How Does a String Telephone Work?

A string telephone might look simple, but it demonstrates one of the most important ideas in science—sound travels as vibrations.

When you speak into the cup, your voice creates tiny vibrations in the air. These vibrations make the bottom of the cup move backwards and forwards. Because the string is attached to the cup, those vibrations travel into the string and begin moving towards the second cup.

Why Must the String Be Tight?

The string only works when it is pulled tight.

A tight string allows the vibrations to travel from one end to the other. If the string becomes loose, those vibrations quickly disappear instead of continuing along the string.

That's why even a small amount of slack can stop your secret spy phone from working.

How Does the Second Cup Produce Sound?

When the vibrations reach the second cup, they make its base vibrate too.

These vibrations push on the air inside the cup, creating sound waves that travel to your friend's ear.

The second cup changes the tiny vibrations in the string back into sounds that you can hear.

Why Did Some Cups Work Better Than Others?

Not all materials vibrate in the same way.

Paper cups, plastic cups and metal cans each transfer vibrations differently. Some materials allow vibrations to travel more efficiently, while others absorb part of the energy.

That's why changing the cup can change how loud or clear your spy phone sounds.

Where Is This Science Used?

Understanding vibrations helps engineers design:

• Mobile phones

• Microphones

• Headphones

•  Loudspeakers

• Communication systems

The same idea that lets two paper cups carry your voice is used in many technologies that allow people to communicate every day.

The Next Question...

Your spy phone carried sound through a string...

Could sound travel through fishing line?

What about wire?

Or even a metal coat hanger?

There's another mystery waiting to be solved.