Sound waves
In this topic you will:
- learn how sound comes from vibrations
- discover how particles vibrate in a sound wave
- find out why sound does not travel in a vacuum.
- learn that sound waves can be reflected
- discover what can happen when sound is reflected.
Key words
- backwards and forwards
- loudness
- medium
- particles
- pitch
- sound wave
- speed of sound
- echo
- effect on the sound
- property
- reflected
- unwanted
Where does sound come from?
Things that vibrate make sounds. To vibrate means to move backwards and forwards very quickly.
The men in the picture are hitting drums to make them vibrate. When the drums vibrate, the drums make a sound.
If you hit a drum with more force, it vibrates more. This makes a louder sound.
If you touch the front of your neck while you are speaking, you can feel a vibration. The vibration comes from your vocal cords, which make the sound when you speak.
Loudspeakers produce sounds from television, radio and music players. If you put small objects into the paper cone of a loudspeaker, the objects will bounce around. This shows that the paper cone in the loudspeaker is vibrating.
Not all sounds are the same.
Sounds can vary in both loudness and pitch.
Thunder makes a sound with a low pitch.
A baby crying makes a sound with a high pitch.
Example
Try placing your fingers lightly on your throat and humming. You’ll feel the vibrations from your vocal cords — the same kind of vibrations that create sound waves.
Questions
Which one of these is needed to make a sound from the guitar?
Write the letter.
- A The guitar is made from wood.
- B The guitar strings vibrate.
- C There is air inside the guitar.
- D There are metal parts on the guitar.
Show Answer
The correct answer is B. Sound is produced when the guitar strings vibrate.
Which statement describes a way that Zara can make the sound louder?
Write the letter.
- A Pluck the string with more force.
- B Pluck the string with less force.
- C Make the string tighter.
- D Make the string looser.
Show Answer
The correct answer is A. Plucking with more force increases the vibration amplitude, making the sound louder.
Describe what causes this sound.
Show Answer
The buzzing sound is caused by the rapid vibration of the insect’s wings as it flies.
Sound waves
Sound travels from a vibrating object to our ears.
This is called a sound wave.
What is a sound wave?
When an object vibrates, it moves backwards and forwards.
Every time the object vibrates forward, the air in front of the object gets pushed forward.
The particles in the air are made to vibrate backwards and forwards in time with the vibrating object.
When the particles in front of the object vibrate, those particles make other particles in front of them vibrate. This makes a sound wave.
The speed of sound waves in air is about 343 metres per second.
The picture shows how a vibrating cone in a loudspeaker makes a sound wave.
Important Concept
A sound wave moves energy — not air — from one place to another. The air particles vibrate but don’t travel with the wave.
The vibrations of the air particles in very loud sounds can cause damage to the ears.
Vibrations from very quiet sounds can be too small for the ears to detect.
Sound at a certain pitch can cause damaging vibrations, even when the sound is not very loud.
You saw in Topic 3.5 that sound is a way of transferring energy. Sound waves transfer sound energy.
The vibration of particles in the air is transferred to other objects. When the vibration is transferred, the other objects will start to vibrate.
The glass in the bottom right picture has broken because of the vibrations of a high-pitch sound.
Common Mistake
Not all sound waves are audible. Just because something makes a sound doesn't mean we can hear it — some vibrations are too quiet or high-pitched for our ears to detect.
Questions
current, wind, wave, stream
Sound travels through air as a sound .................... .
Show Answer
Sound travels through air as a sound wave.
5. Sofia is watching television.
The sound from the television is travelling across the room, as shown in the diagram.
Which of these arrows shows how the particles in air vibrate?
Write the letter.
Show Answer
The correct answer is C. In a sound wave, particles vibrate back and forth in the same direction the wave travels.
Explain what causes the objects to vibrate.
Show Answer
Thunder creates powerful sound waves that vibrate air particles. These vibrations are transferred to nearby objects, making them vibrate too.
Explain why people cannot hear the sound of the fly walking.
Show Answer
The vibrations made by the fly’s feet are too small and quiet for human ears to detect.
Sound waves on the move
Sound waves travel by making particles vibrate. Sound will travel though anything that has particles: gas, liquid or solid.
You can demonstrate this by tapping on a table. Ask another person to listen to the sound. Then ask them to put their ear on the table and listen again.
The first part of this demonstration shows that sound travels through air, which is a gas.
The second part shows that sound travels through the table, which is a solid.
Sound also travels through liquids.
Animals such as whales and dolphins communicate with sounds.
Quick Fact
Sound waves travel faster through solids than through gases or liquids because particles in solids are packed more closely together.
Sound waves move the same way in gases, liquids and solids. The sound wave makes the particles vibrate backwards and forwards.
The substance that the sound wave moves through is called the medium. Therefore, solids, liquids and gases can all be a medium for sound.
Vibrations in a vacuum
To hear a sound, there must be:
- a vibration to make the sound
- a medium containing particles through which the sound wave can travel.
You saw in Topic 3.3 that a vacuum is a space where there are no particles.
As there are no particles in a vacuum, there is nothing to vibrate to make a sound wave.
Therefore, sound will not travel in a vacuum.
Space is a vacuum. If sound waves could travel through space, we would be able to hear the Sun! Scientists think that the Sun would make a high-pitched humming sound, with louder, low-pitched sounds from time to time.
You can demonstrate that sound does not travel in a vacuum. If you put an electric bell in a glass jar, you can see it vibrate as it makes the sound.
If the air is pumped out of the jar to make a vacuum, you can see the bell vibrating but you cannot hear the sound of the bell.
Important Concept
Unlike light, which can travel through a vacuum, sound needs a medium to move. No particles means no sound.
Questions
Select all the correct answers. Write the letters.
- A. solid
- B. liquid
- C. gas
- D. vacuum
Show Answer
Correct answers: A, B, C. Sound can travel through solids, liquids, and gases — but not a vacuum.
The people who went to the Moon worked close together.
They did work such as hammering and digging.
Explain why the people doing this work could not hear it happening.
Show Answer
There is no air on the Moon, so no particles to carry the sound vibrations. Without a medium, sound cannot travel — even if the work created vibrations.
These films often show explosions in space.
There is usually a loud bang when the explosion happens.
Explain whether you would really hear an explosion in space.
Show Answer
No, you would not hear the explosion because space is a vacuum. Sound needs particles to travel, and space has none.
Bridging Idea: What Happens When Sound Meets a Surface?
So far, you’ve learned that sound travels through solids, liquids and gases — but not through a vacuum. We’ve also seen how sound needs a medium made of particles to carry its vibrations.
But what happens when sound waves reach an object in their path?
In the next section, you’ll explore how sound behaves when it hits surfaces — whether it gets absorbed, passes through, or bounces back. This bouncing back is called a reflection of sound.
Get ready to investigate echoes, reflected sound, and how we use this in real life — from sonar to soundproofing.
Reflections
One property of all waves is that they can be reflected from surfaces. Therefore, sound waves can be reflected.
Reflection is like bouncing a ball off a wall. When a wave is reflected, the wave behaves like the ball. The only difference is that a wave is not affected by gravity.
A sound wave travelling towards a wall will hit the wall and come back.
Sound waves reflect best from large, smooth, flat surfaces. Surfaces such as glass, tiles, flat metal and smooth concrete give good reflections of sound.
If you stand between two flat walls you can hear the reflection from sound. You can do this in an empty room.
When you clap your hands, you hear a strange effect on the sound. An effect on a sound means the sound is changed.
The sound of the clap seems to last longer than usual, then fade away.
Clapping your hands makes a sound wave. The sound wave will travel away from your hands in all directions.
When the sound wave hits a wall, it is reflected back.
The reflection of a sound wave is called an echo.
Example
Try clapping your hands in a tiled bathroom or empty hallway, then again in a room with carpets and curtains. The difference you hear is caused by how well the sound reflects off surfaces.
Useful echoes
Echoes can be useful.
Bats use echoes to find insects for food. The bat makes a sound. The sound wave reflects off the insect – there is an echo. The bat can work out where the insect is from the time taken for the echo to reach the bat, and the direction the echo comes from.
Boats can use echoes to find the depth of water under the boat.
A sound is sent from the bottom of the boat. The sound travels through the water and reflects off the solid ground. The echo comes back to the boat. The time taken for the echo to come back can be used to work out the depth.
Notice that the distance travelled by the sound is double the distance from the object making the sound to the reflecting surface. The sound has to travel from the object to the reflecting surface and back again. You can see this in the picture of the bat and insect, and in the picture of the boat.
Echoes can also be used to make images from inside the body. Sounds sent into the mother’s body echo back out of her body. This method is used to make the image of the unborn baby.
Unwanted echoes
Sometimes echoes are unwanted. For example, when recording music, echoes change the sound. A musical note that is played once will repeat with an echo. This effect will spoil the recording.
A room with large flat walls would give many echoes.
The picture on the left shows how the walls of a room are changed to stop echoes. This room can now be used to record music without the effect of echoes.
In a theatre, the audience needs to hear the voices of people on the stage. If there were echoes in a theatre, the voices would not be clear. Theatres are designed to stop echoes. Theatres usually have no large flat surfaces that could cause echoes.
Questions
Write the letter.
A. The pitch of the sound increases.
B. The pitch of the sound decreases.
C. The sound gets reflected.
D. The sound gets louder.
Show Answer
C. The sound gets reflected.
All of the materials have the same area.
Write the letter.
A. soft curtains
B. glass window
C. thick carpet
D. wool blanket
Show Answer
B. glass window
The theatres are shown in the pictures.
Use information in the pictures to explain why the music sounded better in theatre B.
Show Answer
Theatre B has features like more sound-reflective surfaces and better acoustic design, such as wood panelling and curved walls, which help reflect sound and enhance clarity.
Which one of these materials could she put on the walls to stop echoes when she plays?
Write the letter.
A. flat wood sheets
B. shiny metal sheets
C. soft thick curtains
D. large flat mirrors
Show Answer
C. soft thick curtains
Worked Example: Calculating Distance with Echo
You can calculate the distance that a sound wave has travelled using a formula in this question.
You do not need to learn this until you study Cambridge Lower Secondary Science Stage 8.
A fishing boat uses an echo to find the distance from the boat to some fish.
A sound is sent from the ship to the fish. The sound reflects back to the ship.
The speed of sound in water is 1500 metres per second.
The time taken for the sound to go from the ship and back to the ship is 0.2 seconds.
Use this equation to calculate the distance from the boat to the fish:
distance = speed × time
Remember that the distance travelled in 0.2 seconds is from the ship to the fish and back again.
Show Answe
Think Like a Scientist
Sound and vibration
You are going to investigate whether sound requires vibrations to travel.
Equipment you will need:
• 2 disposable cups
• A sharp object to make holes in the bottom of each cup
• A long piece of string (long enough to stretch across the classroom)
• Scissors to cut the string
⚠️ Safety
Take care when using the sharp object to make holes in the cups.
Step 1: Pass the string through both cups so their open ends face away from each other. Make the string as long as possible.
Step 2: Tie knots inside the cups to secure the string.
Step 3: Hold the cups apart to make a string telephone setup (as shown in diagram).
Step 4: Pull the string tight.
Step 5: One person speaks into the cup; the other listens with their cup to their ear.
Step 6: Speak quietly and observe if the listener hears you.
Step 7: Let the string go slack and repeat the same sentence.
Step 8: Pull the string tight again. Have a third person pinch the middle of the string.
Step 9: Say the same sentence with the string being gripped.
Show Answer
The sound wave travels through the string.
Show Answer
When the string was tight, the sound was clearer. When slack, the sound was much weaker or not heard.
Show Answer
The sound was blocked or became quieter when the string was gripped.
Show Answer
Yes, gripping closer to either cup affected the sound heard. It blocked vibration more near the source.
Show Answer
Sound travels as vibrations, and a tight string allows the vibrations to pass clearly.
Show Answer
Use a recording of the same sentence to ensure consistent volume in each trial.
Peer-assessment
- Do your partner’s answers match your investigation observations?
- Do you agree with their conclusion?
- Do you agree with their suggestion for a fair test?
Think Like a Scientist
How is sound reflected?
You are going to investigate how sound is reflected using different surfaces.
Equipment you will need:
• 2 plastic or cardboard tubes
• A mobile phone
• A large sheet of paper (or 2 A4 sheets taped together)
• A block or piece of hard plastic, metal, or tile (as a reflecting surface)
• A piece of wood (as a barrier)
• A soft material (e.g. foam or polystyrene) as an alternative reflecting surface
Step 1: Set up the tubes and reflecting surface as shown in the diagram.
Step 2: Set the mobile phone to play a quiet sound and place it near or inside one tube. Place the other end of this tube close to the reflecting surface.
Step 3: Mark the position of this tube on the paper.
Step 4: Place the second tube so that one open end is near the reflecting surface and the other near your ear.
Step 5: Rotate the second tube slowly to find the angle where the sound is loudest. Keep your ear at the open end.
Step 6: Mark the final position of this second tube.
Step 7: Move the first tube (with the phone) to a new angle and repeat steps 4–6.
Step 8: If time allows, change the reflecting surface and repeat the steps.
Show Answer
The tubes reflect sound best when the angle of incidence equals the angle of reflection — forming a symmetrical V-shape around the reflector.
Show Answer
Hard surfaces like metal reflected sound better than soft materials like foam.
Show Answer
A tile or metal sheet would reflect sound well. Foam or sponge would reflect sound poorly.