Lesson video

Hello, my name is Dr.

George and this lesson is called "Sound Waves." It's part of the unit "Waves." The outcome for the lesson is I can describe the movement of particles in a sound wave travelling through a solid, liquid, or gas, and explain the relative speed of sound in each state of matter.

I'll be using these keywords, which I'll explain as we go along, but you can come back to this slide any time if you want to remind yourself of the meanings.

The lesson has three parts.

Sound waves in air and gases, sound waves in solids, and sound waves in liquids.

So let's take a look at sound waves in air.

The way that sound is actually caused is when objects vibrate.

That means moving back and forth repeatedly.

And when an object, like this guitar string, vibrates, it causes nearby air particles to vibrate as well.

Look at the animation.

It shows vibrating air particles that are moving backwards and forwards and they collide into their neighbours, which sets them vibrating too.

And so we get a pattern of vibrating particles that here is moving from left to right, creating a sound wave.

Can you now complete this sentence? A vibrating object causes nearby air particles to, what? Travel far away from the object, vibrate too, move randomly, or move apart for the sound to get through.

With short questions, I'll wait for five seconds, but if you need more time, press pause and then press play when you've chosen your answer, Did you choose B, vibrate too? So the air particles are made to vibrate by the vibrating object.

They don't all travel away from the object, it doesn't make them move randomly, and they don't move apart for the sound to get through.

A sound wave is the vibrating movement of particles in a pattern.

Now air is a gas, well, it's a mixture of gases, and the top animation shows a representation of the tiny particles in air moving with a range of speeds and directions.

Look at the bottom animation now.

It represents air particle motions where there is a source of sound, a speaker, on the left.

So the particles are naturally moving, although the full movement isn't shown here.

And then extra movement is added on, vibrations, by the sound travelling through.

Now when a loudspeaker cone, that's the moving part inside a loudspeaker, makes a sound wave, it knocks particles of air forwards as shown here in this simplified picture.

These particles slow down, they may change direction as well when they collide into the particles in front of them.

When the loudspeaker cone moves back again to the left, it leaves a gap.

Some nearby particles move back into this gap because the particles have random motion in all directions and some of them happen to be moving towards the left.

This creates a gap further forward, which is also quickly filled by backwards-moving particles of air.

So in this way, vibrations move forward through the air from the left to the right in this picture.

So what has to happen for sound waves to travel? Vibrating air particles have to what into their neighbours, setting them vibrating too? The missing word is collide.

Well done if you got that.

And we usually only represent sound waves showing the back and forth movement of the particles.

This random motion of air particles that is always there isn't shown, as it isn't needed to explain sound.

So this makes our model clearer.

We can see what's happening, how the sound wave is travelling, without the added complication of the random movements of gas particles.

So parts that are not included in a scientific model can always be added in later if needed.

We're aware that there is random motion and, if we want to, we can show that in the model too.

When a sound wave travels through air, air particles aren't transported forwards, they just vibrate back and forth.

Watch one of the particles in this animation.

You can look at one of the red ones and see how it moves.

It's just going backwards and forwards repeatedly, although the sound wave itself is travelling from left to right.

So it's the pulses, which we could also call compressions, that travel forwards.

These pulses in which the particles are more squashed together.

Imagine a lit candle is put in front of a speaker and at the moment the speaker is turned off.

The speaker is then turned on and produces a steady note.

What do you think happens to the flame? Press pause if you need more time to think.

The correct answer is B.

Well done if you picked that.

If you think about how the particles in the air move when a sound wave passes through, they're vibrating, they're moving backwards and forwards, and that's going to make the candle flame move backwards and forwards as well.

And another question now.

A sound from a speaker is heard through air.

Which is the most accurate model for what happens in the air? So in these pictures, the speaker is on the left, and none of these pictures are showing exactly what really happens, but which one do you think best represents what happens? Press pause while you're thinking and press play when you've chosen your answer.

And the correct answer, the most accurate model is C, because here we can see particles, they're actually balls hanging on threads, that are being made to vibrate by the speaker.

And this first particle is vibrating and it's going to hit the next one which will make that vibrate and so on.

That's similar to what happens in a sound wave.

None of the other pictures really shows what's going on in a sound wave in a helpful way.

Like all waves, sound waves transfer energy without transferring material.

Here the direction of energy transfer is shown by the arrow, it's the same as the direction of wave travel.

And the vibrations in the sound wave are parallel to the direction of energy transfer.

Have a look at the arrows.

The vibrations are left-right and right-left, and the energy transfer is left to right.

Any wave with this property, vibrations parallel to direction of energy transfer, is called a longitudinal wave.

Some waves are longitudinal, some are not, but sound waves are longitudinal.

Now, true or false, the wave shown here is a longitudinal wave.

Decide whether that's true or false and then say how you know.

Why do you think that? Press pause while you're thinking about that and press play when you're ready.

Did you realise that it's false? This is not a longitudinal wave, why not? In a longitudinal wave, the vibration is parallel to the direction of energy transfer.

But in this wave, the vibration is 90 degrees to the direction of energy transfer, so right angles, because the vibration here is up-down.

So this wave is transverse, not longitudinal.

And this is like a water wave, for example.

Now a slightly longer task for you.

I'd like you to discuss whether each statement is correct and then tick one column for each to decide whether you're sure it's correct or you think it's correct, or you think it's incorrect or you're sure it's incorrect.

Take as long as you need to do that.

Press pause while you are discussing it and press play when you're finished.

So let's look at the answers.

Air particles can be pushed forward by other air particles that hit them.

That's true, that can happen in collisions.

In a sound wave, air particles vibrate and move forward slowly.

That's not true.

Particles vibrate, but they aren't all moving forwards.

Air particles can bounce backwards off the air particles they bump into.

That's also true.

So collisions can push particles forward in the direction they're already going or it can change their direction.

And finally, air particles in the sound wave move backwards and forwards repeatedly, they vibrate.

That's true, so well done if you've got most or all of those right.

Let's move on to the second part of this lesson, sound waves in solids.

Now you already know that sound waves consist of particles vibrating.

So what if there's a vacuum? A vacuum means a completely empty space.

So in a vacuum there's nothing at all.

No solid, liquid, or gas, no air, so no particles.

Here we have a gas spreading into a vacuum.

There's gas on the left.

And then when the wall disappears, the gas moves into the vacuum on the right.

And after that, the vacuum no longer exists.

We just have a box of gas.

Sound waves can't travel through a vacuum because in a vacuum there are no particles that can be set vibrating, and sound waves are caused by vibrating particles.

So sound waves require a medium to travel through, and a medium means the material or the substance that a wave is travelling through.

And the medium for sound waves can be a gas, a liquid, or a solid.

It can travel through any of those things.

So two questions for you now.

Which of the following words also means the material a wave is travelling through? And then question two, which of the following can sound waves travel through? The correct word for the material a wave is travelling through is the medium.

And sound waves can travel through brick, glass, liquid water, wood, or helium gas, any solid liquid or gas.

What it can't travel through is a vacuum which has no particles.

So well done if you've got those.

Let's take a closer look at what's happening in a solid.

In a solid, the particles are very close together and there are strong electrostatic forces of attraction holding the particles of a solid close to their neighbours.

Electrostatic forces of attraction are the forces between positive and negative charges.

Particles can't move around each other or apart, they can't change positions.

They can only vibrate around their fixed positions.

If one end of the solid is vibrated, it makes the particles in that end of the solid vibrate, and then those particles collide with neighbouring particles, passing the vibrations through the solid like this.

As you can see, the direction of wave travel is left to right and the vibration is backwards and forwards, left-right.

The particles themselves don't travel through the solid, it's the sound wave that travels through.

And the vibrations due to a sound wave add on to the smaller vibrations of particles that are already happening in any solid.

So how does a source of sound affect the particles of a solid? Press pause while you read these and press play when you've chosen your answer.

The correct answer is it makes the size of vibration of the particles of the solid increase along one direction.

If you're wondering why A isn't the right answer, it makes the particles of the solid start vibrating.

Well, it doesn't make them start vibrating.

Particles in the solid are always vibrating anyway in random directions.

What the sound wave does is increase the vibrations in one particular direction.

That's the direction the sound wave is travelling.

Now these electrostatic forces of attraction between the particles of a solid can be imagined as acting like tiny springs, as represented here.

When one particle moves out of position, neighbouring particles are pulled a bit out of position too.

And this happens very quickly because the particles are so close together and the forces between them are so strong.

This animation shows a representation of a sound wave moving through a solid lattice, irregular arrangement of particles in a solid.

So when one particle moves, it pulls on the other particles around it or pushes on them.

Particles of a gas are too far apart for electrostatic forces to act between them.

They only affect each other when they actually collide.

So if a particle in a gas is caused to move more in one direction by a sound wave, it has to travel some distance before it hits any nearby particle and passes on some of that motion.

And that's why sound waves travel through rigid solids faster than through gases and the sound can be heard more clearly through a solid.

Now, is this true or false? Putting your ear against a wall helps you hear what's on the other side more clearly.

So choose true or false and then decide why you think that.

Press pause while you answer both of those questions and press play when you're ready.

Did you realise that this is actually true? Maybe you've tried this.

And the reason is because sound waves travel faster through a solid, the wall, than through gases like air.

And the sound that is heard is also more clear.

The particles in the solid are very close together and are attracted to each other with strong electrostatic forces.

This means that when some particles in a solid move, they easily make the particles next to them move in the same way.

You can also notice this if you put your ear to a table and gently tap the table.

You'll find that the ear that's next to the table hears that sound louder and clearer than the ear that's next to the air.

But be careful not to tap too loudly because the sound will be transferred very well to the ear that's against the table.

Which of the following reasons for why sound waves travel faster through solids than through air is incorrect? Press pause while you read these and press play when you've chosen your answer.

And the correct answer, the incorrect statement is D, particles move more quickly in a solid than in a gas.

Particles actually move faster in a gas.

The other three statements are correct.

Particles in a solid are closer together than in a gas.

There are much stronger electrostatic forces of attraction between the particles of a solid than a gas.

And the particles are in a regular arrangement in a solid but in a random arrangement in a gas.

Now can you explain the difference between the speed of sound in air and the speed of sound in iron as shown in this table? Press pause while you write down your explanation and press play when you're finished.

Now I'll show you some of the points that you could have included in your answer.

At room temperature, air is a mixture of gases but iron is a solid.

So it's important to mention that one is a gas, one is a solid.

The speed of sound is greater in solids than gases.

And now for the explanation of that.

The particles of a solid are closer together than gas particles.

There are strong electrostatic forces between the particles of a solid.

So when one particle is disturbed, its neighbours are also pulled out of position.

Both of the above two points mean it takes less time for a vibrating particle to pass on a vibration to its neighbours in a solid than in a gas.

Well done if you got many of those points into your answer.

That word disturbed, by the way, means moved out of its normal position.

If you just said moved, that's fine.

And moving on to the third part of the lesson, let's look at sound waves in liquids.

Sound waves do also travel well through liquids.

Now you may have noticed if you ever swim underwater that sounds quieter and muffled, but that's only because human ears don't work well underwater.

Vibrations actually travel faster and further through liquids than through gases like the air, and there are animals that make use of that.

For instance, blue whales, they make sounds underwater that travel very long distances.

Their hearing organs work well in water so they can hear each other making these sounds and it's their way of communicating.

Particles of a liquid have enough energy to partially overcome the electrostatic forces of attraction between them.

So they can move around each other, they can change places within the liquid, but they can't move apart from each other.

They stay close together, as this animation shows.

And when particles of a liquid are disturbed by a sound, the neighbouring particles are not affected as much as in a solid.

They do collide with nearby particles much more easily than particles in a gas, but they're not held tightly together as particles are in a solid.

And that's why in liquids sound waves travel slower than in solids but faster than in gases.

Two questions for you.

Which option from A, B, C, explains why sound waves are faster in solids than liquids? And then which option explains why sound waves are faster in solids and liquids than in gases? Press pause while you choose your answers to these, press play when you're ready.

The correct answer to one is C.

Sound waves are faster in solids than liquids because the particles are held together by very strong forces in solids.

So when one particle is disturbed, many are disturbed.

And question two, the answer is B.

Sound waves are faster in solid and liquids than gases because the particles are much closer together in solids and liquids, so it can collide more easily to pass on particle movements caused by a sound.

Well done if you picked out both of those.

And now a longer written task for you.

Can you describe the properties of solids, liquids, and gases that cause sound waves to travel fastest through solids and slowest through gases? Press pause while you're writing your answer and press play when you're ready to check it.

I'm gonna show you an example answer.

Yours doesn't have to be written in exactly the same words, of course.

You could say particles in a solid are held together by strong electrostatic forces of attraction.

Disturbing one particle very quickly disturbs many particles, so movements caused by a sound wave travel very quickly.

Particles in a liquid are very close together, but they have enough energy to partially overcome the electrostatic forces of attraction between them.

They can easily collide to pass on movements caused by a sound wave, but they cannot shake each other to pass on the movement.

Particles in a gas are very far apart and have enough energy to completely overcome the electrostatic forces of attraction between them.

It's harder for them to collide and pass on movement caused by a sound wave.

Compare this with your answer and see if you missed any points and try to remember them for next time.

And we're at the end of the lesson now, so here's a summary of it for you.

Sound waves are caused when objects vibrate, causing nearby air particles to vibrate as well.

As a sound wave travels, each bit of air is vibrating parallel to the direction of wave travel, not travelling forwards.

Sound is a longitudinal wave because the direction of vibration is parallel to the direction of energy transfer.

Sound waves are faster in solids and liquids than in gases.

In solids and liquids, the particles are very close, so vibrations are more easily passed on.

Sound waves are faster in solids than liquids.

Strong forces holding the particles of a solid firmly together mean vibrations travel even faster.

Well done for working through this lesson.

I hope you enjoyed learning all about sound waves and I hope to see you again in a future lesson.

Bye for now.