Chapter 2 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

S2.7.2 Chapter 2

Sound travels

How does a sound get from one place to another?

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 sound travels through air, water and solids
  • Compare a tap heard through air and through a table
  • Explain that sound cannot travel through empty space

A drum is hit across a field. You hear it. How does the sound get to you?

The drum skin vibrates. It pushes the air next to it. That air pushes the air next to it. The vibration passes on, bit by bit, until it reaches your ear.

A hatched drum on the left, labelled drum. A row of six small circles follows, labelled bits of air, with a short arrow from each one to the next. A person on the right is labelled you.youdrumbits of air
Each bit of air pushes the next one.

Draw it first

Draw sound going through three things

  1. Three rows. In each row a small black square is a tap on the left, and an arrow shows the sound going through a block. The first block is air and is empty. The second block is water and has dots. The third block is wood and has diagonal lines. Labels: air, water, wood.air

    1 Draw a tap and a block of air. Draw an arrow through it.

  2. Three rows. In each row a small black square is a tap on the left, and an arrow shows the sound going through a block. The first block is air and is empty. The second block is water and has dots. The third block is wood and has diagonal lines. Labels: air, water, wood.airwater

    2 Draw a block of water. Draw an arrow through it.

  3. Three rows. In each row a small black square is a tap on the left, and an arrow shows the sound going through a block. The first block is air and is empty. The second block is water and has dots. The third block is wood and has diagonal lines. Labels: air, water, wood.airwaterwood

    3 Draw a block of wood. Draw an arrow through it.

Sound passes through air. It passes through water too. Under water you can hear two stones knock together.

Sound passes through a solid as well. A solid keeps its own shape. A table, a wall and a metal bar are solids.

Sound goes faster in water and iron than in air. In air it goes about 340 metres each second.

See it three ways

As a picture

A table with two columns, Through and Metres a second, and three rows. Air is 340. Water is 1 500. Iron is 5 000. Labels: Through, Metres a second, air, 340, water, 1 500, iron, 5 000.ThroughMetres a secondair340water1 500iron5 000

In words

Sound goes through air, water and solids. It is faster in water and in iron than in air. These numbers are about right.

Sound cannot travel through empty space. Empty space has no air at all. With nothing to pass the vibration on, the sound stops.

A small black square on the left is a tap. A short arrow from it ends in the middle of a gap between two dashed lines. The gap is labelled empty space. The word stops sits under the end of the arrow.empty spacestops
In empty space there is nothing to pass the sound on.

People test this with a bell in a sealed glass jar. A pump takes the air out. The ringing fades away.

Light is much faster than sound. In a storm you see lightning first. The thunder comes later.

In 3 seconds, sound goes about 1 km. Count the seconds from the flash to the thunder. Divide by 3. That is about how many km away the storm is.

A hatched box on the left is labelled storm, with a zigzag of lightning below it. A person on the right is labelled you. A top arrow from the storm to the person is labelled light, at once. A lower arrow is labelled sound, 3 s. A dashed line below them is labelled 1 km.stormyoulight, at oncesound, 3 s1 km
Light arrives at once. Sound takes about 3 s for each km.

Try it

Hear a tap through air and through a table

You need: a wooden table, or a strong wooden bench, a quiet room.

  1. Sit at the table. Reach one arm out along it.
  2. Keep your head up. Tap the table softly with a finger. Listen.
  3. Now rest one ear flat on the table. Keep your arm out.
  4. Tap the table softly again, with the same soft tap. Listen.
  5. Which tap sounded louder?

What you will see: The tap sounds louder with your ear on the table. The table passes the vibration on better than air does.

If you cannot try it, this is what happens.
A table with two columns, Ear and Sound, and two rows. In the air, the sound is quiet. On the table, the sound is louder. Labels: Ear, Sound, in the air, quiet, on the table, louder.EarSoundin the airquieton the tablelouder

A wrong idea many people have

Sound only travels through air.

Why it feels right: You hear most sounds through the air all day. You rarely think about anything else.

The test that breaks it: Press one ear to a table. Tap the far end softly. The tap is loud and clear. Lift your head and tap again. It is quieter. The table carried the sound.

What is true: Sound travels through air, water and solids. It cannot travel through empty space.

Key words

solid
keeps its own shape.
empty space
a place with no air at all. Sound cannot travel there.

Back to your guess

The answer is D: Sound travels through air, water and solids.

Sound travels through air, water and solids. It cannot cross empty space, where there is no air.

Check yourself

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

  1. What can sound travel through?

    Tap your answer.

    1. Not this one. You hear most sounds through air. Sound also goes through water and through a table. Try another.

    2. Yes, that is right. Each bit of air, water or solid passes the vibration on. In empty space there is nothing to pass it on.

    3. Not this one. Empty space has no air. The vibration has nothing to pass it on. Try another.

    See the answer

    Air, water and solids

    Each bit of air, water or solid passes the vibration on. In empty space there is nothing to pass it on.

    • If you chose A: You hear most sounds through air. Sound also goes through water and through a table.
    • If you chose C: Empty space has no air. The vibration has nothing to pass it on.

  2. Amina rests her ear on a table. Kofi taps the far end. The tap sounds louder than in the air. Why?

    Tap your answer.

    1. Not this one. Sound does go through air. You hear it. The table just passes it on better. Try another.

    2. Not this one. The tap made the sound. The table only passes the vibration on. Try another.

    3. Yes, that is right. A solid carries the vibration well. Her ear on the table gets more of it.

    See the answer

    The table passes the vibration on better than air does.

    A solid carries the vibration well. Her ear on the table gets more of it.

    • If you chose A: Sound does go through air. You hear it. The table just passes it on better.
    • If you chose B: The tap made the sound. The table only passes the vibration on.

  3. You count 9 seconds from a flash of lightning to its thunder. About how far away is the storm?

    Tap your answer.

    1. Yes, that is right. Sound goes about 1 km in 3 seconds. So divide the seconds by 3.

    2. Not this one. You used the number of seconds as the km. Divide the seconds by 3. Try another.

    3. Not this one. You multiplied the seconds by 3. Divide them by 3 instead. Try another.

    See the answer

    3 km

    Sound goes about 1 km in 3 seconds. So divide the seconds by 3.

    • If you chose B: You used the number of seconds as the km. Divide the seconds by 3.
    • If you chose C: You multiplied the seconds by 3. Divide them by 3 instead.

  4. A storm is 6 km away. About how many seconds pass between the flash and the thunder?

    See the answer

    About 18 seconds

    Sound takes about 3 seconds for each km. So 6 km takes about 3 × 6 = 18 seconds.

  5. Why can you not hear a sound in empty space?

    See the answer

    There is no air to pass the vibration on.

    Sound passes a vibration from one bit of matter to the next. In empty space there is nothing to do that.

  6. From an earlier chapter

    A drum is hit and it makes a sound. What is the drum skin doing?

    Tap your answer.

    1. Not this one. A drum skin looks still. But it moves back and forth too fast to see. Go to S2.7.1. Try another.

    2. Yes, that is right. Every sound starts with something that vibrates. The skin is the drum's vibrating part.

    3. Not this one. Cooling has nothing to do with sound. A sound starts with a vibration. Go to S2.7.1. Try another.

    See the answer

    Vibrating

    Every sound starts with something that vibrates. The skin is the drum's vibrating part.

    • If you chose A: A drum skin looks still. But it moves back and forth too fast to see. Go to S2.7.1.
    • If you chose C: Cooling has nothing to do with sound. A sound starts with a vibration. Go to S2.7.1.

  7. Look at the table. Through which does sound go fastest?

    A table with two columns, Through and Metres a second, and three rows. Air is 340. Water is 1 500. Iron is 5 000. Labels: Through, Metres a second, air, 340, water, 1 500, iron, 5 000.ThroughMetres a secondair340water1 500iron5 000
    Tap your answer.

    1. Not this one. Air has the smallest number. You hear most sounds through air, but sound is slowest there. Try another.

    2. Not this one. Water is faster than air, but iron has a bigger number still. Try another.

    3. Not this one. The numbers are not the same. Sound goes faster through some things than others. Try another.

    4. Yes, that is right. Iron has the biggest number. Of the three, sound goes fastest through iron.

    See the answer

    Iron

    Iron has the biggest number. Of the three, sound goes fastest through iron.

    • If you chose A: Air has the smallest number. You hear most sounds through air, but sound is slowest there.
    • If you chose B: Water is faster than air, but iron has a bigger number still.
    • If you chose C: The numbers are not the same. Sound goes faster through some things than others.

  8. A bell rings inside a sealed glass jar. A pump takes the air out of the jar. What do you hear?

    Tap your answer.

    1. Yes, that is right. Sound needs air, water or a solid to pass it on. Take the air out and the sound fades.

    2. Not this one. Sound needs something to pass the vibration on. With no air, it cannot reach you. Try another.

    3. Not this one. The bell still shakes. But the air that carries the shake to you has gone. Try another.

    See the answer

    The ringing fades away, because the air has gone.

    Sound needs air, water or a solid to pass it on. Take the air out and the sound fades.

    • If you chose B: Sound needs something to pass the vibration on. With no air, it cannot reach you.
    • If you chose C: The bell still shakes. But the air that carries the shake to you has gone.

Keep it

Copy this into your exercise book.

Sound travels by passing a vibration through air, water and solids. It cannot cross empty space. In air it goes about 340 metres each second. A table can carry a tap better than air.

Draw: A tap, a block, and an arrow through it. Write air, water and wood.

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.