Lesson video

Hello and welcome to this lesson.

My name's Dr.

George, and this lesson is part of the unit sound, light and vision.

The lesson is called travelling sound, and it's about how sound changes as it travels and what it can actually travel through.

Let's look at the outcome, which I'm going to help you achieve by the end of this lesson.

I can explain why sounds are quieter at a greater distance and why sound waves travel faster through solids and liquids than gases, but not at all through a vacuum.

This slide shows the keywords.

Don't worry about knowing and understanding them now.

I'll introduce them through the lesson.

But if you want to come back at any point to check the meanings, you can come back to this slide.

This lesson has three parts.

The first is called sounds are quieter from further away.

The second is sound waves in solids, liquids, and gases.

And the third is called in a vacuum.

Let's get started on the first part.

In the video, you can see a guitar string being plucked, and when it's plucked it vibrates, it moves quickly backwards and forwards.

And a vibrating object like this causes nearby air particles to vibrate too.

Remember, particles are the tiny, tiny things that everything is made of.

Air particles get knocked forwards by the string when it moves one way, but they then spread backwards into the space that's made when the string moves back again.

And then these vibrating air particles knock into their neighbours and set them vibrating too, as shown in this animation.

So we end up with lots of air particles, all moving back and forth.

And this pattern of vibrating particles is what we call a sound wave.

Now here's a question for you.

Which of the following travels outwards from a source of sound? A source of sound is something that makes a sound.

I'll give you five seconds, but if you need a little longer, press pause and then press play when you're ready.

And the answer is C, sound waves.

The other two are not right.

Air particles don't all travel outwards from a source of sound.

They're travelling backwards and forwards vibrating, and there's no such thing as sound particles.

Now, as you get further away from a vibrating object like a guitar string, the amplitude of vibrations of the air particles is smaller.

They don't travel as far backwards and forwards, and that actually means the volume of the sound further away is lower.

Let's see that shown in an animation.

On the left is a loudspeaker cone vibrating.

It's making a sound that travels through the air.

Look at the particle, the red particle, the darker one in this box.

Look how far it's travelling backwards and forwards.

Look at the one in the box on the right.

When the sound reaches it, it doesn't travel as far backwards and forwards.

It's got a smaller amplitude of vibration, and that's what makes the sound closer to the speaker louder and quieter further away from the speaker.

I'm sure you already knew that if you move further away from something that's making a sound, the sound gets quieter.

But here we actually see why that's happening.

And here's the next question for you.

A clock, you can see it in the picture, is making a sound.

Why does the clock sound louder when you are closer to it? If you need more than five seconds, pause the video and press play when you're ready.

And the reason why the clock sounds louder when you're closer is the sound has a higher amplitude.

As you saw in the animation, the air particles are moving further backwards and forwards closer to the object that's making the sound.

Now we can think of this by thinking about energy as well, that when an air particle is pushed forwards by something like this speaker cone, energy is transferred to it.

That air particle then transfers some of its energy to the next particle it knocks into, but it might not just knock into one single particle.

There may be many collisions before the particle transfers all of the energy it was given.

So further from the vibrating object, the energy is shared out among more particles.

On the left, you can see a loudspeaker cone, the moving part of a loudspeaker making a sound.

As the sound wave travels, the energy becomes more spread out.

Here are pictures of ways of representing the sound wave after a short time.

So it hasn't travelled very far, and not many of the particles have been affected yet.

Only these particles are vibrating so far.

But after some time, the sound wave has travelled further and more of the particles are now vibrating.

The energy has been shared between more particles.

And before a sound wave passes, particles in the air are already moving, but they're moving in random ways like in this animation.

Different directions, different speeds, this is what particles and gases do.

But a sound wave then adds extra movement, vibration, this to and fro movement in addition to this random motion.

And as the sound wave moves through the air, it causes the random movement of air particles to increase a little bit.

They're moving just a little bit faster than before, and this means that the air is heated up a little bit and that's where some of the energy from the sound wave goes as it travels along.

Now here's another question for you.

You are walking towards a clock as it strikes 12 at midday.

Which of the following correctly explains why the bongs get louder as you walk closer? Pause if you need more than five seconds to think, and press play when you're ready.

And the only correct answer here is that the sound is less spread out the closer you are.

Well done if you got that right.

Now a longer task for you.

I want you to discuss whether each of these four statements is correct or incorrect, and then tick one column for each.

So decide whether it's correct or incorrect, and then decide whether you're sure or whether you just think so.

Take as long as you need to do this, press pause.

And when you're finished, press play.

And here are the answers.

Two of them are definitely correct and two are definitely incorrect.

The energy of a sound is used up as it travels, not correct.

The energy isn't used up, it hasn't disappeared, it's spread out.

Sound spreads out as it travels.

Yes, we saw that.

We saw the wave spreading out as it travelled away from the loudspeaker.

Air particles always transfer all their energy to the next particle.

No, they make lots of collisions and transfer some of their energy to lots of particles.

Sound waves can produce a small heating effect.

That is true.

When a sound wave passes through, some of the energy goes into heating up the air a little bit.

So well done.

And now let's move on to the next part of the lesson.

Sound waves can travel through solids and liquids as well as gases like air.

We have a name, one of the keywords today, for any material that sound is travelling through, we call it the medium.

So if sound travels through air, then air is the medium.

It might surprise you to hear that sound can also travel through solids and liquids.

But when you think about it, you'll realise that you've experienced this.

You can sometimes hear someone in the next room through a solid wall.

So the sound must be travelling through the wall.

And you can hear sounds from outside when you're indoors, even when all the doors and windows are closed, and they're solid.

So the sound must be travelling through those solid objects.

Here's a question: Jacob is inside with all the doors and windows closed, but he can still hear his mum mowing the grass outside.

How does the sound wave reach Jacob? Pause the video if you need longer than five seconds, and press play when you're ready.

Did you realise that there was more than one correct answer? The sound wave can reach Jacob through the glass in the windows, through the wooden back door, and through the wall, because sound can travel through all of these solid objects.

Well done if you got some of those or all of those.

Let's look more closely at solids and their particles.

Particles of a solid are very close together, as shown in this animation.

And there are strong forces holding the particles to their neighbours, but they don't stay absolutely still.

They can't move around each other or move apart, but they vibrate in their fixed positions.

There is a little bit of randomness to how they vibrate.

They're not all vibrating in exactly the same direction or in exactly the same speed, and they vibrate because of their temperature.

If the solid got hotter, it would vibrate further and faster.

If you can make one end of a solid object vibrate, then the particles at that end will start vibrating.

This animation shows a representation of particles in a solid, and you can see that the vibrations move along from left to right as particles bump into their neighbours and make them vibrate too.

But the particles themselves, they don't all move from the left hand side to the right hand side.

They're just moving backwards and forwards.

So the particles vibrate, but they don't travel along the solid, but the sound wave does.

And the vibrations due to a sound wave add to those smaller random vibrations that the particles have because of their temperature.

And that was similar with gases, that as the sound wave passes through, some of its energy goes into slightly heating up the substance.

Now this question might look the same as the last one, but it's not, so listen carefully.

Jacob's mum is using a loud lawnmower in the garden.

Jacob is inside with all the doors and windows closed, but he can still hear the lawnmower.

What is caused to vibrate? There's more than one correct answer, so think carefully about each option.

If you need more than five seconds, press pause, and play when you're ready.

Did you realise that all of these things vibrate because the sound is passing through all of them? Well done if you did.

Here's something you might not have realised from everyday life.

Sound waves actually travel faster through solids than they do through liquids and gases.

And that's because vibrations are passed along from one particle to the next very easily, very quickly in solids.

And we know why.

It's because the particles in solids are so close together and because of these forces of attraction between the particles that are so strong that when one particle moves out of place, it pulls the neighbouring particles a bit out of position as well.

It's almost as if they are connected by strings.

When some move, the others have to move along with them.

Sounds from outside are clearer and louder when heard through an open window than a closed one.

But that's not about speed of sound.

That's because when the window's closed, some of the sound makes it through the solid glass, but some of the sound waves reflect off the glass, so they don't all make it through to you.

But it's still true that the sound waves that do pass through the glass are travelling faster in the glass than they do through the air.

Here's a question with two parts.

True or false? Sound waves travel better through solids than gases.

So choose your answer and then choose an explanation.

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

Are you ready? It's true.

Sound waves travel better, more easily, faster through solids than gases.

Now what's the reason for this? Well, there are two here.

Particles of a solid are strongly attracted to each other, so many of them vibrate when one of them vibrates, and particles of a solid are very close together, so they easily collide when they're vibrating.

Well done if you've got all those.

Now on to liquids.

Sound waves also travel very well through liquids.

If you're swimming under water, sounds are quieter and they're muffled.

But that's just because your ears don't work very well in water.

The sound is still travelling well through the water.

In fact, vibrations travel faster and further through liquids than they can through gases like air.

Blue whales make sounds underwater, and those sounds travel very long distances.

And their hearing organs, their kind of ears work well in water, so they can hear really well.

Let's look at what the particles would look like in a liquid if we could see them.

You can see the animation on the right.

The particles have enough energy to partly escape the strong forces of attraction.

So in a solid, the particles couldn't move away from each other, but in the liquid, the particles can move around.

But they're not separated, they're still very close together, they're still touching.

And because the particles of a liquid are very close together, they can easily bump into each other and so they can pass along vibrations from a sound wave very well.

And that's why sound waves travel faster through liquids than through gases.

However, if we compare solids and liquids, sound waves are faster when they travel through solids, and that's because particles of a liquid are not bonded, not actually joined to other particles as they are in a solid.

In a liquid, this movement, this motion of particles around each other complicates the way that a sound wave travels.

Try filling in these gaps, and the words you can use are solids, liquids, gases, and you'll need to use some of them more than once.

Pause the video and press play when you're ready to see the answers.

And here are the words.

Sounds travel slowest through gases, and sound waves travel faster through solids and liquids because the particles are very close together, so can easily collide to pass on vibrations.

Sound waves travel fastest through solids because the particles are bonded together.

They're joined together by strong forces.

In liquids and gases, particles are not bonded, which complicates how easily vibrations can be passed forwards.

Well done if you got most or all of those.

Here's something for you to try.

A metal object is hanging on a string.

In the picture on the left, it's a spoon.

It makes a sound when it's tapped gently.

Now you're going to predict what you think will happen to the sound if you put your finger in your ear, that's the finger with the string on it.

And explain why you think this will happen.

Then watch a demonstration of this or ideally try it out yourself.

And afterwards, if you want to change what you've written, you can do that.

So press pause, try this out, and when you're ready, press play.

Now I'll show you example answers to these questions.

The prediction.

When I put my finger in my ear, I will hear the sound more clearly and it may be louder too.

Is that what you predicted? And is that what happened? Explanation: without my finger in my ear, I heard the sound through the air, a gas.

With my finger in my ear, I heard the sound through the string, a solid, and my finger.

Sound waves travel better through solids than gases, so the sound was clearer and louder.

In a solid, the particles are closer together and bonded together.

The vibrations are passed from particle to particle very easily.

In a gas, the particles are further apart and are not bonded together, making it harder for vibrations to travel.

Did your explanation include those ideas, that the sound travels better through the solid? And why? Well done if you got some or all of those ideas into your answers.

Now let's go to the last part of this lesson, which is called in a vacuum.

And if you don't know what that is, you'll find out in a minute.

A vacuum means a completely empty area of space.

There's nothing at all in a vacuum, no solid, liquid or gas.

So there's no air in it.

There are no particles.

The animation shows a gas on the left behind a wall, and then when the wall is taken away, the particles spread out into the vacuum.

They don't do it because they want to fill the empty space.

They don't want anything.

They're not alive.

They're not trying to fill the space.

They're just moving around randomly as usual.

And as they move around randomly, they tend to spread out.

Now remember, sound waves consist of particles, vibrating particles moving backwards and forwards in a pattern.

So sound waves can't travel through a vacuum because there are no particles in a vacuum, there's nothing to vibrate.

So sound waves require a medium.

Remember, a medium is something that waves travel through.

Above the Earth's atmosphere out there in space, it's a vacuum.

So let's think about astronauts.

Two astronauts are on a spacewalk, they're out in space, and one astronaut taps two tools together.

Which of the following explains why the other astronaut does not hear a sound? If you need more than five seconds, press pause.

Press play when you're ready for the answer.

Did you choose B? There are no particles between the two astronauts to vibrate.

There can be no sound wave when there's no particles that can move backwards and forwards, so nothing travels between the two astronauts.

Final task for you.

A semi-inflated balloon, a partly-inflated balloon is placed in a sealed glass jar as shown in the left hand diagram.

Then air is pumped out of the jar and the balloon gets bigger.

And eventually, the balloon gets so big that the pins at the top pop it.

Predict what you think you'll hear when the balloon pops, explain why you think it, and then watch a demonstration of this actually happening.

And then afterwards, if you need to, you can make changes to what you've written and write an improved explanation.

So take as long as you need, pause the video, and press play when you're ready to see the example answers.

I hope you enjoyed watching that.

Now, the prediction.

This student has written: when the balloon pops, it will make a much quieter noise than it would if popped in air or perhaps even no noise at all.

Were you surprised by that? The explanation: pumping the air out of the jar means there are far fewer air particles in the jar.

Inside the jar it may be close to being a vacuum.

When the balloon pops, there will be very few or no particles within the jar to transmit vibrations outwards towards the walls of the jar.

So very little or no sound waves will travel outwards through the jar.

If you've got the idea that the sound doesn't travel very well or perhaps not at all because there aren't air particles or hardly any air particles in the jar, then well done, you've got the key point.

And that's the end of this lesson.

Here's a summary of what it was all about.

Sound spreads out as it travels, so further from a source of sound, there are smaller amplitude vibrations of the particles and the volume of the sound is lower.

Sound waves can travel through solids, liquids, and gases.

The material travelled through is called the medium.

Sound waves travel faster through a liquid than a gas, as the particles are closer together in a liquid.

And sound waves travel even faster through solids, as particles in solids are bonded, joined together.

Sound waves cannot travel through a vacuum because in a vacuum there are no particles to vibrate.

So well done for getting through this lesson.

I hope you learned some new things, I hope you enjoyed it, and I hope to see you again in a future lesson.

Bye.