Hello, my name's Dr.

George.

This lesson is called speed of sound, and it's part of the unit, waves.

The outcome for the lesson is I can measure the speed of sound waves in air using an echo method, and you'll be doing that later in this lesson.

Here are the key words for the lesson.

I'll introduce them as we go along, but if you need to remind yourself of any of the meanings later, you can come back to this slide.

The lesson has three parts, and they're called the speed of sound waves, measuring the speed of sound in air, and echo location.

Let's start.

Speed is usually measured as how many metres will be travelled every second.

And the word per is used instead of every.

So the units of speed are metres per second, which is written m/s.

And that forward slash means per.

It's used in some other units too.

A person walking might travel 1.

5 metres every second, so their speed is 1.

5 metres per second.

A cyclist might travel six metres every second, so their speed is six metres per second.

A car on the motorway travels 24 metres every second.

Can you write down its speed in the shortest possible way? And when I ask a short question, I'll wait five seconds, but if you need longer than that, just press pause and press play when you have your answer ready.

And here's the correct answer, 24 m/s, 24 metres per second.

Now car travels 50 metres with steady speed.

If it goes 50 metres in two seconds, the speed is 25 metres per second.

And if you're not sure why, have a look at this.

50 metres in two seconds, cut that in half to find how far each second, and it's 25 metres each second.

If the car goes 50 metres in five seconds, the speed is 10 metres per second.

And let's just see why.

50 metres in five seconds.

If we break that up into five, how far do we go each second? 10 metres.

And that's why we calculate speed using speed is distance divided by time because that tells you how much distance in each one unit of time.

We usually write that equation as speed is distance over time, which means the same thing.

Now a couple of questions.

I'm going to show you how to answer the first one and then you can try the second one.

Laura takes 40 seconds to walk across a school field that is 64 metres wide.

Calculate her speed.

So when you're gonna do a calculation, it's a good idea to start by writing down the equation you're going to use and picking out the quantities that you're going to substitute into the equation.

There we have the time and the distance, so we put those into the right places in the equation, do the calculation, and we get speed as 1.

6 metres per second.

And now here's one for you.

Aisha cycles the last 600 metres to school in 125 seconds.

Calculate her speed.

Press pause while you're writing out your answer and press play when you've finished.

So again, we write down the equation.

We pick out the values we need from the question.

The distance is 600 metres and the time is 125 seconds and we get 4.

8 metres per second.

Not surprisingly, cycling is usually faster than walking.

Now let's use what you've learned about speed to think about the speed of sound.

First, a reminder of what sound waves are.

They're pulses caused by forward and backward vibrations that travel through a medium.

So here we have particles.

This could be a sound wave in air, for example, and they're vibrating, they're moving repeatedly back and forth.

and sound travels through a medium.

And the medium just means a material that a wave is travelling through.

And we hear sounds when pulses like these arrive at our ear through the air.

This particular sound wave is travelling from left to right and the particle vibrations are parallel to the direction of travel.

The particles move repeatedly right and left.

When a sound wave is created, the particles of a medium are set vibrating in a pattern to produce pulses that move forwards.

The particles of the medium are not transported forwards.

So when the sound wave travels, it's not that all the air moves left to right.

It doesn't.

The wave itself moves left to right whereas the particles vibrate.

Let's recap.

When sound travels through air, what happens to the particles of air? Are they moving away from the source of the sound, are they carried along with the sound wave or do they vibrate back and forth and not travel? Press pause if you need more time to think.

And did you choose the answer C? So that's the correct answer.

They vibrate back and forth and do not travel.

Now a bit more recap about sound, different types of sound.

So a squeaky sound has a high pitch, a sound that a whistle might make for example.

And a deep sound, we say that it has a low pitch, for example, a rumble of thunder.

And with a high pitch sound, the vibrations of the sound wave have a higher frequency.

That means they're happening more times per second.

Here's a representation of the moving part of a loudspeaker making a sound.

And if we hear a low pitch sound, the vibrations of that sound wave have a lower frequency.

The vibrations are happening fewer times per second, and this animation represents that.

But sound waves of different frequencies actually all travel at the same speed in a particular medium.

So what decides the speed of a sound wave is the medium that it's travelling through.

In a higher frequency sound wave, the particles will vibrate back and forth more quickly, but the wave pulses would still travel forward at the same speed.

So higher frequency doesn't mean a faster travelling wave, it means more pulses per second, but the wave moves forwards the same number of metres per second.

The speed of sound waves depends on the properties of the medium, how quickly the vibrations pass through it from particle to particle.

And here are some examples.

You'll see that the speed of sound in air is actually relatively slow compared with the speed of sound in liquids and solids.

In steel, it's nearly 6,000 metres per second.

So for a particular medium, all sound waves travel at the same speed through that medium, but they might travel at a different speed through another medium.

What does it mean if sound waves are higher frequency? Tick all that apply.

So we have the sound heard will be higher pitched, the sound waves are faster, there are more vibrations per second, the sound heard will be louder.

Press pause if you need more time to think.

Did you notice there are two correct statements here? The sound heard will be higher pitched and higher frequency means more vibrations per second.

Sound waves are faster in solids and liquids than they are in gases, and that's because the particles are much closer together in solids and liquids.

So the vibrations are passed on from particle to particle much more quickly.

The strong forces holding particles of a solid family together make the speed of sound even faster in solids than it is in liquids.

So can you remember the order? Starting with the lowest speed, write down the three states of matter in order of the speed of sound through each one.

Here is the correct order.

Slowest in gases, fastest in solids.

The speed of sound in air at the earth's surface is about 340 metres per second, although it does vary with the temperature and it does vary as you go up higher because the air pressure is lower and that affects the sound speed as well.

But 340 metres per second is about right near the ground.

So sound waves through air are travelling over 300 metres every second, and that's more than the length of three football pitches every second.

So sound is very fast, even through air, which is a gas.

And because sound is so fast, it often seems as though we hear sounds from nearby objects as soon as we see the sound being made.

It seems as though it doesn't take any time at all for the sound to reach us.

It does.

It's just a very short time.

So have a look at the table.

If something happens one metre away and the sound is made, it only takes 0.

003 seconds, three thousandths of a second for the sound to reach us.

But at larger distances it can become noticeable that there's a delay.

For example, if a sound is made a hundred metres away from you, it will take 0.

3 seconds to reach you.

You might be able to see what's happening that's making that sound.

And you might notice that the sound just isn't quite in time with what you're seeing.

It's a little bit later.

It's even more obvious as you go further away.

If something happens 340 metres away, for example, it would take a second for the sound to reach you.

Now thunder is of course the sound of a lightning strike happening.

But when a lightning strike happens, there can be a delay between seeing the lightning and hearing the thunder.

You may have noticed that.

And that's because light travels through air about a million times faster than sound waves do.

So say a lightning strike happens a thousand metres away, a kilometre away, the light takes about 0.

000003 seconds to reach you.

That's about three millionths of a second.

But the sound takes around three seconds.

So that's a very noticeable delay.

There's nearly three seconds between the light reaching you from the lightning and the sound reaching you.

You can actually use that as a way of estimating how far away a storm is.

And a short question.

You always hear the sound of a lightning strike, the thunder, at the same time as the lightning strike happens, as you see the flash of light, the same time as the sound waves are created, or the same time as the sound waves arrive at your ears.

Correct answer is D.

You hear the sound when the sound waves arrive at your ears.

You don't hear the sound when the sound waves created.

That's when the lightning happens.

But the sound then takes time to reach you.

Now a longer question, true or false, thunder often arrives before you see the flash of light from a lightning strike.

So choose whether that's true or false and then I'd like you to write an explanation of why do you think that? How do you know? So press pause while you're doing that and press play when you're ready to check your answer.

Well, this is false.

Did you realise it's the wrong way round? So a way you could write the explanation is this: thunder is the sound of a lightning strike.

It often arrives after you see the flash of light from a lightning strike because light travels much, much faster through air than sound waves do.

Now let's look at how to measure the speed of sound in air.

You can do it using what could be called a one-way method.

We measure the distance between these people.

We could use a trundle wheel or a long tape measure, and on the left hand side we have someone with two blocks of wood and they're going to hit the blocks together and the person with the timer will start it when they see the two blocks hitting and they'll stop the timer when they hear the sound of the two blocks hitting.

And then you can calculate speed as distance over time.

Things you have to think about if you use this method.

The person who's doing the timing needs to be quite a long way away, over 150 metres really, so that the sound waves take about half a second or more to arrive.

For shorter distances, the person's reaction time will make the timing errors too large.

In fact, the timing errors might be so large that you really don't measure the speed of sound accurately at all.

And making the sound has to happen at the same time as starting the stopwatch.

So you can have a person with a flag next to the person with the blocks, and the person with a flag lowers the flag or moves the flag when the blocks hit.

And that's easier for the person with the stopwatch to see than the blocks hitting.

But all of this needs to happen at the same time.

The person with the flag needs to be in time with the person with the blocks and the person with the timer has to very quickly start their stopwatch when they see the flag.

And there will be timing errors in all of this.

So there's another method you could use called an echo method, and it's often more accurate.

It gives you a value of speed of sound that is closer to the true value.

So you need a large, flat, hard outdoor wall and sound waves will reflect from that.

They'll bounce back off the wall when they hit it.

So we still have someone with two blocks of wood and they hit those together.

The sound travels to the wall, bounces back off and comes back.

And the person with the timer presses start when the person hits the blocks together, and they're standing right next to them, so that should be easy to see, and they press stop when they hear the echo come back from the wall.

You have to be a little bit careful here.

The distance travelled by the sound is double the distance to the wall.

It's gone there and back.

And then you use speed as distance over time again.

And you can use a greater distance because you are getting double the distance.

So even if you're only standing say a hundred metres from the wall, you are actually letting the sound travel for 200 metres, a longer time and a longer distance.

And so any errors in the distance or the timing are a smaller proportion, a smaller percentage of the measurement itself, they're less significant.

Which of the following should reduce the error, the inaccuracy, in the value found for speed of sound? Using a longer distance to the wall, filming the stopwatch and watching in slow motion with sound on to measure the time the sound arrives, repeating measurements and calculating a mean, or making the sound as short and sharp as possible.

There might be more than one right answer here.

Press pause while you're thinking and press play when you're ready.

The correct answer is all of them.

All of these should reduce the error in the value found for the speed of sound.

Remember, error is the difference between your measured value and the true value.

Filming the stopwatch and watching in slow motion is helpful because you don't have to rely on someone standing there pressing the stopwatch at the right moment.

You can watch the video in slow motion, see what the stopwatch says when the sound returns.

Two questions.

I'll show you how to do one of them and then you can try the other one.

Izzy and Sam use an echo method to find the speed of sound in air.

They are 120 metres from a wall.

Sam measures the time between the sound and its echo as 0.

66 seconds.

Calculate the speed of sound.

We can pick out the two measured quantities here that we need to use.

We have the distance and the time there, write down the equation.

And now we have to be careful.

The distance travelled by the sound, it went there and back, to the wall and back.

So we double that distance and then we calculate the speed.

Now we can't really claim to know this speed to six significant figures to the nearest thousands of a metre per second because we only measured the distance to the nearest metre and we only measured the time to the nearest hundredth of a second.

So if we write our answer to three significant figures, that's reasonable because we had the distance to three significant figures, we had the time to two, so we get 360 metres per second, which is a little off, but actually not bad for a simple method like this.

Now one for you to try.

Aisha and Lucas stand 180 metres from a wall.

Aisha measures the time between a sound and its echo as 1.

09 seconds.

Calculate the speed of sound.

Press pause while you're writing out your answer and press play when you're ready to check it.

You pick out the values that you need from the question.

There's a distance and a time there.

Write out your equation.

Substitute in remembering that distance travelled by the sound wave is twice the distance to the wall.

And calculate.

And we'll write a sensible number of significant figures in our answer, three, because there were three significant figures in the measurements used.

And we get 330 metres per second.

So well done if you got that right.

And now it's your turn.

You're going to use an echo method to measure the speed of sound in air for a low pitch sound, which you'll get by hitting two blocks of wood together and a high pitch sound which you can use a whistle for.

So here's the equipment you need and a diagram of how you set this up.

So go and try this investigation, record your times and distances and then calculate the speed of sound for the high pitched and the low pitched sound.

And then come back and I'll show you some example results.

Well, I hope that went well.

And here are some example results.

They may not be the same as yours.

Did you think of repeating your measurements and taking a mean? 'Cause a mean of several measurements is likely to be more accurate than an individual measurement.

So here the distance of the wall was 120 metres.

So the distance travelled by the sound wave was twice that.

And you can see the speeds have been calculated.

You might ask yourself, does this mean that the speed of low pitch sound is different from the speed of high pitch sound? Well, probably not because remember there are various reasons why there might be errors in this experiment, particularly in the timings.

So these speeds are not significantly different.

And now the third part of this lesson, echo location.

Dolphins make short clicking sounds that echo off objects underwater.

So they hit objects and bounce off and come back to the dolphin.

The longer the echo takes to return to the dolphin, the further away the object is.

And this is called echo location.

And dolphins are actually able to work out how far away things are using this technique.

The same idea is used in sonar technology to find distances from objects underwater.

The distance can be calculated because the speed of sound through water is known.

Presumably dolphins instinctively understand the speed of sound in water.

So even though they won't be doing a formal calculation in their heads, they sense how far away things are from the return of the echoes.

So if you're using sonar and you want to work out a distance, you're going to use this same equation.

Speed is distance over time, but you're trying to calculate a distance, so you need to rearrange.

If you multiply both sides by time taken, you get this version of the equation.

Speed times time equals distance.

Remembering though that the object's distance away is actually half of the distance travelled by the sound.

The sound had to go to the object and back.

And another pair of questions I'll show you how to solve the first one.

A dolphin looking for a shoal of fish makes a click.

The echo returns after 0.

3 seconds.

The speed of sound in water is 1,500 metres per second.

How far away is a shoal of fish? Let's pick out the values we need from there.

We have a time and we have a speed and we know that speed is distance over time.

We could rearrange the equation first or we could substitute the values in and then rearrange.

Either way is okay.

Here's how we find the distance travelled.

And it turns out to be 450 metres, but remember that's the distance travelled by the sound.

It's gone to the object and back.

So the actual distance to the fish is half of that, 225 metres.

Now your turn.

A sonar system on a boat sends a sound wave vertically downwards.

An echo is detected after 0.

7 seconds.

The speed of sound in water is 1500 metres per second.

How deep is the water? And the echo returned because it hit the bottom.

So press pause while you write your solution and press play when you're finished.

So first, pick out the quantities we need to use.

Here's the equation.

We could substitute those in or we could rearrange the equation first if you want to.

Find the distance travelled, which is the speed times the time, and that's 1,050 metres.

And then we have to remember the sound went there and back.

So the actual depth of water is half of that, 525 metres.

Well done, if you got that answer.

And now a couple more questions for you to try.

Press pause while you read these and write to your answers and then press play when you're ready to check them.

So first of all, the explanation of how to find the depth of the water.

Here are some points that your answer could include.

Emitted sound waves travel downwards and reflect from the seabed.

The time taken for the echo to return is measured.

The distance the sound waves travel there and back is calculated using the speed of sound in water.

The depth of the water is equal to half of this distance.

So well done if you included those points and you might find it useful to read your answer out loud to yourself to check that it's as clear as it could be.

Question two, we find the useful measurements here.

We use this equation, we could rearrange it or we could substitute the values in straight away.

Distance travelled is the speed times the time.

So that's 270 metres, but that's the distance there and back.

So the distance to the object is actually half of that, which is 135 metres.

And now we're at the end of the lesson.

So I'll give you a summary.

The speed of sound is the distance travelled by a sound wave pulse every second, measured in metres per second.

The speed of sound waves depends on the properties of the medium.

It is about 340 metres per second in air, faster in liquids, and faster again in rigid solids.

Sound waves can reflect from hard surfaces, travel back, and be heard again as an echo.

The speed of sound waves or the distance they travel can be calculated using the equation speed equals distance over time.

In an experiment, measuring longer distances and times means any errors are a smaller proportion of the measurements.

So well done for working through this.

I hope you learned a few new things and I hope you enjoyed measuring the speed of sound.

I hope see you again in a future lesson.

Bye for now.