KS3 · Physics

How sound travels through a medium

You can hear music through a wall, yet there is no sound at all between two astronauts in space. What is the difference? Particles.

Physics · Particle model

Why does the medium change how fast sound travels?

Sound has to travel through something, and that something is called the medium. Whatever the medium is, it is made of particles, and the vibration is handed from one particle to the next. Switch between a gas, a liquid and a solid. For each one, ask: how easily can one particle pass a push on to its neighbour?

Selected state

Gas (for example air)

Spacing

Further apart

The particles are further apart, so vibrations are passed on least easily. Sound travels slowest through gases.

Watch out: The particles here are drawn as tiny spheres. That is a model, and a model only reflects some aspects of what a gas, liquid or solid is really like. Models of sound usually leave out the random motion of the particles.

From speaker to ear

?

Reason it through

How does a vibrating loudspeaker cone end up as a sound in your ear?

Link 1 of 4

First link · your turn

The cone moves forwards. What does that do to the air particles right next to it?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Physics · Sound

What is actually crossing the room?

A loudspeaker is playing music on the other side of the room, and you can hear it.

Which of these is closest to what you think is happening in the air between the speaker and your ear?
How sure are you?

Predict, then check

Picture a jar with a balloon inside. Most of the air has been pumped out of the jar.

The balloon is popped inside the jar that has had most of the air pumped out. What do you hear?

Physics · Sound

Thunder takes its time

Lightning strikes about three kilometres away. How many seconds pass before you hear the thunder?

Your estimate

15 seconds

0 seconds30 seconds

Near and far

Close to the sourcevsFurther from the source

The same sound, listened to close to the source and then further away.

Focus

What has happened to the energy?

Close to the source

It has barely begun to spread out

Further from the source

It is spread out, not used up, and some slightly heats the medium

The insight

Quieter does not mean the energy has disappeared. It has been spread out.

Particles sharing the energy

Close to the source

Fewer particles

Further from the source

More particles

Amplitude of the particles' vibration

Close to the source

Larger

Further from the source

Smaller

How loud it sounds

Close to the source

Louder

Further from the source

Quieter (lower volume)

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Sound is a vibration handed on from particle to particle, so it needs something to travel through.

What you need to know

  • Sound is a vibration, which means something moving repeatedly backwards and forwards. It travels by being passed on from particle to particle through a medium.
  • The particles themselves don't travel from the source to your ear. A pulse of squashed-together particles (a compression) moves along, while each particle only vibrates backwards and forwards.
  • Sound travels fastest through solids, slower through liquids and slowest through gases, because of how close the particles are and how strongly they are held together.
  • A vacuum has no particles, so sound can't travel through it. Sound takes time to travel, and it gets quieter with distance because its energy is spread out, not used up.

The big picture

Sound is a vibration that is passed on from particle to particle through a material called the medium. That is why it can't cross a vacuum, and why it travels fastest in solids, a bit slower in liquids and slowest in gases.

Key points

1A sound wave is a pulse that travels through a material, caused by the vibrations of particles. Sound waves transfer energy from one place to another.
2The material that sound travels through is called the medium. Sound can travel through solids, liquids and gases.
3Sound is fastest in solids (particles very close and bonded), slower in liquids (very close) and slowest in gases (further apart).
4A vacuum is a space with no particles, so there is nothing to vibrate and sound cannot cross it.
5We hear a sound when the vibrations arrive at the ear and make parts of it vibrate.
6Further from a source the same energy is shared among more particles, so they vibrate with a smaller amplitude and the sound is quieter.

Worked example

Problem

Put air, water and a brick wall in order for how quickly sound travels through them, slowest first. Give a reason for each position.

⚠ Watch out

Thinking the air itself travels from the loudspeaker to your ear, like wind carrying the sound. It doesn't. The compressions travel, but each air particle only vibrates backwards and forwards.

🧠

Memory hook

A shove ripples through a packed crowd, yet nobody walks anywhere: everyone just sways on the spot. The ripple travels, the people stay. A tight crowd passes a shove on fast; a thin one slowly. (Real air particles also jostle at random, which this picture leaves out.)

✓

Check yourself

A friend says, 'Sound is just air rushing from the speaker to your ear.' Use what you know about particles to tell them what really happens, and say what is carried across the room.

Flashcards

(14)
What does 'vibrating' mean?
Moving repeatedly backwards and forwards.
What is the medium for a sound?
The material that sound travels through and sets vibrating. Solids, liquids and gases can all be a medium.
How does sound get through a medium?
Vibrations are passed on from particle to particle: each vibrating particle knocks into its neighbours and sets them vibrating too.
What happens to nearby air particles when a vibrating object moves forwards?
It knocks them forwards and squashes them together.
What is a compression?
A place where the particles are squashed closer together. Compressions travel away from the source as a pulse.
Do air particles travel from a loudspeaker to your ear?
No. They only vibrate backwards and forwards, while the compressions travel.
What does a sound wave carry from one place to another?
Energy.
When do we hear a sound?
When the vibrations arrive at the ear and the particles make parts of the ear vibrate.
In which state of matter does sound travel fastest, and in which slowest?
Fastest in solids and slowest in gases, with liquids in between.
Why does sound travel fastest in a solid?
The particles are very close together and held by strong attractive forces, so when one vibrates many of its neighbours vibrate with it.
What is a vacuum, and why can't sound cross one?
A space with no particles. With nothing to vibrate, there is nothing to pass the vibration on.
Why is a sound quieter further from its source?
The energy is shared among more particles, so each vibrates with a smaller amplitude.
What happens to the energy of a sound as it spreads out?
It is spread out, not used up, and some of it slightly heats the medium.
What do models of sound leave out?
Usually the random motion of the air particles. A model, like particles drawn as tiny spheres, only reflects some aspects of the real thing.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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