Lesson video

Hello, my name's Dr.

George and I'm going to be teaching you this lesson today.

It's part of the unit sound, light and vision, and it's called reflecting and absorbing sound.

Let's get started.

Here's the outcome for this lesson, which I'm going to help you achieve by the end.

I can describe factors that affect how much sound a material or object might reflect or absorb and explain ways to reduce noise.

Here are the key words for the lesson.

Don't worry if you don't know them all already.

I'm going to introduce them as we go along, but this slide is here in case you want to come back at any time and check the meanings.

The lesson has three parts, reflecting sound, absorbing sound, and reducing noise.

Let's start with reflecting sound.

First, a reminder of what sound is and how it's made.

Sound is caused when objects vibrate and this guitar string is vibrating when it's plucked, it moves quickly, backwards and forwards, and that vibration causes nearby air particles to vibrate as well.

Here's an animation, vibrating air particles knock into their neighbours and set them vibrating too.

So this pattern of pulses is created that moves along.

And this pattern of vibrating particles creates a sound wave.

Look carefully at one of the particles and watch it as the sound wave moves through.

See if you can see how the particle moves.

And here's a question.

When sound travels through air, what happens to the particles of the air? Think about what you just saw in the animation.

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

Have you got your answer? Correct answer is they vibrate back and forth and do not travel.

So if you watch the animation and go back and look at it again if you want to, the particles do not all move from the left ending up all on the right.

Each particle moves backwards and forwards.

It vibrates.

Well done if you got that right.

Now we're going to think about reflecting sound.

And to do that we might want to look at some diagrams. We've been representing sounds using animations, using little circles representing particles that move.

But an easier way to represent sound sometimes is using lines showing how the sound is travelling out from the source, the thing that's making the sound.

The lines represent the travelling pulses of sound waves created by particle vibrations.

This animation is switching between representing sound, using lines and representing it using moving particles.

So let's see what happens when sound reflects.

This can happen if sound hits a hard surface like a brick wall or a cliff, it can reflect.

If you know anything about light, you might know that light can also reflect.

With light, you can reflect it using a mirror or some other shiny surface.

With sound, we should need a hard surface.

So here we have a person hitting two wooden block together to make a sound and that sound travels outwards from the source, from the blocks and we're representing that using these lines, they're circles because the sound travels outwards in all directions.

After a while, some of that sound is going to hit the wall, here we've just sort of represented that part of the sound with parts of the lines.

And the sound may reflect off the wall and travel back towards the person.

And if it gets back to the person's ears, they may hear the sound of the blocks hitting each other again.

And that's what we call an echo.

So when a sound comes back to you 'cause it's been reflected.

Here's a question about that.

I want you to choose the point or points where the person hears the sound of the block knocking together.

So look at each one carefully.

There might be one answer, there might be more than one answer.

When does a person hear the sound? Five seconds.

But if you need longer, press pause and press play when you're ready.

And the correct answers, and the first one is A, the person will hear the sound of the blocks just after they knock them together.

Just when the sound from the blocks reaches their ears.

But they will hear the sound again after it has reflected off the wall and come back to their ears.

So well done if you've got both of those.

You're more likely to hear an echo if a wall is face on to you.

And what I mean is facing the way it's shown in the top diagram, that sound is reflecting straight back towards the person and they're going to hear an echo.

If the wall's at a different angle such as in the lower diagram, less of the sound will reflect back to the person.

The arrows show the direction the sound is going and when it reflects it's going off in a different direction.

It's not going back to towards the person.

So the way that solid surfaces are angled affects whether the sound comes back to you, the type of surface matters as well.

Smooth flat surfaces can produce a clear echo.

So on the left is a representation of sound bouncing off a smooth flat surface.

The arrows represent the direction the sound is travelling.

Each part of that surface reflects sound in the same direction.

And that gives us a clear echo.

An echo that sounds similar to the original sound.

A bumpy surface reflects sound in many different directions.

Here's a picture representing that, that spreads the sound out in all directions and the echo that we hear is not going to be as clear.

It might sound a bit muffled, a bit sort of blurred, bit unclear.

On the left is a picture of somewhere where we want sound to be nice and clear.

It's a recording studio.

They might be recording interviews in there, for example, people talking.

And if you look at the walls, they're rather strange, they're not smooth and flat.

Smooth flat walls would be good at reflecting sound back into the centre of the room.

And you don't want that, if you're trying to record clear sounds of people speaking, you don't want echoes coming back as well.

So these walls are covered with shapes, they are bits like pyramid shapes that reflect sound in different directions because of these different angles on the shapes.

So less sound reflects right back to the centre of the room.

We don't get so much echo when we're trying to record something.

On the right we have something that naturally will reflect sound in some different directions because these cliffs, they're not smooth, they're not flat, but they might still give some echo.

It just won't be very clear.

Now echos are quieter than the original sound.

You might have noticed this if you've ever heard echos and there are several reasons for this.

One is that sound waves spread out.

So as they spread out, their energy becomes spread out as they travel.

Another is that sound waves transfer some of their energy to the air as they travel.

So the sound wave itself loses some of the energy and instead it goes into some of the random motions of air particles.

That actually means the air warms up very slightly as the sound travels through it.

And so the sound is losing some of its energy to the thermal energy of the air, the heat energy of the air.

Also surfaces can reflect sound in different directions.

So when you hear an echo, not all of the sound that reflected may come back to your ears.

So it's quieter and surfaces absorb some of the sound wave, you'll learn more about that later.

But when a surface absorbs some of the wave, that wave never comes back out again.

An echo is also not as clear as the original sound, partly because echoes from different surfaces can return at different times.

And also because if there are air currents moving air, if there's a draught in the room or if there's gusts of wind, that can distort the sound wave.

So the sound that comes back might not quite sound like the original, might be a bit unclear or a bit muffled.

So there are a lot of things to think about here.

Here's a question.

True or false, echoes make a room noisier.

Choose true or false and then choose a reason that explains or justifies the answer you chose.

Press pause if you need longer than five seconds.

Let's check the answer.

It's true.

Echoes make a room noisier.

And the right reason is that an echo means sounds will be heard more than once.

And usually we don't want that if we're trying to have a conversation or if we're trying to record some music.

We don't want echoes bouncing around.

Here's a task for you.

Jun is at a beach and finds a cave.

When he stands in the cave entrance and claps, he hears an echo.

I want you to explain why Jun hears an echo and explain why the echo isn't as loud or as clear as the original sound.

You'll be using ideas that I've mentioned.

So I want you to pause the video, try writing your answers and press play when you're ready.

And I'll show you some example answers.

And here are some example answers.

Explain why Jun hears an echo.

The sound reflects off the back of the cave and travels back to Jun.

That's when he hears the sound again, that's the echo.

And explain why the echo isn't as loud or as clear as the original sound.

There are a lot of different reasons you could give here.

Here are some.

Sound spreads out and becomes quieter as it travels to the cave wall and back.

Some of the sound is absorbed by the walls of the cave.

The sound is reflected in many directions.

Some of the sound may bounce around between the walls of the cave and arrive back at Jun at slightly different times.

Similarly, echoes from different parts of the cave arrive back at Jun at slightly different times.

Did you get some of those ideas into your answer? If you did, well done.

And did you use some of the scientific words such as reflect, echo, absorb? If you feel like you didn't do so well on that one, don't worry.

Go back and have another look and what I talked about and see if you can then have another go.

Now for the second part of this lesson, absorbing sound.

When a sound wave in air arrives at a solid surface, the vibrating air particles collide, hit the particles of the solid.

And remember that particles in a solid are already vibrating a little bit.

Just normally in a solid, the particles are moving back and forwards a bit because of their temperature.

But the collision increases the vibrations of the particles in the solid.

So on the left here we have a representation of particles in a gas, they're not attached to each other.

And on the right we have particles in a solid.

They are very close and they're attached to each other, they're bonded.

And we have a sound wave in the gas travelling and then reaching the solid and the vibrating air particles are hitting the edge of the solid and making the particles at the edge of the solid vibrate.

And then those particles make the next one vibrate and so on.

So the animation showed that a few times.

So what happens is some of the energy is transferred from the sound wave into the solid.

A question, what happens when sound is transmitted from air to a solid? Pause the video if you need more than five seconds and press play when you're ready.

And the right answer is B.

When sound is transmitted from air to a solid, particles in the solid vibrate quicker.

None of the other three options are right.

And we've seen sound reflecting off brick earlier.

Brick is a hard rigid solid.

Rigid means it doesn't change its shape easily and that's because the particles the brick is made of are held together by very strong attractive forces, forces that attract the particles to each other and stop them just moving apart.

So because of that sound waves can't make the particles of brick vibrate very much and that means very little sound is absorbed by the brick.

Not much of the energy of the sound passes into the brick.

Most of it is reflected.

So a sound wave is mostly reflected off brick back into the air.

But if you have a softer material, the particles are not as strongly bonded together.

The attractive forces between them are not as strong and it's actually easier for sound waves to make the particles vibrate in a softer material.

Here's an animation again of sound in air passing into a solid.

So we have that sort of thing happening.

We have energy being absorbed from the sound wave by the material, by the solid material on the right.

So softer or more flexible materials, materials that change shape easily, they can take in, absorb more energy from sound waves and they reflect less sound than harder materials.

Question, which best describes what happens when a surface or object absorbs sound? I'll give you five seconds, but pause if you need longer, then press play.

And the correct answer is C.

Sound increases the vibration of its particles.

When a surface or object absorbs sound, it's particles vibrate more.

Well done if you've got that.

Now let's think about some different materials.

I want you to try to order these materials by their increasing ability to absorb sound, starting with the material that absorbs the most sound.

And then the next one.

And finally the one that absorbs sound least well.

Five seconds but pause if you need longer.

Have you put these in order? Correct order is B, C, A.

I hope you realise B is going to be the softest of these materials.

Wood is quite hard but brick is even harder.

So B is going to be the best at absorbing sound.

The softest material is best at absorbing sound.

Again, well done if you got that right.

A longer task, explain how carpet on a floor would absorb more sound than a bare wooden floor.

And there's some help here.

Pay attention to these points 'cause you should try to include each one in your answer if you can.

A key property that is different for carpet and wood.

Something about the particles of the carpet and what they're doing, and what the particles of the wood are doing and also the particles of the air.

So take as long as you need, write down your answer, pause and press play when you're ready.

So here are some examples of things that you could have said such as when vibrating particles in the sound hit the bare wooden floor, they make the particles in the wood vibrate more, they transfer energy to the wood.

The reason why more is involved there is it wouldn't be quite right to say they make the particles in the wood vibrate.

Particles in solids vibrate anyway all the time, but sound hitting the wood makes the particles vibrate more.

The next point, the same sound wave would cause particles in the carpet to vibrate more than the particles in the wood because carpet is softer and more flexible.

A sound wave transfers more energy to carpet than to wood and more sound reflects off a wooden floor than off a carpet.

So well done if you've got those key ideas that the sound wave hitting a solid transfers some energy to the solid, it makes the solid particles vibrate more.

And if the solid is softer, they vibrate even more than if the solid is hard.

So more energy from a sound transfers to a softer solid.

Here's the third part of the lesson, reducing noise.

Noise means unwanted sound.

Any sound that you don't want to be hearing.

A noise from outside, for example, can be reduced by using materials or objects that reflect sound waves away reflect them back to the outside instead of letting them come through.

A lawnmower outside sounds a lot quieter inside if you've got the windows closed.

And that's because some of the sound waves that would've passed through the air, the open window, now reflect away off the glass.

So not so much of the sound makes it into the room.

Another way to reduce sound is absorption.

So if you have bushes and trees forming a sort of barrier alongside a busy road.

Plant material is quite soft.

So it's good at absorbing a lot of the noise.

And so it's not so noisy if you're on the other side of that barrier away from the road.

So windows are good at reflecting, plants are good at absorbing sound.

Snow is a lot softer than normal ground.

So when there's snow on the ground, more sound gets absorbed than usual.

So snowy days can be noticeably quieter than other days, even if the amount of traffic is the same, listen out for that next time it snows.

Snow is helpful to predators, if they're hunting, they can move more quietly through snow than on normal ground.

Here's a question.

Five people are walking around a room and talking, which of these three rooms would be the quietest? So think about that, pause if you need to and press play when you have your answer ready.

So did you choose the lounge? I hope you can see in the picture there are a lot of soft surfaces, soft furniture, carpet, curtains, good at absorbing sound.

So there won't be a lot of echoes reflected sounds around that room.

It won't be so noisy.

In a tiled bathroom, we have a lot of hard surfaces and also in this unfurnished room.

So in an unfurnished room where we have these flat, smooth, hard surfaces, they reflect sound very well.

It's going to be very echoey.

And we could call that noisy because usually those echoes are not wanted.

Same thing with the tiled bathroom, whereas the soft furnishings absorb the sound well, making the room quieter and you're talking, you won't hear the echo of what you're saying.

Another question.

Which of these five options explain why curtains can help make a room quieter? Might be more than one correct answer here.

So think carefully and pause the video if you need longer than five seconds.

And the first answer, sounds make the particles of the curtains vibrate more.

That's what sound does when it hits a solid.

The folds in the curtain help them absorb sound.

Having folds mean there's more surface there to absorb energy from the sound.

And curtains are soft and flexible and that's the sort of material that's good at absorbing sound.

Well done if you've got some or all of those right.

The final task of this lesson is about reducing noise.

Here we have some pupils and they're discussing how to reduce the noise from a road outside their school and they each have a suggestion.

I'll let you read those and then the questions ask, which pupil's ideas would work to reduce the noise and explain why each pupil's idea would work well or not so well.

Pause the video and when you've thought about your answers, press play.

Let's have a look at some examples of what you could say.

All of the pupil's ideas reduce the noise to some extent, so nobody had an idea that wouldn't work at all.

And here are some things you could say about what they're saying.

So Aisha said we could plant a row of bushes and Alex said we could plant a row of trees.

Well, those are both helpful because plants are fairly soft, so both bushes and trees would absorb sound.

Trees are bigger so it would absorb more sound in total.

However, with trees there would be gaps between the ground and the lowest leaves where sound heading to the school would not be absorbed.

Thick, tall bushes might be the best option.

But then Laura said we could put up a metal fence and Andeep said we could put up a wooden fence and Sam said we could put up a fence with bumps or panels that face different directions.

Well, metal and wood fences are quite hard and so they could reflect noise back towards the road instead of letting it go through to the school.

The wood is softer, might also absorb more of the noise.

And a fence with bumps would reflect noise in different directions so that it spreads out more.

Remember the wall of that recording studio with a strange bumpiness that helps make the sound reflect in different directions.

Well done if you've got some of those ideas and you may have got some other ideas that are also correct.

Here's a summary of the whole lesson.

Sound reflects from hard surfaces.

An echo is when a sound is heard again after reflecting and travelling back.

The particles of hard materials are held together by strong forces and sound waves cannot make them vibrate easily.

Sound waves reflect off hard surfaces.

Sound waves can more easily cause the particles of a soft material to vibrate.

Sound waves are absorbed more easily by soft surfaces.

Noise is unwanted sound.

It can be reduced by using soft materials to absorb it or hard materials to reflect it away.

Well done for working through the whole lesson and I hope you'll remember some of these things about sound reflecting and absorbing and about how we reduce noise.

I hope to see you again in future lesson.

Bye.