Lesson video

Hello, my name's Dr.

George, and this lesson is part of the Sound, Light, and Vision unit.

The lesson is called Travelling Vibrations, and it's all about how sounds get from one place to another.

So let's start.

Here's the outcome for the lesson.

So I'll be helping you to be able to explain how vibrations are passed to the air and through the air to allow sounds to be heard.

And here are the key words for the lesson.

I'm not expecting you to know them all now.

I'll be introducing them throughout the lesson, but this slide is here in case you need to come back at any point and check the meanings.

And this lesson has three parts.

The first is called vibrating the air, the second is about how vibrations travel, and the third is about models of sound, and you'll learn what that means later.

Let's start with vibrating the air.

We should remind ourselves what air is.

Well, it's a gas or rather a mixture of gases, including oxygen, which we breathe.

And air, like all gases and liquids and solids, is made up of tiny, tiny particles, many of them.

They're too small to see and they're moving all the time.

This animation shows the way they move with different speeds, different directions.

There's a kind of randomness about how they move.

And you may notice sometimes they collide, they hit each other.

So that's what we're thinking of when we think of air.

Now let's look at something that makes a sound.

Well, a speaker, for example, makes sound by vibrating.

And vibrating means moving repeatedly backwards and forwards.

And the vibrations are usually too small and too fast to see.

But I can show you an animation of moving part of the speaker called the cone vibrating, moving backwards and forwards.

And these vibrations have been slowed down and made bigger so that you can actually see them.

So now, let's put the two together.

Let's see a vibrating speaker cone and see the effect that it has on the surrounding air particles.

And you're going to see that the vibrating cone makes the air particles vibrate as well.

So when I show the animation, I want you to watch one of the air particles.

Some of them have been highlighted, they're in a different colour so that it's easier to keep your eye on one of them.

So look at one of those and watch carefully what it does.

So here we have the speaker cone, and it starts vibrating, And look at the effect on the air particles.

Watch one of them, see how it moves is a repeat of the animation.

Okay, did you see that the air particles move backwards and forwards? So when the speaker cone vibrates, it's making the air particles vibrate as well.

There was something missing from this animation.

What it didn't show is all of that random motion that the air particles have.

So it's been simplified so that we just see the vibration of the air particles because of the cone.

In reality, there's also random motion that they're doing as well.

And now a question for you.

Complete the sentence.

A vibrating object causes nearby air particles to what? I'm going to give you five seconds.

But if you need longer to think, just pause the video, and press Play when you are ready.

And the answer is a vibrating object causes nearby air particles to vibrate too.

It doesn't cause them to travel far away from the speaker.

You didn't see all the air particles moving away from the speaker.

It doesn't make them move randomly.

They do that anyway.

And it doesn't make them move apart for the sound to get through.

That makes it sound as though the sound somehow travels between the particles, but it doesn't.

It's the vibrations of the particles that make the sound.

You may have noticed in the animation that when the vibrating speaker cone moves forwards, it pushes into nearby air particles, squashing them together.

Watch this again.

So we get areas where the particles are close together, squashed together.

And when the cone move backwards, it leaves a space in front of itself.

Now some of the air particles quickly move back into that space, watch carefully and you'll see it.

Now they're not moving back into the space because they think they should.

They don't think anything.

They're not alive.

They're not trying to fill the space.

But we've already seen particles move in different directions, and so they spread out and they end up filling the available space.

So we've got particles being pushed forwards and then falling backwards into the space that it makes.

All vibrating objects make sound, and they all make the air vibrate in the same way as the speaker cone.

So here we have a guitar string being made to vibrate.

And like the speaker cone, it alternates between squashing nearby air particles together and then allowing them to spread out.

There's a similar effect in the lower video, where the person pushes the plunger into the syringe, squashing the air inside, but the random movement of the air particles makes them spread out and they push the plunger back out again.

And our question for you.

When a vibrating object may moves forwards, it knocks nearby air particles forwards.

What then causes the particles to move back again? I'll give you five seconds, but if you need longer, pause the video, and press Play when you are ready.

And let's check the answer.

Air particles are moving in all directions.

They have that random motion, so some of them happen to move back.

And a task for you.

I want you to describe what happens to nearby air particles when a speaker cone vibrates and explain why this happens.

So you're going to write your answer using what you've learned so far.

And when you are ready, I'll show you an example answer.

So pause the video, and press Play when you're finished.

Here's an example answer.

A vibrating speaker cone causes nearby air particles to vibrate too.

When the speaker cone moves forward, this knocks into nearby air particles, pushing them forward.

They squash together or bunch up.

When the speaker cone moves backwards, this creates a space.

Nearby air particles quickly spread into the available space due to their free movement in a range of directions.

Then the speaker cone moves forwards again.

Compare your answer with this one.

Now this is a very detailed answer.

Don't worry if yours is not as long, but did you include the idea that the speaker cone makes the air particles vibrate and that it pushes them forwards when it moves forwards and they fall back into the space when the speaker cone moves back? If you've got all of those ideas into your answer, well done.

If you've got some of the ideas, that's good.

Now, look at how you could have improved it.

And now on to the second part of this lesson, how vibrations travel.

We've seen how vibrations get started in the air, but how do they then move from one place to another? Let's look at a speaker cone again.

We know that when it moves forward, this knocks into nearby air particles, pushing them forward.

Now, let's look at what happens next.

The particles that got pushed forward are going to hit some of the air particles in front of them and knock them forwards as well.

And those particles then knock into air particles in front of them, and so on.

And this is how a vibration, sound, travels forward through the air, particles knocking into the next ones, knocking into the next ones, and this keeps going.

So which of the following has to happen for a sound to travel? Five seconds, but if you need longer, pause the video, and press Play when you're ready.

And here's the answer.

Particles have to collide, hit, knock into each other as they vibrate.

That's what makes the sound move forwards.

You know that air particles are also moving in all directions.

They have their random movement all the time.

And when a speaker cone moves backwards, it creates a gap, and some of the air particles nearby quickly move back into this space because of their random motion.

And this creates a space further along now, that you can see in the diagram.

The particles that were knocked forwards earlier will now some of them move back into that space.

And now the speaker pushes the first particles forwards again to produce the next vibration.

So we have this forwards and backwards movement of particles pushed forward, falling backwards.

And this repeats every time the speaker cone vibrates.

Any vibrating object, not just a loudspeaker, makes the surrounding air particles vibrate too in the same way.

Let's see an animation representing what happens overall.

So we have circles again representing air particles, and they're being pushed forwards starting on the left, and their random motion makes them go back into the space that's made.

So what we have is sound travelling from left to right here.

But the particles themselves, they're just vibrating.

They're moving backwards and forwards, left, right, over and over.

I hope you can see that happening.

So no air particles are travelling away from the sound source, the thing that's making the sound, which must be over to the left somewhere, they're travelling backwards and forwards.

So which of the images most accurately represents sound travelling through air? Think carefully about this.

And if you need more than five seconds, pause the video, press Play when you're ready.

And the correct answer is B.

We have a loudspeaker cone on the left, and then we seem to have something like spheres or balls hanging from strings, and the vibrating loudspeaker makes the first one move, and vibrate and it's going to hit the next one, and that will hit the next one, and so on.

So that's a bit like the way sound travels, with air particles hitting the next air particles, and so on.

None of the other pictures really seems to look like what's happening when sound travels.

So the air particles vibrate in a kind of pattern, and this pattern makes travelling pulses of air, which we call compressions.

So these places that are marked here, where the air particles are closer together, compressed, these are compressions.

And it's these pulses that travel, not the air particles themselves.

You only hear a sound when the pulses, the vibrations, arrive at your ear, and they take time to get there.

And when they do arrive, they will travel into your ear, and the particles will then cause parts of your ear to vibrate by colliding with them, by hitting them.

So that pattern of pulses enters your ear, and that's when you hear something.

This picture makes it look as though the particles of air inside your ear are smaller.

They're not really.

A question for you.

Lightning makes an extremely loud sound, which we call thunder.

And is this true or false? If a lightning strike happens three kilometres away, you'll hear it as soon as it happens.

So choose True or False, and then choose A or B to explain why you chose that answer.

Five seconds, or pause if you need longer to think.

And the answer is False.

You don't hear the lightning strike straight away.

And the reason is the vibrations take time to reach you through the air.

They take time to travel to you.

In fact, if a lightning strike is three kilometres away, you'll hear thunder about nine seconds later.

It's time for a demonstration and something for you to do.

So you're going to see a lip candle put in front of a speaker, and then the speaker's going to be switched on, so they make sound.

And you are going to predict what you think you'll see when this happens and explain why you think that.

And then watch the demonstration, either in real life or perhaps on a video, and think about whether your prediction and explanation were correct.

And if you want to make any changes, any improvements, then do that.

So pause the video for that.

And when you've finished, press Play.

Let's see some example answers.

So the prediction.

When the speaker is turned on, the candle flame will vibrate, the candle will continue to burn.

Did you predict that? And I hope that's what you saw.

And here's an example explanation.

When a speaker makes a sound, it vibrates.

This causes air particles near the speaker to vibrate too.

Vibrating air particles knock into the particles next to them, causing them to vibrate as well.

The air particles around the candle flame will end up vibrating too.

These vibrating air particles cause the vibration of the flame.

The candle continues to burn as there is always oxygen in the air nearby.

Sound does not remove this or blow anything away.

Did your explanation cover the same ideas that the air particles vibrate and that pushes the flame around that makes it vibrate? But what doesn't happen is the air doesn't all move out of the way.

If it did, the candle would blow out.

Now, let's learn about models of sound.

So remember the air particles can be represented by tiny spheres, and they have different speeds and directions.

And sometimes they collide, they hit each other, and change direction, as shown in the animation again.

Well, this is an example of a scientific model.

That's one of the key words for this lesson.

Picturing air and other gases in this way can help us explain and predict the properties, the behaviours of gases.

Scientific models don't have to show every single detail of how something is.

They only accurately reflect some aspects of what they represent.

So for example, this model of air is simplified.

It doesn't show that actually there are different kinds of particles in air because it's made of different gases.

And in fact these particles are not all spheres.

So it simplifies, but this model of air and gases is still useful, though it can't explain every property that they have.

So which of the following statements about scientific models are correct? And there's more than one answer here.

If you need more than five seconds, pause, and then press Play.

Scientific models can be pictures on paper, on screen, or in our minds.

So they're a way of thinking about something.

And scientific models, they're useful because they can produce predictions.

We can use them to think what's going to happen, in data and images and visualisations, ways of picturing something.

To model sound, it's useful to simplify by ignoring the usual random motion of air particles, 'cause that's not needed to explain how sound actually works.

So in this animation and in our way of thinking about sound, we only show the back and forth movement of the particles as the sound passes.

It makes the model clearer and easier to understand.

Parts that are not included in a scientific model can often be added in later, if we need to, if we want to.

Now, notice that the forwards and backwards movement of air particles as a sound passes is a bit like the up and down movement of water when a water wave passes by.

They're both a kind of repeated movement backwards and forwards.

So the vibrations or pulses of sound that travel through air and other materials are called sound waves because of their similarities with water waves.

Let's look at representations, models of sound wave and water wave, both caused by moving particles.

Water is also made of particles.

And look at how they move.

Look carefully at one of the particles in each wave.

They're vibrating in both cases.

They're doing repeated movements, but there are differences.

Both waves of travelling left to right, but in the sound wave the particles are vibrating left to right, and in the water wave, the particles are vibrating up down.

But they're similar enough that we can call them both waves.

Another similarity is that they both transfer energy.

Waves carry energy from one place to another.

Here's a question about this.

Which of the following statements are correct? Now, there's more than one correct answer.

So you'll have to think carefully about whether each statement is true or not.

If you need longer than five seconds, pause the video, and press Play when you're ready.

The first correct answer is sounds or caused by vibrations.

We've been seeing that throughout this lesson.

For example, the speaker vibrating to make sound.

It's also true that sources of sound, things that make sound, create sound waves that travel.

Sound waves travel from one place to another.

And finally, it's true that sound waves transfer energy.

They carry energy along.

Well done if you've got all of those.

And our final task for this lesson, imagine that group of pupils make a model to show how we can hear a loudspeaker.

And you can see the model in the picture.

They're thinking of a loudspeaker with table tennis balls hanging on threads next to it.

And there are some questions here for you to read and answer.

You might like to write your answers down.

Take as long as you need, press Pause.

And when you're ready, press Play, and I'll show you the answers.

Are you ready? Example answers.

What do the table tennis balls represent? Well, air particles.

And what happens to the table tennis balls when the loudspeaker vibrates? The loudspeaker knocks into the first ball, pushing it forward.

This ball then bumps into the next one, and so on, until the last one is knocked.

The sound, vibration, can be felt at the last ball.

Each ball swings backwards after being knocked forwards, where it gets knocked forwards again by the next vibration.

So each ball ends up moving backwards and forwards, vibrating.

Your answer doesn't have to be identical to that, but did you get the idea that the speaker knocks the first ball, which starts vibrating, knocks the second, and so on, until they're all vibrating.

How is this model similar to what really happens when sound moves through the air? Well, a loudspeaker vibrates air particles in the same way by knocking into them.

Air particles also collide into each other and move back and forth like the balls do.

So in that way, the model is similar to the real thing, and that makes it useful.

And the last question, how is this model different to what happens when sound moves through the air? Because it's not perfect and it doesn't match everything about reality.

So the answer here is air particles are not held in place.

They move with a range of speeds and directions, as well as being caused to vibrate by the speaker.

Air particles do not swing back into place, they move backwards or get knocked backwards by other air particles.

And air particles are not all in a line.

They're far smaller and there are far more of them in the same space.

So there are a lot of things you could have said in your answer to four about the differences between the model and the real sound.

Well done if you've got some of them.

And we've reached the end of the lesson, so well done for working through it.

And let's look at a summary of what it was all about.

A vibrating object causes nearby air particles to vibrate too.

Air particles are knocked forwards, but then spread backwards when the object moves back.

Vibrating air particles knock into their neighbours and set them vibrating too in the same way.

This pattern of vibrating particles creates a sound wave.

And scientific models often only accurately represent some aspects of what they represent.

The usual motions of gas particles, random, are often ignored in models of sound waves.

Well done for working through this lesson about sound.

I hope you enjoyed it and found it interesting.

And maybe next time there's a thunderstorm, you'll check whether there's time in between seeing the lightning and hearing the thunder, which shows you that the sound is taking some time to travel to you.

I hope to see you again.

Bye!.