Chapter 6 of 8

All 8 chapters
  1. 1Reflection, readunderstood
  2. 2Refraction, readunderstood
  3. 3Lenses and the eye, readunderstood
  4. 4The colours of light, readunderstood
  5. 5Why things have colour, readunderstood
  6. 6Sound as a wave, readunderstood
  7. 7The speed of sound, readunderstood
  8. 8Pitch, loudness and hearing, readunderstood

S3.9.6 Chapter 6

Sound as a wave

How does sound get from a drum to my ear, and why is space silent?

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

  • Explain why sound cannot cross empty space
  • Model a sound wave with a stretched spring or a row of people

Mei beats a drum. Ravi hears it from across the yard. Nothing flew from the drum to Ravi. So how did the sound travel?

Air is made of tiny particles, too small to see. They move about all the time and bump into each other. A drum skin that moves out pushes the particles next to it together. A drum skin that moves back leaves them spread out.

A place where the particles are crowded together is squashed. A place where they are spread out is stretched. The squashed parts push the next particles. Those push the next ones. This pattern of squashed and stretched parts is a sound wave.

Draw it first

Draw a sound wave in air

  1. In the first picture, a row of thirteen evenly spaced lines is labelled layers of air at rest. In the second picture, the lines crowd together in some places, labelled squashed, and spread apart in others, labelled stretched. An arrow is labelled sound travels. In the third picture, a second row below shows the same pattern a moment later, with the squashed parts moved along to the right. It is labelled a moment later. Labels: layers of air at rest, squashed, stretched, sound travels, a moment later.layers of air at rest

    1 Draw a row of evenly spaced lines. Each line is a layer of air at rest.

  2. In the first picture, a row of thirteen evenly spaced lines is labelled layers of air at rest. In the second picture, the lines crowd together in some places, labelled squashed, and spread apart in others, labelled stretched. An arrow is labelled sound travels. In the third picture, a second row below shows the same pattern a moment later, with the squashed parts moved along to the right. It is labelled a moment later. Labels: layers of air at rest, squashed, stretched, sound travels, a moment later.squashedstretchedsound travels

    2 The drum pushes. Draw some lines close together, which is squashed, and some far apart, which is stretched.

  3. In the first picture, a row of thirteen evenly spaced lines is labelled layers of air at rest. In the second picture, the lines crowd together in some places, labelled squashed, and spread apart in others, labelled stretched. An arrow is labelled sound travels. In the third picture, a second row below shows the same pattern a moment later, with the squashed parts moved along to the right. It is labelled a moment later. Labels: layers of air at rest, squashed, stretched, sound travels, a moment later.squashedstretcheda moment latersound travels

    3 A moment later the pattern has moved on. The squashed parts are farther along, but the lines themselves have only shaken a little.

Look at the third picture. The squashed parts moved to the right. But each layer of air only moves a tiny way to and fro. The pattern travels. The air stays about where it was.

Rows of dots stand for air particles. In some places the columns of dots crowd together, labelled squashed, and in other places they are spread apart, labelled stretched. An arrow is labelled sound travels. The distance from one squashed part to the next is marked and labelled wavelength. Labels: wavelength, squashed, stretched, sound travels.wavelengthsquashedstretchedsound travels
The distance from one squashed part to the next is the wavelength.

The distance from one squashed part to the next is the wavelength. A sound wave is a pattern that moves. The wave carries the shake from the drum to your ear.

See it three ways

As a picture

A table with two columns, material and sound. Air: yes. Water: yes. Wood: yes. Empty space: no. Labels: material, sound, air, water, wood, empty space, yes, no.materialsoundairyeswateryeswoodyesempty spaceno

In words

Sound can pass through air, water and solids, because they are made of particles. It cannot pass through empty space, which has none.

In symbols

sound needs particles. No particles, no sound.

Far above the air, space is a vacuum. A vacuum has almost no particles. There is nothing to squash and stretch. So sound cannot cross it. Space is silent.

Two boxes side by side. The left box is filled with dots, labelled air, and an arrow runs through it, labelled sound passes. The right box is empty, labelled empty space, and is labelled no sound. Labels: air, sound passes, empty space, no sound.airsound passesempty spaceno sound
Sound goes through air. It cannot go through empty space.

Light is different. Light is not a squashing of particles, so it can cross empty space. That is how sunlight reaches us across the space between the Sun and the Earth.

Try it

Model a sound wave with people

You need: six or more people, or a few friends and some space, flat ground with room to stand in a line.

Be safe: Push very gently, and stand on flat ground away from roads. Stop if anyone feels unsteady. No one should run.

  1. Stand in a line, a short step apart, all facing the same way. Each person puts both hands lightly on the shoulders of the person in front.
  2. The person at the back gives a gentle push forward, and then steps back.
  3. Watch the push pass along the line. Each person is pushed forward a little, then comes back.

What you will see: The push travels all the way along the line. No one walks from the back to the front. Each person only moves a little forward and back. This is how a sound wave moves through air. In air, a loud sound and a quiet sound travel at the same speed.

If you cannot try it, this is what happens.
Six people stand in a line. An arrow runs along under them, labelled the push goes this way. Labels: the push goes this way.the push goes this way

A wrong idea many people have

Air rushes from the drum to your ear and carries the sound.

Why it feels right: You can feel a breath of air from a loud drum, so it seems that the air travels.

The test that breaks it: Hold a thin strip of paper a hand away from a drum while a friend beats it. The paper trembles in place. It is not blown away along the room.

What is true: Each particle of air only shakes a tiny way to and fro. The pattern of squashed and stretched parts travels, not the air.

Key words

particles
the tiny bits that air, water and solids are made of.
sound wave
a pattern of squashed and stretched parts that moves outwards from a source.
wavelength
the distance from one squashed part to the next.
vacuum
a place with almost no particles, such as space.

Back to your guess

The answer is C: The drum shakes the air next to it. The shake passes from one bit of air to the next.

The drum shakes the air next to it. The shake passes from one bit of air to the next, like a push through a crowd. With no air, there is nothing to pass it on.

Check yourself

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

  1. Why can sound not cross empty space?

    Tap your answer.

    1. Yes, that is right. A sound wave is squashed and stretched particles. In a vacuum there are almost none, so a sound cannot cross it.

    2. Not this one. Distance is not the reason. In space a sound cannot even start to travel, because there is nothing to shake. Go to S3.9.6. Try another.

    3. Not this one. Cold air still carries sound. What is missing in space is the particles themselves. Go to S3.9.6. Try another.

    See the answer

    There are no particles to pass the shake along.

    A sound wave is squashed and stretched particles. In a vacuum there are almost none, so a sound cannot cross it.

    • If you chose B: Distance is not the reason. In space a sound cannot even start to travel, because there is nothing to shake. Go to S3.9.6.
    • If you chose C: Cold air still carries sound. What is missing in space is the particles themselves. Go to S3.9.6.

  2. A sound wave passes through air. What do the air particles do?

    Tap your answer.

    1. Not this one. The particles do not travel the whole way. Only the pattern does. Go to S3.9.6. Try another.

    2. Yes, that is right. A particle is pushed forward, then comes back. It passes the push to the next particle. The pattern moves, but the particles stay near their places.

    3. Not this one. They must move, or they could not pass the shake on. They move a tiny way and come back. Go to S3.9.6. Try another.

    See the answer

    Each one shakes a tiny way to and fro.

    A particle is pushed forward, then comes back. It passes the push to the next particle. The pattern moves, but the particles stay near their places.

    • If you chose A: The particles do not travel the whole way. Only the pattern does. Go to S3.9.6.
    • If you chose C: They must move, or they could not pass the shake on. They move a tiny way and come back. Go to S3.9.6.

  3. What is a squashed part of a sound wave?

    Tap your answer.

    1. Not this one. That is a stretched part. A squashed part is the opposite. Go to S3.9.6. Try another.

    2. Not this one. Air in a squashed part is still moving. It is just crowded together. Go to S3.9.6. Try another.

    3. Yes, that is right. A squashed part has the particles close together. A stretched part has them far apart. The wave is the two of them moving outwards.

    See the answer

    A place where the air particles are crowded close together.

    A squashed part has the particles close together. A stretched part has them far apart. The wave is the two of them moving outwards.

    • If you chose A: That is a stretched part. A squashed part is the opposite. Go to S3.9.6.
    • If you chose B: Air in a squashed part is still moving. It is just crowded together. Go to S3.9.6.

  4. Six friends stand in a line. The first one pushes the second, who pushes the third, and so on. What does this model?

    Six people stand in a line. An arrow runs along under them, labelled the push goes this way. Labels: the push goes this way.the push goes this way
    Tap your answer.

    1. Yes, that is right. The people are like particles. The push passes along the line, as a sound wave does along air. Each person only moves a little.

    2. Not this one. No one walks to the end. Each person only moves a little, like a particle of air. Go to S3.9.6. Try another.

    3. Not this one. Light does not need a material. A sound wave does, and the people stand in for the particles. Go to S3.9.6. Try another.

    See the answer

    A sound wave. The push moves along, but each person stays near the same place.

    The people are like particles. The push passes along the line, as a sound wave does along air. Each person only moves a little.

    • If you chose B: No one walks to the end. Each person only moves a little, like a particle of air. Go to S3.9.6.
    • If you chose C: Light does not need a material. A sound wave does, and the people stand in for the particles. Go to S3.9.6.

  5. Which materials can sound travel through?

    Tap your answer.

    1. Not this one. Sound also travels through water and through solids. You can hear a knock on a table through the wood. Go to S3.9.6. Try another.

    2. Yes, that is right. Sound needs particles to pass the shake along. Air, water and solids have them. Empty space does not.

    3. Not this one. Sound needs particles. Empty space has none, so sound cannot cross it. Go to S3.9.6. Try another.

    See the answer

    Air, water and solids, but not empty space.

    Sound needs particles to pass the shake along. Air, water and solids have them. Empty space does not.

    • If you chose A: Sound also travels through water and through solids. You can hear a knock on a table through the wood. Go to S3.9.6.
    • If you chose C: Sound needs particles. Empty space has none, so sound cannot cross it. Go to S3.9.6.

  6. From an earlier chapter

    Review: a white shirt is in red light. What colour does it look?

    Tap your answer.

    1. Not this one. White reflects every colour that falls on it, but only red falls on it here. Go to S3.9.5. Try another.

    2. Not this one. White reflects all colours. There is red in the light, so it reflects red. Go to S3.9.5. Try another.

    3. Yes, that is right. White reflects all colours that fall on it. Red light has only red, so the shirt reflects red and looks red.

    See the answer

    Red.

    White reflects all colours that fall on it. Red light has only red, so the shirt reflects red and looks red.

    • If you chose A: White reflects every colour that falls on it, but only red falls on it here. Go to S3.9.5.
    • If you chose B: White reflects all colours. There is red in the light, so it reflects red. Go to S3.9.5.

Keep it

Copy this into your exercise book.

Sound is a pattern of squashed and stretched parts that moves through particles. The particles only shake to and fro. It needs air, water or a solid, so space is silent.

Draw: A row of lines with some close together and some far apart. Label squashed and stretched. Draw an arrow for the way the sound goes.

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 (720 KB). It opens on any phone.