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← Little Scientists TechnologySending sound20 minutesStd 1–2

Cup and String Phone

✨ A whisper travels along a piece of string and comes out clearly in a cup five metres away.

Top: two cups joined by a tight string with a zigzag vibration travelling along it from a whisper to an ear. Bottom: the same cups with a slack string that only bends, so no sound arrives.
How it works

🧰 You need

  • two paper cups or steel tumblers
  • about five metres of string
  • a pin or nail for the hole
  • sticky tape
  • a friend

⚠️ Ask an adult to make the hole in each cup.

👀 What you will see

With the string pulled tight, a whisper is surprisingly clear at the far end. With the string slack, almost nothing comes through. If a finger touches the string, the sound stops at once.

🧪 Do it

  1. Ask an adult to make one small hole in the base of each cup.
  2. Thread the string through and tie a big knot inside each cup so it cannot slip out.
  3. Walk apart until the string is straight and tight, with nothing touching it.
  4. One of you whispers into a cup while the other holds the second cup to an ear.
  5. Swap over so you both get a turn.
  6. Let the string go slack and try again.

💡 Why it happens

Sound is made by something shaking quickly back and forth, and that shaking is called a vibration. Your voice makes the bottom of the cup vibrate, the tight string passes those vibrations along, and the far cup shakes the air inside itself so your friend's ear picks them up. A slack string bends instead of passing the shaking along, and a finger on the string soaks up the vibration. Telephone wires do the same job using electrical signals instead of shakes.

🔁 Now try this

Try the same call with thin sewing thread and then with thick wool string, and decide which carries your whisper better.

🤔 Think about it

Why can you hear a train through the rails before you hear it through the air?

🧘 Ask the Acharya

“Why can you hear a train through the rails before you hear it through the air?” — ask him and he will explain it from your own chapters.

Ask Guru →

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Ask GuruAcharya Samayeshwar