Chapter 4 of 8

All 8 chapters
  1. 1Sounds come from vibrations, readunderstood
  2. 2Sound travels, readunderstood
  3. 3Echoes, readunderstood
  4. 4A string telephone, readunderstood
  5. 5Loud and quiet, readunderstood
  6. 6High and low notes, readunderstood
  7. 7How we hear, readunderstood
  8. 8Noise and our ears, readunderstood

S2.7.4 Chapter 4

A string telephone

How can a string carry my voice to a friend?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Make a string telephone with two cups and string
  • Explain why the string must be tight for it to work
  • Explain that the string itself vibrates

Two cups and a string can carry your voice. You can make one today.

Your voice makes the air vibrate. The air makes the bottom of the cup vibrate.

Draw it first

Draw a string telephone

  1. Two cups with their open ends facing out, joined by a straight string, labelled cup and string. An arrow labelled voice points into the left cup. Dashed lines show the string moving up and down, labelled vibrates. An arrow labelled sound leaves the right cup.cupstring

    1 Draw two cups with a straight string between them. Write cup and string.

  2. Two cups with their open ends facing out, joined by a straight string, labelled cup and string. An arrow labelled voice points into the left cup. Dashed lines show the string moving up and down, labelled vibrates. An arrow labelled sound leaves the right cup.cupstringvoice

    2 Draw an arrow into the first cup. Write voice.

  3. Two cups with their open ends facing out, joined by a straight string, labelled cup and string. An arrow labelled voice points into the left cup. Dashed lines show the string moving up and down, labelled vibrates. An arrow labelled sound leaves the right cup.cupstringvoicevibrates

    3 Draw dashed lines for the string moving. Write vibrates.

  4. Two cups with their open ends facing out, joined by a straight string, labelled cup and string. An arrow labelled voice points into the left cup. Dashed lines show the string moving up and down, labelled vibrates. An arrow labelled sound leaves the right cup.cupstringvoicevibratessound

    4 Draw an arrow out of the far cup. Write sound.

The first cup bottom shakes the string. The vibration runs along the string. At the far end it shakes the other cup bottom.

That cup bottom makes the air in the cup vibrate. Your friend hears your voice.

Six boxes in a column, with an arrow pointing down between each pair. From the top the boxes are labelled: your voice, cup bottom, tight string, far cup bottom, air in the cup, friend's ear.your voicecup bottomtight stringfar cup bottomair in the cupfriend's ear
Each part passes the vibration to the next.

The string must be tight. A tight string is pulled straight. A slack string is loose and sags.

A slack string soaks up the vibration. It does not pass it on.

The string must touch nothing, too. A hand, a fence or a wall can stop the vibration.

Two cups joined by a straight string, with the left cup labelled cup. An arrow labelled voice points into the left cup, because that is the cup spoken into. Two short bars pinch the middle of the string, labelled pinch. Dashed lines show the string moving between the left cup and the pinch, labelled vibrates. The string right of the pinch is straight, labelled still.cupvoicepinchvibratesstill
A pinched string cannot pass the vibration on.

See it three ways

As a picture

Two strings, one above the other. The top string is straight, labelled tight, and an arrow above it shows the vibration passing along it. The bottom string sags in the middle, labelled slack.tightslack

In words

A tight string passes the vibration along. A slack string does not.

Try it

Make a string telephone

You need: two paper cups, or two empty tins with smooth edges, string, 5 to 10 metres long, a nail, for a grown up to use, a friend.

Be safe: A grown up makes the holes. Check tin edges are smooth. Never put string around anyone's neck.

  1. A grown up makes a small hole in each cup bottom.
  2. Push one end of the string into a hole, from outside.
  3. Tie a big knot inside the cup. It stops the string slipping out.
  4. Do the same with the other cup and the other end.
  5. Walk apart until the string is tight. Keep it off the ground and away from walls.
  6. Talk quietly into your cup. Your friend holds their cup to their ear.
  7. Take two steps closer, so the string sags. Talk again.
  8. Step back until it is tight. Pinch the string while you talk. Then let go.

What you will see: Your friend hears you when the string is tight. When it sags, they hear little or nothing. When you pinch the string, the sound stops at once. When you let go, it comes back.

If you cannot try it, this is what happens.
A table with two columns, String and Sound, and three rows. A tight string gives clear sound. A slack string gives little or none. A pinched string gives none. Labels: String, Sound, tight, clear, slack, little or none, pinched, none.StringSoundtightclearslacklittle or nonepinchednone

A wrong idea many people have

The sound goes along inside the string, like water in a pipe.

Why it feels right: The voice goes in one end and comes out the other. It seems to run through the string.

The test that breaks it: Water goes through a loose pipe as well as a tight one. But a slack string carries almost no sound. Pull it tight and the sound comes back. Hold the tight string lightly while your friend talks. You feel it buzz.

What is true: The string itself vibrates. It passes the vibration along. Nothing runs inside it.

Key words

tight
pulled straight, with no sag.
slack
loose, so it sags.

Back to your guess

The answer is C: The string shakes, and the shaking carries the sound.

The string shakes. Your voice makes the first cup vibrate. The tight string passes the vibration to the far cup.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Why must the string of a string telephone be tight?

    Tap your answer.

    1. Not this one. No sound runs inside the string. The string vibrates. A slack string soaks up the vibration. Try another.

    2. Not this one. A slack string still holds the cups. It just does not carry the vibration. Try another.

    3. Not this one. A tight string does not change your voice. It only passes the vibration along. Try another.

    4. Yes, that is right. The vibration runs along a tight string. A slack string soaks it up.

    See the answer

    A tight string can pass the vibration along.

    The vibration runs along a tight string. A slack string soaks it up.

    • If you chose A: No sound runs inside the string. The string vibrates. A slack string soaks up the vibration.
    • If you chose B: A slack string still holds the cups. It just does not carry the vibration.
    • If you chose C: A tight string does not change your voice. It only passes the vibration along.

  2. Ravi talks into his cup. What happens right after the cup bottom vibrates?

    Tap your answer.

    1. Yes, that is right. The cup bottom shakes the string. Then the string shakes the far cup.

    2. Not this one. The vibration must pass along the string first. The far cup shakes last. Try another.

    3. Not this one. The string does not fill up. It shakes and passes the shaking on. Try another.

    See the answer

    The tight string vibrates.

    The cup bottom shakes the string. Then the string shakes the far cup.

    • If you chose B: The vibration must pass along the string first. The far cup shakes last.
    • If you chose C: The string does not fill up. It shakes and passes the shaking on.

  3. Mei's telephone works. Then her friend steps on the string. The sound stops. Why?

    Tap your answer.

    1. Not this one. A string does not store sound. It passes vibrations on, and a pressed place cannot shake. Try another.

    2. Yes, that is right. A pinched or pressed string cannot shake. The vibration stops there.

    3. Not this one. Sound does go through solids like a tight string. The pressed place stops the vibration. Go to S2.7.2. Try another.

    See the answer

    The string cannot vibrate where it is pressed.

    A pinched or pressed string cannot shake. The vibration stops there.

    • If you chose A: A string does not store sound. It passes vibrations on, and a pressed place cannot shake.
    • If you chose C: Sound does go through solids like a tight string. The pressed place stops the vibration. Go to S2.7.2.

  4. Ana's string is 10 metres long. Joseph stands 7 metres from her, holding his cup. The string sags. How far must he move to pull it tight?

    See the answer

    Walk 3 metres further apart.

    The string is 10 metres long. He is 7 metres away. 10 − 7 = 3, so he walks 3 metres further.

  5. From an earlier chapter

    You talk into the cup. What does the bottom of the cup do?

    Tap your answer.

    1. Yes, that is right. Your voice makes the air vibrate. The air makes the cup bottom vibrate.

    2. Not this one. It looks still, but it vibrates too fast to see. Every sound comes from a vibration. Go to S2.7.1. Try another.

    3. Not this one. Cooling has nothing to do with it. Your voice makes the cup bottom vibrate. Go to S2.7.1. Try another.

    See the answer

    It vibrates.

    Your voice makes the air vibrate. The air makes the cup bottom vibrate.

    • If you chose B: It looks still, but it vibrates too fast to see. Every sound comes from a vibration. Go to S2.7.1.
    • If you chose C: Cooling has nothing to do with it. Your voice makes the cup bottom vibrate. Go to S2.7.1.

  6. Draw a string telephone. Show how the vibration goes from one cup to the other.

    See the answer

    Two cups joined by a tight string. Dashed lines show the string moving. An arrow goes from the first cup to the second.

    The vibration starts at the first cup bottom. It runs along the tight string to the far cup.

  7. Kofi holds the tight string lightly while Ana talks into her cup. What does he feel?

    Tap your answer.

    1. Not this one. The sound does not run inside the string. The string itself shakes, and his fingers feel it. Try another.

    2. Yes, that is right. The string passes the sound on by vibrating. A light touch feels that buzz.

    3. Not this one. A voice does not pull. It shakes the string, and he feels a buzz. Try another.

    See the answer

    The string buzzing, because it vibrates.

    The string passes the sound on by vibrating. A light touch feels that buzz.

    • If you chose A: The sound does not run inside the string. The string itself shakes, and his fingers feel it.
    • If you chose C: A voice does not pull. It shakes the string, and he feels a buzz.

  8. Look at the picture. Two string telephones, P and Q. Which one carries a voice better?

    Two string telephones, one above the other. The top one is marked P and has a straight string. Its parts are labelled cup and string. The bottom one is marked Q, and its string sags in the middle. Labels: P, Q, cup, string.PcupstringQ
    Tap your answer.

    1. Not this one. The string of Q sags. A slack string soaks up the vibration. Try another.

    2. Not this one. A string that sags soaks up the vibration. Only the straight string passes it on well. Try another.

    3. Yes, that is right. The string of P is tight, so it passes the vibration on. The string of Q sags.

    See the answer

    Telephone P

    The string of P is tight, so it passes the vibration on. The string of Q sags.

    • If you chose A: The string of Q sags. A slack string soaks up the vibration.
    • If you chose B: A string that sags soaks up the vibration. Only the straight string passes it on well.

Keep it

Copy this into your exercise book.

Your voice makes a cup bottom vibrate. A tight string carries the vibration to the far cup. A slack string, or a pinched one, stops it. The sound does not go inside the string.

Draw: Two cups and a tight string. Dashed lines for the string moving. Arrows for voice and sound.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (610 KB). It opens on any phone.