KS3 · Physics

Sound waves as longitudinal waves

A loudspeaker across the room reaches your ear. Is air flying out of it to get there? Follow one air particle and you'll find the answer is a surprising no.

Follow one particle

1Cone pushes forward23456

Stage 1 of 6: Cone pushes forward. paused

Cone pushes forward · 1/6Scrub along: the dot is the compression moving on, the words are the one particle that stays.

A loudspeaker cone moves forwards and knocks the air particles in front of it forwards. They get squashed together, and that squashed patch is called a compression. Pick one of the knocked particles to follow. We'll call it the marked particle.

Scrub through six moments, or press Play. The glowing dot shows how far the compression has got; the words follow one air particle.

Physics · Waves

Parallel, or at right angles?

Sort each wave: longitudinal or transverse?

Still to sort

Longitudinal (0)

The vibration is parallel to the direction of energy transfer.

Where the line is: Parallel to the direction of travel, never at right angles to it.

Transverse (0)

The vibration is at right angles to the direction of energy transfer.

Where the line is: At right angles to the direction of travel, never parallel to it.

5 of 5 still to sort.

You've seen which way the particles vibrate in sound. Now use the same test on other waves: compare the direction of the vibration with the direction the energy is being transferred.

Physics · Sound

What do you think is happening?

A loudspeaker is playing music across a room, and you can hear it.

Which is closest to what you think is happening to the air between the loudspeaker and you?
How sure are you?

Predict, then check

Use what you know about how a sound wave is passed from particle to particle.

A loudspeaker is playing in a vacuum, a space with no particles at all. What happens to the sound wave?

Physics · Sound through materials

Fastest to slowest

How fast a sound wave travels through the material

1 · Fastest3 · Slowest

  1. A liquid, such as water

  2. A gas, such as air

  3. A solid, such as a wall

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Follow one air particle and see what a sound wave really does.

What you need to know

  • Sound starts with a vibrating object. It sets nearby air particles vibrating, they set their neighbours vibrating, and that pattern of vibrating particles moves through the air as a sound wave.
  • Each particle only vibrates backwards and forwards, parallel to the direction the wave travels. It is the compressions, where particles are squashed together, that travel forwards. The air itself isn't carried along.
  • Sound is a longitudinal wave: the vibration is parallel to the direction of energy transfer. A transverse wave vibrates at right angles to it.
  • Sound needs a medium (a solid, a liquid or a gas) and can't travel through a vacuum.
  • Sound travels fastest through solids, then liquids, and slowest through gases, because of how close together the particles are and how tightly they are held.

The big picture

A sound wave is a pattern of vibrating particles passed from one set to the next. Each particle vibrates backwards and forwards parallel to the direction the wave travels, while the compressions and the energy move on. That makes sound a longitudinal wave. It needs a medium, and travels fastest through solids and slowest through gases.

Key points

1A sound wave transfers energy without transferring material, and the energy moves in the same direction as the wave.
2In a longitudinal wave, the vibration is parallel to the direction of energy transfer.
3In a transverse wave, such as a water wave, the vibration is at right angles to the direction of energy transfer.
4No particles means no sound, because there is nothing to be set vibrating.
5Solid, then liquid, then gas: close and firmly held passes a vibration on fastest.

Worked example

Problem

A student writes: 'In a sound wave, air particles travel from the loudspeaker to your ear, moving up and down as they go.' Find the two mistakes and write a correct sentence.

⚠ Watch out

Picturing sound as moving air, or as an up-and-down ripple. The particles don't travel with the wave, and they don't vibrate at right angles to it. They vibrate backwards and forwards along the direction the sound is going.

🧠

Memory hook

The squash travels, the air just shuffles. The wave runs ahead; each particle only moves back and forth along the way it goes.

✓

Check yourself

Without looking back: what does one air particle do as a sound wave passes, and what is it that actually travels along?

Flashcards

(15)
What makes a sound?
A vibrating object. Vibrating means moving back and forth repeatedly.
How does a vibrating object make a sound wave in air?
It makes nearby air particles vibrate. They collide with their neighbours and set those vibrating too, so a pattern of vibrating particles moves through the air.
What is a compression in a sound wave?
A patch where the particles are more squashed together. Compressions are what travel forwards.
Does the air itself travel from a loudspeaker to your ear?
No. Each bit of air vibrates backwards and forwards and is not transported forwards.
What does a loudspeaker cone do to the air as it moves forwards, and then back?
Forwards, it knocks air particles forwards. Back, it leaves a gap that nearby particles move into because of their random motion.
What do all waves, including sound, transfer?
Energy, without transferring material. The energy is transferred in the same direction as the wave travels.
What is a longitudinal wave?
A wave where the direction of vibration is parallel to the direction of energy transfer. Sound is one.
What is a transverse wave?
A wave where the vibration is at right angles to the direction of energy transfer, like up-down vibration with left-to-right travel in a water wave.
What is a medium?
The material a wave travels through. For sound it can be a solid, a liquid or a gas.
Can sound travel through a vacuum?
No. A vacuum has no particles to be set vibrating.
Which materials let sound travel fastest and slowest?
Fastest in solids, slower in liquids, slowest in gases.
Why does sound travel quickly through a solid?
Its particles are very close together and held by strong forces, so disturbing one quickly disturbs many.
Why does sound travel slowly through a gas?
Gas particles are far apart and only affect each other when they collide, so a moved particle travels some distance before it passes the motion on.
Why are liquids in the middle for the speed of sound?
Their particles are close together, so vibrations pass on easily, but they can move around each other and aren't held as tightly as in a solid.
What do models of sound waves usually leave out, and why is that allowed?
The random motion of air particles. It's always there but isn't needed to explain sound, and parts left out of a model can be added later if needed.

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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