Is there noise in the background? If so, go to a quiet area.

The Science Behind the DIY String Telephone

The string telephone is the perfect experiment to explain to your kids about sound waves. 

When we speak, our vocal cords make molecules in the air vibrate. Have your kids hum a song while holding their hands against their throats to feel the vibration. The vibration is what creates sound waves.

When you speak into a cup, your voice vibrates the air inside the cup, which in turn vibrates the bottom of the cup. These vibrations travel along the string and then vibrates the cup on the other end. The vibrations are converted to sound waves, and that’s why the person can hear through the other cup.

You will find that keeping the line taut is the key to making the string telephone experiment work. When the string is slack, the vibrations dissipate along the way and the never reach the cup on the other end. 

Another tip is to make sure the string doesn’t touch anything along the way. Otherwise, the sound waves will travel the wrong way.

You can use the DIY string telephone to hear surprisingly far! 

Here are a few experiments for your kids to try:

  • Use a plastic cup instead of a paper cup or vice versa. Does the type of cup matter?
  • Use different types of strings and observe how the strings change the quality of sound. You can try yarn, cotton twine, fishing line, kite string, etc.
  • Try different string lengths. How long can you make the line and still hear each other?
  • Try making the string slack. Can you still hear anything? 
  • Touch the string to an object. Can you still hear anything?
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Final Thoughts on the Paper Cup String Telephone Activity

My kids cannot grasp that there was once when mommy and daddy had to use a landline telephone.

In case your kid asks why a landline telephone doesn’t have a straight, taut wire between the two telephones, here is the answer: Landline telephones have microphones that convert sound waves into electrical signals. The electrical signals can travel through wires and the telephone on the other end has an earphone inside that converts the electrical signals back to sound waves.

I hope your kid enjoyed traveling back in time and talking through a string telephone. The activity certainly brought back good memories for me!

For a sound wave activity that you can SEE, check out the salt vibration experiment.

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