How To Make A Simple String Telephone | Mombrite

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Make a string telephone and explore how sound travels through vibrations in this fun, hands-on science experiment kids love to try at home or school.

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. What a different world we live in!

Get your kids talking with this string telephone you can make with just a few items you probably already have around the house. My kids loved this paper cup phone STEM project and used the string telephone to play educational games.

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

Supplies:

• 2 paper cups (you can also use disposable plastic cups)
• String (I used yarn because that’s what I had available at home)
• 2 paper clips

Instructions:

1. Cut a long piece of string. You can experiment with different lengths, but I recommend no more than 6-8 feet to start for younger kids. It’s difficult for the younger ones to understand that they need to keep the string taut for this experiment to work, and a longer string would be harder to work with.

2. Cut a small slit on the bottom of each paper cup.

3. Thread the string through each cup.

If you are having trouble, you can do step #4 first and insert the end of the string through a paper clip. The paper clip can help you push the string through the small slit.

4. Slip a paper clip through each end of the string and tie a knot to prevent the string from falling out.

3. Have one or both kids walk slowly away from each other until the string is taut. The string should be tight but not touch anything.

4. Have one kid put the paper cup to his/her ear and have the other one softly speak into his/her paper cup.

If your kid can’t hear the other person with the paper cup telephone, you might be asking yourself, “Do cup phones actually work?” I can tell you that they really do, but make sure you 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 sound waves to your kids.

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. Vibration 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 vibrate the cup on the other end. The vibrations are converted to sound waves, so 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, vibrations dissipate along the way and 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!

Science Extensions: Take the Experiment Further

Once kids understand how the string telephone works, try these simple extensions to explore sound and vibrations in more depth.

Tight vs. Loose String

Question: Does string tension affect sound?

Try talking with the string pulled tight, then slightly loose.

What to notice: Sound is clearer when the string is tight because vibrations travel better through a stretched string.

Short String vs. Long String

Question: Does distance change how well sound travels?

Test a shorter string and a longer string using the same cups.

What to notice: Shorter strings usually carry sound better because vibrations don’t have to travel as far.

Try Different Types of String

Question: Does the type of string matter?

Test cotton string, twine, yarn, or fishing line.

What to notice: Smooth, sturdy strings carry sound better than fuzzy or stretchy ones, which absorb vibrations.

Try Different Types of Cups

Question: Do different cups affect the sound?

Use paper cups, plastic cups, foam cups, or metal cups (if you can manage to drill a hole).

What to notice: Stiffer cups usually carry sound more clearly because they vibrate better than soft or flimsy cups.

Loud Voice vs. Quiet Voice

Question: Do louder sounds travel differently?

Have one person whisper and then speak loudly into the cup.

What to notice: Louder sounds create stronger vibrations, making them easier to hear on the other end.

What Happens If the String Touches Something?

Question: Can sound still travel if the string touches a chair or table?

Let the string rest against an object and try again.

What to notice: The sound becomes weaker or disappears because vibrations escape into the object.

Prediction & Reflection

Before testing, ask:

• What do you think will happen?
• Which setup will work best?

After testing, ask:

• What worked the best?
• What changed when you adjusted the cups or string?
• What would you try next?

Encourage kids to change only one thing at a time (string type, cup type, length, or tension). This helps introduce fair testing, an important science skill.

Frequently Asked Questions About the String Telephone Experiment

Why does the string need to be tight?

The string needs to be tight so sound vibrations can travel through it. When the string is loose, the vibrations weaken, and the sound becomes hard to hear.

Does the string telephone work with any type of string?

It works best with smooth, sturdy string, like cotton string, twine, or fishing line. A very stretchy yarn or fuzzy string can absorb vibrations and make the sound quieter.

How long can the string be and still work?

Shorter strings usually work better because the sound vibrations don’t have to travel as far. As the string gets longer, the sound may become quieter or harder to hear.

Why can’t you hear the sound if the string touches something?

When the string touches an object, the vibrations escape into that object instead of traveling straight to the other cup. This weakens the sound or stops it completely.

Final Thoughts on the Paper Cup String Telephone Activity

My kids cannot grasp that there was once a time when mommy and daddy had to use a landline telephone. The closest thing they had was a walkie-talkie, but even then, it was wireless.

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 them back into 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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• Fun and Easy Echolocation Activity for Kids
• Easy Sound Wave Water Experiment for Kids
• How to Build Balloon-Powered LEGO Cars

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