Chapter 3 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

S2.7.3 Chapter 3

Echoes

Why do I hear my shout again from a far wall?

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.

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

By the end you can

  • Explain that an echo is sound bouncing back from a hard surface
  • Say which things bounce sound back and which take it in
  • Explain how bats and dolphins use echoes to find food

Shout at a big wall far away. A moment later, you hear your shout again.

That sound is an echo. An echo is sound that bounces back.

A hard surface is a hard, flat face. Walls, cliffs and the inside of a well are hard surfaces. They bounce sound back.

Draw it first

Draw an echo

  1. A person on the left and a wall on the right. A top arrow goes from the person to the wall, labelled sound. A lower arrow goes back from the wall to the person, labelled echo. A dashed line under them is labelled 17 m or more.sound

    1 Draw a person and a wall. Draw an arrow to the wall. Write sound.

  2. A person on the left and a wall on the right. A top arrow goes from the person to the wall, labelled sound. A lower arrow goes back from the wall to the person, labelled echo. A dashed line under them is labelled 17 m or more.soundecho

    2 Draw an arrow coming back from the wall. Write echo.

  3. A person on the left and a wall on the right. A top arrow goes from the person to the wall, labelled sound. A lower arrow goes back from the wall to the person, labelled echo. A dashed line under them is labelled 17 m or more.soundecho17 m or more

    3 Draw a dashed line between them. Write 17 m or more.

The sound goes to the wall and comes back. It makes the trip twice.

You hear a separate echo only when the wall is far. It must be about 17 m away or more. Then the echo comes back a moment after your shout.

If the wall is closer, the echo comes back too fast. It joins your shout.

Soft things take in sound. Cloth, cushions and grass soak it up. They bounce back much less sound.

A bare room has many echoes. A room with cushions and curtains has fewer.

Two rooms side by side. In the left room, labelled bare room, arrows bounce from wall to wall. In the right room, labelled soft room, arrows go to a dotted wall of cloth, labelled cloth, and stop there.bare roomsoft roomcloth
A bare room bounces sound about. Cloth takes it in.

See it three ways

As a picture

Two pictures side by side. On the left, an arrow goes to a hatched wall and a second arrow comes back from it, labelled wall. On the right, an arrow goes to a dotted cloth and stops, labelled cloth.wallcloth

In words

A hard wall bounces sound back. Soft cloth takes the sound in.

Animals use echoes. Many bats make high squeaks as they fly. Most are too high for people to hear.

The squeak hits an insect and bounces back. The bat hears the echo. It knows where the insect is, even in the dark.

Dolphins make clicks under water. They listen for the echoes. The echoes show where the fish are.

Two rows. In the top row, a box labelled bat sends an arrow labelled squeak to a circle labelled insect. A second arrow labelled echo comes back. In the bottom row, a box labelled dolphin sends an arrow labelled click to a circle labelled fish. A second arrow labelled echo comes back.batinsectsqueakechodolphinfishclickecho
Bats and dolphins send out sound and listen for the echo.

Try it

Find an echo

You need: a big hard wall, such as the side of a large building, open ground in front of it, a friend, if you like.

Be safe: Keep away from roads, deep water and the edge of a drop. Never lean into a well.

  1. Find a big wall with open ground in front of it. Keep away from roads.
  2. Walk about 40 big steps from the wall. Turn and face it.
  3. Clap once, hard and sharp. Listen.
  4. Walk on until you are about 10 steps from the wall. Clap again. Listen.
  5. No big wall? Clap in a bare room. Then hang a blanket on one wall and clap again.

What you will see: At 40 steps you hear the clap. A moment later you hear it again. Near the wall, the echo joins the clap. In the bare room, the blanket takes in sound. The echo is weaker.

If you cannot try it, this is what happens.
A table with two columns, Distance and You hear, and two rows. Far from the wall, you hear the clap, then an echo. Near the wall, you hear one long clap. Labels: Distance, You hear, far, clap, then echo, near, one long clap.DistanceYou hearfarclap, then echonearone long clap

A wrong idea many people have

Bats are blind, so they use echoes.

Why it feels right: Bats fly in the dark and they hunt by sound. People say "blind as a bat".

The test that breaks it: Scientists have watched and tested many kinds of bat. Every kind has eyes that work. Many fruit bats make no echo squeaks at all. They find fruit by sight and smell. A blind bat could not do that.

What is true: No bat is blind. All bats can see. Many also use echoes in the dark.

Key words

echo
sound that bounces back from a hard surface.
hard surface
a hard, flat face that bounces sound back, such as a wall.
take in
to soak up sound instead of bouncing it back.

Back to your guess

The answer is D: The sound bounces back from the wall.

The sound bounces back from the wall, like a ball. You hear it a second time. That is an echo.

Check yourself

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

  1. What is an echo?

    Tap your answer.

    1. Not this one. A quiet clap makes an echo too. An echo is about bouncing, not about loudness. Try another.

    2. Not this one. An echo is your own sound coming back. No second person is needed. Try another.

    3. Yes, that is right. The sound hits a hard surface and bounces back. You hear it a second time.

    See the answer

    Sound that bounces back from a hard surface

    The sound hits a hard surface and bounces back. You hear it a second time.

    • If you chose A: A quiet clap makes an echo too. An echo is about bouncing, not about loudness.
    • If you chose B: An echo is your own sound coming back. No second person is needed.

  2. Which room has the fewest echoes?

    Tap your answer.

    1. Yes, that is right. Soft things take in sound. Less sound bounces back, so there are fewer echoes.

    2. Not this one. Hard walls bounce sound back. This room has many echoes. Try another.

    3. Not this one. A hard roof bounces the sound back down. A high roof can even give a later echo. Soft things take sound in. Try another.

    See the answer

    A small room with thick curtains and cushions

    Soft things take in sound. Less sound bounces back, so there are fewer echoes.

    • If you chose B: Hard walls bounce sound back. This room has many echoes.
    • If you chose C: A hard roof bounces the sound back down. A high roof can even give a later echo. Soft things take sound in.

  3. Sound goes 340 metres each second. A wall is 170 metres away. How long until the echo of a clap comes back?

    A person on the left, labelled you, and a hard wall on the right, labelled wall. Labels: you, wall.youwall
    Tap your answer.

    1. Not this one. That is the time to reach the wall. The sound must come back too. Try another.

    2. Yes, that is right. The sound goes 170 metres there and 170 metres back. That is 340 metres. At 340 metres each second, it takes 1 second.

    3. Not this one. You may have divided 340 by 170. Divide the whole trip, 340 metres, by 340 metres each second. Try another.

    See the answer

    1 second

    The sound goes 170 metres there and 170 metres back. That is 340 metres. At 340 metres each second, it takes 1 second.

    • If you chose A: That is the time to reach the wall. The sound must come back too.
    • If you chose C: You may have divided 340 by 170. Divide the whole trip, 340 metres, by 340 metres each second.

  4. Mei claps 5 metres from a wall. She hears no separate echo. Why?

    A person on the left, labelled Mei, and a hard wall close to her on the right, labelled wall. A dashed line between them is labelled 5 m. Labels: Mei, wall, 5 m.Meiwall5 m
    Tap your answer.

    1. Not this one. It does bounce sound. The echo is there, but it joins the clap. Try another.

    2. Not this one. A quiet clap makes an echo too. Here the echo comes back too fast to hear apart. Try another.

    3. Yes, that is right. A separate echo needs a wall about 17 m away or more. A near wall gives an echo that joins the sound.

    See the answer

    The echo comes back so fast that it joins the clap.

    A separate echo needs a wall about 17 m away or more. A near wall gives an echo that joins the sound.

    • If you chose A: It does bounce sound. The echo is there, but it joins the clap.
    • If you chose B: A quiet clap makes an echo too. Here the echo comes back too fast to hear apart.

  5. How does a dolphin find fish in the water?

    Tap your answer.

    1. Not this one. The dolphin sends out its own clicks. The echoes from the fish tell it where they are. Try another.

    2. Not this one. A dolphin does not make light. It uses sound and echoes. Try another.

    3. Not this one. A dolphin does not find fish by smell. It sends out clicks and listens for echoes. Try another.

    4. Yes, that is right. The clicks bounce off the fish and come back as echoes. The dolphin hears where the fish are.

    See the answer

    It makes clicks and listens for the echoes.

    The clicks bounce off the fish and come back as echoes. The dolphin hears where the fish are.

    • If you chose A: The dolphin sends out its own clicks. The echoes from the fish tell it where they are.
    • If you chose B: A dolphin does not make light. It uses sound and echoes.
    • If you chose C: A dolphin does not find fish by smell. It sends out clicks and listens for echoes.

  6. From an earlier chapter

    A dolphin's clicks must travel through water. Can sound do that?

    Tap your answer.

    1. Not this one. Sound also goes through water and solids. It goes faster in water than in air. Go to S2.7.2. Try another.

    2. Yes, that is right. A vibration can pass through water. It goes faster there than in air.

    See the answer

    Yes. Sound travels through water.

    A vibration can pass through water. It goes faster there than in air.

    • If you chose A: Sound also goes through water and solids. It goes faster in water than in air. Go to S2.7.2.

  7. Kofi says bats use echoes because they are blind. What is true?

    Tap your answer.

    1. Yes, that is right. Bats are not blind. Many use squeaks and echoes to find insects.

    2. Not this one. No bat is blind. Every kind has eyes that work. Try another.

    3. Not this one. Echoes work well in the dark. That is when most bats hunt. Try another.

    See the answer

    All bats can see. Many also use echoes in the dark.

    Bats are not blind. Many use squeaks and echoes to find insects.

    • If you chose B: No bat is blind. Every kind has eyes that work.
    • If you chose C: Echoes work well in the dark. That is when most bats hunt.

Keep it

Copy this into your exercise book.

An echo is sound that bounces back from a hard surface. Soft things take sound in. Dolphins and many bats send out sounds and listen for echoes. They find food this way.

Draw: A person, a wall, and two arrows. One goes to the wall. One comes back. Write echo.

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.