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Hello, my name's Mr Norris, and this lesson is all about methods for measuring the speed of sound in air, but also the speed of sound in solids, because of course, sound waves travel at different speeds in different materials.

So, this lesson will build on what you've studied previously about sound waves and how waves travel, and about wave frequency, wave length, and wave speed, with a focus on practical methods.

The outcome of this lesson is that hopefully by the end of it, you'll be able to describe how to measure the speed of sound in air, but also how to measure the speed of sound in a metal rod.

Some keywords we'll be focusing on this lesson are synchronisation, period, wavelength, accurate, and systematic error.

Now the next slide will give an example sentence of each word being used.

Once I've gone through each example sentence in turn, you might want to pause the video and have a re-read to try and get as prepared for the lesson as you can.

So, synchronisation is the process of ensuring two or more things happen at the same time.

The period of a wave is the time taken for one oscillation.

The wavelength of a wave is the distance a wave travels during one period.

A result is accurate if it's close to the true value.

And a systematic error affects all results by the same amount.

So repeatable and precise results, which cluster closely together, they're repeatable.

They still need to be checked for systematic errors before they can be considered as accurate.

So, each of those key terms will be explained as they come up in the lesson, but you might want to pause the video now just to read over and get as prepared as you can for the lesson.

This lesson is divided into three sections.

The first section focuses on different methods for how we can measure the speed of sound in air.

The second section focuses on a method for how we can measure the speed of sound in a metal rod.

And the third section introduces you to what kind of things can we consider when we want to find out how accurate or think about how accurate our results are.

So, let's get going with the first section on measuring the speed of sound in air.

So, we need to start by just going over what we mean by speed, which I'm sure you'll be familiar with, but let's just check.

So, speed is the distance travelled for every unit of time.

And you can calculate something's speed by doing the distance travelled divided by the time taken.

You can, of course, change the subject of that to distance travelled is given by the speed times the time taken.

And of course, we use the letter s for distance travelled and v for speed, and t for time taken.

But I think actually today we're gonna be, we'll only be calculating speed, so we'll keep it in the first rearrangement with speed equals the distance divided by the time.

And of course, the unit for speed can depend on the units for distance and time.

If you have distance in metres and time in seconds, then speed is the distance travelled every unit of time.

So, it's in metres every second, because metres is the unit of distance and seconds is the unit of time.

Whereas if distance is in kilometres and time is in hours, then speed would be in kilometres per hour, because kilometres is the unit for distance and hours is the unit used for time, and speed is distance travelled every unit of time.

So, distance travelled every hour in kilometres, every hour in that case.

And of course, the speed of a wave depends on the medium, the material that it's passing through.

The speed of a wave also depends on what kind of wave it is, because of course, sounds and light have very different speeds, different types of wave have different speeds even in the same material.

So, if the wave medium, the material is air, you can see the speed of sound is 340 metres every second whereas the speed of light is 300 million metres travelled every second.

And there are other speeds of sound and speeds of light given for different materials there.

Notice on the bottom row, the speed of sound in steel, so an alloy, a metal alloy, the speed of sound in steel is 5940 metres per second, whereas light doesn't pass through steel.

We say that light is opaque to steel.

So, there is no speed of light in steel, no speed of visible light in steel because visible light isn't transmitted through steel.

And it should be obvious from the table that light waves travel much faster than sound waves in the same wave medium, in the same material.

So, let's do some practise calculations.

I'm sure you'll have done these before.

So, this is just a check and reactivate those old memories.

So sound waves, which is the thunder from a lightning strike one kilometre away, so that's where the lightning strikes, one kilometre away from where we are, that takes about 2.

9 seconds to arrive at us, okay, because the sound waves and the light waves both have to travel that one kilometre from where the lightning strike happens to where we are hearing the thunder.

So, let's calculate the wave speed of the sound waves in metres per second.

There's the distance travelled, there's the time taken.

We've got to calculate the wave speed in metres per second.

So, we're gonna do, we're gonna need distance in metres.

We're gonna use speed is the distance divided by the time.

So, the speed is the distance is 1000 metres because we did that unit conversion from 1 kilometre is 1000 metres.

The time is 2.

9 seconds.

That's going to give us the speed in metres per second and then we do the division 1000 divided by 2.

9.

Gives you 344.

82.

And then the number goes on and on.

So, it's good practise to round it.

And I've rounded it to 340 metres per second.

That's two significant figures.

Okay, you have a go at this one following the same pattern.

If sound waves, which are the thunder from a lightning strike 0.

7 kilometres away, if they take about 2.

1 seconds to arrive, then what is the wave speed based on that data for those sound waves? So, follow exactly the same pattern.

Pause the video now and have a go at that.

Okay, let's see how you got on.

There's the distance, 0.

7 kilometres.

There's the time, 2.

1 seconds.

We've got to calculate the wave speed in metres per second.

So, we need to turn the distance to metres first.

So, 0.

7 kilometres, that's 700 metres.

You do that by timesing by 1000.

Then speed is distance divided by time.

The distance in metres is 700 metres.

The time is 2.

1 seconds.

That gives you a speed of 333.

3 recurring metres per second.

And if we round it to a sensible number of significant figures, that's gonna round to 330 metres per second based on those measurements.

So, both of those give pretty much the expected value for the speed of sound in air, which is 340 metres per second, and 330 metres per second is pretty close.

So, both of those are pretty accurate results for the speed of the sound waves, which is the thunder from the lightning strike.

So, let's now look at a first method that you could actually use to go out and measure the speed of sound.

So, this method involves three people and they're a set distance apart.

That distance has got to be measured using a long tape measure or using a trundle wheel.

This person is holding two blocks of wood, which they strike together to make a sound.

And this person is holding a timer.

It's their job to measure how long does it take for those sound waves to travel that distance.

That's so you can calculate the speed.

So, they need to know when the sound was made.

That's when they start the stopwatch.

So, that's why you've got a third person holding the flag because it's their job to lower the flag at the exact moment that the blocks of wood are struck together to make the sound.

So, that gives a visual signal that the sound has been made and the person on the right can start the timer, and then they stop the timer when the sound waves actually arrive at them and that's when they hear the sound.

So, start the timer when the sound is made, that's when the flag drops, and stop the timer when they hear the sound because that's when the sound has got to them, that's when the sound waves have travelled the distance and then they've measured the time taken.

Then you can get the speed of sound from the distance that you measured divided by the time taken as measured by the timer.

However, there are some things to consider.

The time taken for those sound waves to travel needs to be at least half a second because any smaller, and it will be too difficult to time.

Now, because the expected speed of sound in air, because air is the medium for the sound waves here, the sound waves are travelling from air across that distance, are travelling through air across that distance, the expected speed of sound in air is about 340 metres per second.

So how long, how far apart do the people need to be for the time taken for the sound waves to travel to be at least half a second? Well, we can calculate that.

Distance travelled is speed times time, or s is the distance travelled, v is the speed, t is the time taken.

The expected speed of sound in air is 340 metres per second.

We need it to take about half a second.

So how far apart, how far back does the person with the timer need to be? Well, that's the distance the waves would travel in that minimum time of half a second, which is 170 metres.

So, you need a really significant distance, 170 metres, for this method to give a measurable time taken for the sound waves to travel.

If you're any closer than that, then the sound waves will take less than half a second to travel, and it could be too difficult to measure.

So, that's a consideration.

Have you got that significant distance available to you to try this method? So, we'll now look at a second method for measuring the speed of sound in air.

This would be called the echo method.

You can see you need one less person and you're going to create an echo of the sound made from the two blocks of wood.

An echo is when sound reflects from a surface and comes back to you and you hear the sound again.

So, the person with the timer can start the stopwatch when they hear the sound being made, and then the sound waves will travel to the wall, and then travel back, and you'll hear the sound again, and that's when the person with the stopwatch stops the timer.

So again you can measure the, you need to measure the distance to the wall using a long tape measure or trundle wheel, but then the distance travelled by the waves is double the distance to the wall because the waves are going there and back, so you need to do double the distance.

And then, of course, you're measuring the time taken between the sound being made, that's when sounds start to travel, and the sound being heard again, that's when the sounds have travelled back to you.

They've gone that distance, double the distance to the wall, or to the wall and back.

So, the speed of the waves is then going to be given by the distance travelled divided by the time taken.

So, let's do some practise calculations using the echo method.

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

They're 120 metres from a wall.

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

66 seconds.

Calculate the speed of sound.

Well, the distance to the wall is 120 metres.

The time taken for the echo to return is then 0.

66 seconds.

Speed is distance divided by time, but of course, the distance to the wall needs to be doubled to get the distance travelled by the waves.

So, the distance travelled by the waves is 2 times 120 metres, and then you divide by the time taken for the sound to travel that distance, type it in your calculator, you get 363.

636 metres per second, so a sensible number for significant figures, about 360 metres per second second for Izzy and Sam.

So later that lesson, Aisha and Lucas stand 180 metres from the wall, so a bit further back.

Aisha measures the time between a sound and the echo is 1.

09 seconds.

Can you calculate the speed of sound that Aisha and Lucas would get from that data? Use exactly the same steps as on the left-hand side, but you do this one, so pause the video now.

Okay, let's see how you got on.

So, there's the distance, there's the time.

Speed is distance travelled by time taken, but the distance travelled by the waves is the distance there and back.

So that's two lots of 180 metres divided by the time taken for the waves to travel.

That gives you about 330 metres per second.

So, a sensible number of significant figures, about 330 metres per second.

So, again, both of these pairs of students have got a reasonably accurate value for the speed of sound.

We're expecting about 340 metres per second, but Aisha and Lucas are a bit closer.

That's probably because they stood further from the wall.

And we'll look at why that improved their accuracy, made them a bit closer to the expected value of 340 metres per second later in the lesson.

Right, I'm just going to talk through a few possible sources of error in this experiment.

First one is if there's wind that would cause additional air movement and it might make the sound waves take slightly longer or slightly shorter than they should have done to return from the wall.

There might be synchronisation errors such as not starting the timer at the exact moment the sound is made.

You could try and combat that by using a countdown.

So, the two people say three, two, one start.

And that on start is when they make the sound with the bricks, and they should start the timer at the same time.

But that's not gonna be perfect.

There could be errors due to not getting the synchronisation exactly right.

Errors in stopping the timer when the sound waves arrive, because that is a pure like reaction.

Oh, heard the sound, stop the timer.

That could be, that won't happen, perhaps at exactly the right moment.

There could be errors in stopping the timer due to reaction time.

Right, I mentioned about using a greater distance earlier and why that might benefit, make results more accurate.

Now using a greater distance, it's not gonna reduce the actual errors, but it will make them a smaller proportion of a longer distance measurement and a longer time taken because you've used a longer distance.

So, there's always a benefit to using longer measurements where you can.

It doesn't actually reduce the size of the errors, but it does make them a smaller proportion of what you're measuring.

So, a lower percentage.

So, it kind of affects the results by a smaller proportion.

And of course, if you repeated results and took an average, a mean, that reduces the effect of the errors.

It doesn't reduce the errors, but it reduces their effect on a final result.

Okay, so let's check those ideas about reducing errors.

Draw a line to link each experimental step to the correct reason why it would improve the measurement.

So I think you should pause the video now, read through each experimental step on the left carefully, and link it to the correct reason it would improve the measurement, which are the three on the right.

I'll pause the video now and have a go at that.

Okay, let's see how you got on.

So, the first experimental step is using as great a distance as possible for the echo.

That makes timing errors a smaller proportion of a longer measurement.

The measure's longer, the timing, the measurement's longer, the timing errors will be the same.

So, those same timing errors will be a smaller proportion of a longer measurement.

The second experimental step was taking repeat measurements and calculating a mean.

That reduces the effect of random errors on the final result.

It doesn't reduce the actual errors in the measurements, but it reduces the effect of those errors on the final result that you give.

And that leaves using a countdown to synchronise starting the timer that actually should reduce the size of timing errors if the synchronisation is actually better.

So, very well done if you've got all of those three correct matchings.

Okay, time for you to do a task now.

I would like you to describe that method for measuring the speed of sound in air using an echo.

So, aiming for four to five succinct steps in a logical order, just describe that method for measuring the speed of sound in air using the echo.

Now, in a moment, I'm going to put up a writing frame, which you could use to help you.

But I'm sure lots of you will be really confident to just give that a go yourself without using the writing frame.

So if you'd like to start now, pause the video now and have a go.

If you'd like to use the writing frame, keep watching.

Okay, so if needed, you could use this writing framework to help you write this method for measuring the speed of sound in air using an echo.

So, pause the video now and have a go at that task.

Okay, so well done for the effort you've been into writing, having a go at writing that method for measuring the speed of sound in air using an echo method.

I'm gonna show you a example answer now.

So yours won't be exactly like this, but you should make sure that yours covers as many of these points as possible, and is also in a similar order as this as possible.

So, it's a well-sequenced method.

So, let's have a look.

So, step one would be use a trundle wheel.

I went for a trundle wheel.

You could have said a long tape measure to measure a distance of at least 100 metres from a vertical wall.

It has to be a very long tape measure if you went for that option.

Step two, two people should stand at that distance from the wall.

Step three, one person should make a sudden noise by banging two blocks of wood together, but any sudden noise would do.

Step four, the second person should start a timer when the noise is made.

So use a countdown to synchronise that, and stop the timer when the echo is heard.

And then step five, the speed of sound can then be calculated by speed is distance travelled divided by time taken.

But the distance travelled is double the distance to the wall.

So, that contains all of the steps.

So, try and make sure that yours has got as much detail as that, even if it's not identical.

Okay, well done.

We're now onto the second key method that we can use for measuring the speed of sound.

And this one is about measuring the speed of sound in a different medium, a different material.

It's measuring the speed of sound in a metal rod.

So, let's have a look at that.

Now, this method is gonna use the other equation for calculating the speed of waves.

The wave equation, which of course is wave speed is frequency times wavelength.

V for the wave speed, F for the frequency, and the Greek lambda symbol means wavelength.

And of course, the units of wave speed can depend on the units used for wavelength.

So frequency is always in hertz, but if wavelength is in metres, then speed will be in metres per second.

If wavelength is in centimetres, then wave speed will be in centimetres per second.

Let's do some practise at using the wave equation to calculate the speed of sound.

So in aluminium, sound waves of frequency 4,600 hertz have a wavelength of 140 centimetres.

What is the speed of sound in aluminium in metres per second? So the first thing is, it's gonna be, well, there's the frequency.

Let's circle the data from the question.

There's the frequency.

There's the wavelength of the waves.

But we need to get the speed in metres per second.

The wavelength is in centimetres.

We need to put it in metres.

So, we need to divide by 100 to go from centimetres to metres.

That's 100 centimetres in a metre.

Wave speed is then frequency times wavelength.

Frequency is 4,600.

The wavelength is 1.

4 metres.

And that'll give the wave speed in metres per second.

You get 6,440 metres per second.

So, the speed of sound in a metal is much faster than the speed of sound in air, which was only 340 metres per second.

This is in the thousands of metres per second.

Right, you have a go at this one.

This is for copper.

Sound waves of frequency 2600 hertz have a wavelength of 190 centimetres.

What is the speed of sound in copper in metres per second? So, follow exactly the same steps as I showed you to make sure you can calculate wave speed using the wave equation.

Pause the video now and have a go at that question, please.

Okay, let's see how you got on.

Well, here are the numbers.

There's frequency, there's wavelength, but we need speed in metres per second, so we need to convert centimetres to metres.

So, that's divide by 100.

Then wave speed is going to be the frequency times the wavelength.

The frequency was 2,600.

The wavelength is 1.

90 metres.

That gives a wave speed of 4,940 metres per second for the speed of sound in copper.

So, speed of sound in metals generally looks like it's gonna be in the thousands of metres per second compared to the speed of sound in air, which is 340 metres per second.

What we need to do now then is we're going to look at a demonstration of this method for how the speed of sound in a metal rod can be measured.

It's going to involve getting the frequency of the sound waves, and getting the wavelength of the sound waves in the metal rod.

During this demonstration, you should be ready to note down the following measurements.

The demonstration will show you the frequency of sound produced when the sound travels through a metal rod.

And we'll also need to note down the length of the metal rod.

So, be ready to write those down as you're watching the video.

<v Tutor>In this demonstration,</v> we're going to measure the speed of sound along this aluminium rod.

And to do so, we're going to use the wave equation, speed is frequency times wavelength.

So in order to do that, we need to know the wavelength.

We're going to measure that by first of all, measuring the length of the rod, which is 60 centimetres.

And we're going to match that to the sort of wave that we have travelling through the rod.

More of that later on.

As well as the wavelength, we need to measure the frequency.

And for this, we've got an app on this iPhone.

And as you can see here, the background frequency at the moment is 31 hertz.

That's the low hum of the room and all the machinery and stuff around.

So what we're going to do, we're gonna hit the rod against the metal plate that you can see there.

And each time, record the frequency of the sound wave in the metal rod.

Now that sound wave starts at the right-hand end of the rod.

It moves along the rod to my hand, reflects off my hand and goes back along the rod to the other end.

And what we get is a shape of wave that looks like this, which is a representation of the backwards and forwards movement of the particles in the longitudinal sound wave.

Now, as you can see, this is equal to half of one wavelength.

So the rod, if you remember, was 60 centimetres long.

And this wavelength then is going to be twice that.

The whole wavelength is going to be twice that, which would be one metre 20.

So, let's hear what that sounds like.

And whilst we're doing so, double check the frequency measurements that we saw then.

(rod clanking) (rod clanking) So we have a wavelength of 4.

20 metres, and we have a frequency of 4,156 hertz.

And from those two measurements, we can use the wave equation to calculate the speed of the sound wave moving through this aluminium rod.

Right, I just want to recap what happened in that demonstration.

So, the strike of the rod on that metal plate created a compression that travelled down the rod this way.

And then the compression reflects from the other end of the rod, the left-hand end.

But when the compression reaches that right-hand end again that I've labelled A, that's one vibration of the rod.

So there, back, the rod's vibrated once.

And then those compressions continue up and down the rod.

And that's the rod vibrating.

It happens very quickly.

The time between vibrations is the period of the sound wave.

And of course, the wavelength is the distance travelled by waves during one period.

So the distance travelled by waves during one vibration, during one period, that's there and back, twice the length of the rod.

That's why wavelength is twice the length of the rod.

So quick check, which gives the wavelength of the sound waves in the rod in this experiment? That's right, it's b double the length of the rod gives the wavelength of the sound waves in this experiment.

Right, your task then is let's calculate the speed of sound in the metal rod from the measurements that you made.

So the example measurements from the video, the frequency of sound produced was 4156 hertz, and the length of the metal rod was 0.

60 metres.

And we've got to use the wave equation, wave speed is frequency times wavelength, to get the speed of sound using everything we've just talked about.

So, pause the video now and get a value by doing calculations to get the speed of sound in that metal rod.

Okay, let's see how you got on.

So, first thing is the wavelength is double the length of the rod in this experiment.

So the length of the rod was 0.

60 centimetres, so 0.

60 metres.

Double that length is 1.

20 metres, but the wavelength of the sound waves in the rod.

And then wave speed is frequency times wavelength.

Frequency is 4,156 from the demonstration.

Wavelength, we just calculated double the length of the rod, 1.

20 metres.

That gives a wave speed based on that data for the sound waves in the rod of 4,987.

2 metres per second, but we're going to round it to a sensible number of significant figures.

So, about 4,990 metres per second is our result for the speed of sound in that aluminium rod.

That is the same result that you should have got if you also used the data from the video demonstration.

So, well done.

We've covered the two methods for measuring the speed of sound.

First method was in air.

Second method was in a metal rod.

Let's just have a think about what it means to consider the accuracy of our results.

So, a result is accurate if it's close to the true value for whatever's being measured.

Like, what value does it actually have, and was our result the same as that or very close to that? Then it's accurate.

So, this kind of dartboard graphic represents accuracy.

The black dots are measurements and the true value is the bullseye, like what it should be.

So these measurements, those black dots are quite accurate.

They're quite close to the bullseye, the true value.

These results are very accurate because they're really close to the true value where we want them to be.

These results are clustered closely together, but the accuracy is not good, so they're far away from the true value of the bullseye where we want them to be.

And these results are very poor accuracy.

So that's a representation of what we mean by accuracy, being close to the true value.

Now, if results cluster closely together, there's a word for that as well.

The word is precise or repeatable, if repeat results cluster closely together.

Now, if that happens, You can call the results precise, but that's a different thing to being accurate.

So, those first results on that first dartboard graphic are quite precise.

They're quite close together.

These results are very precise because they cluster very closely together.

On the third dartboard, those results are also very precise, even though the accuracy is poor, but because they're all very similar, then they're still precise even though the accuracy is poor.

And on the last dartboard, the precision is poor as well as the accuracy being poor.

So, you can see that accuracy is a different thing to precision or repeatability.

Just because results are precise or repeatable, it doesn't make them accurate.

You can have results with poor accuracy, which are actually very precise.

Okay, quick check.

Which represents the most accurate set of measurements? Is it represented by graphic A, graphic B, or graphic C? Which represents the most accurate set of measurements? The measurements which kind of imply they're closest to what they should have been, the true value.

The answer is B.

These are most accurate because they're closest to the bullseye, closest to where they should be.

If you said answer A, they are precise but not as accurate as the set of results in B.

B is closest to the bullseye, the results are closest to what they should be, the true value A, the results are close together, but they're not close to what they should be, or not as close as the results in B.

So when scientists try to measure something, they do lots of repeats.

Because if results are repeatable and precise, they're likely to be accurate unless there's a systematic error that affects all results by the same amount.

So, those are accurate results.

They're repeatable and precise as well, so they cluster closely together.

And they're accurate because they're close to where they should be, close to the true value.

These results are repeatable and precise, but they're inaccurate because something, a systematic error, whatever that is, or whatever it happens to be, something has made all of those results off from where they should be, but by about the same amount every time.

So that's systematic, about the same every time.

So once you've got a repeatable and precise result, scientists would publish it.

And then other scientists can try to reproduce the experiment to check for those kind of errors, the systematic errors.

And that's part of this process called peer review, where scientists review other scientists' work who are all experts in the same area.

So, that first scientist's result gets added to by another scientist's result and another scientist's set of results.

They're all trying to reproduce the same experiments, check they all get the same results.

And if everyone does get very similar results, then the result is reproducible.

Other scientists can reproduce it.

And that is then what can lead to a result becoming accepted as accurate, as in representative of the true value.

Okay, so all of that data, all of that evidence leads to the final result or a final value being accepted as this is the value, we've measured it, everyone gets the same, it's reproducible, so this is what we think it really actually is, it's accurate.

And then accepted accurate results then get published in books, and on websites of official scientific organisations.

So, let's do a little check now of what we just talked about, about accuracy.

Which of the below, there might be more than one, is enough to allow a result to be considered accurate? So A, if a result is repeatable and precise, does that mean it's considered accurate? B, if it's repeatable, precise, and reproducible by others who've also checked for systematic errors, does that then mean it can be considered accurate? And then option C, if a result is very close to previous published measurements of the same thing, does that suggest that it's accurate as well? So, which of those actually can allow a result to be considered accurate? There might be more than one.

Make decisions about which now.

Might need to pause the video.

Okay, let's review these.

So, A is not enough for a result to be considered accurate because there may be a systematic error.

So in B, a repeatable and precise result, if it's reproduced by others and they've also checked for systematic errors and everybody gets the same, then everyone's convinced, yeah, that there isn't a systematic error in this result.

It's right.

Then that is enough for a result to be considered accurate.

And actually C is enough for a result to be considered accurate as well.

So if it's very close to previous measurements, then that suggests it's reproducible and there aren't systematic errors because everybody's checked and everyone agrees.

So, it's probably right.

It's probably accurate.

So, well done if you got both of those.

Okay, so now let's apply what we've learned about accuracy to our own result for the speed of sound in the metal rod.

So, I want you to state whether you think your result, our result, is accurate or not, and explain why you think this.

So, here's our result.

It was 4,990 metres per second, and we had an aluminium rod.

So have a look in the table, and then I'd like you to write just a couple of bullet points.

Do you think our result is accurate or not? And explain why you think this.

Pause the video now to have a go at that task.

It should be quite quick to do.

So, here is an example answer based on the example result.

You should compare yours to this and check that it's kind of along the same lines.

I do not think the example result is accurate.

This is because it is significantly different to a published accepted value for the speed of sound in aluminium.

So you can see that in the table, the published accepted value was 6420 and ours was significantly different to that, 4990.

So, not close to the true value.

So, probably not accurate.

That was it.

Simple as that.

Well done if you got something along those lines.

So, well done, you've made it to the end of this lesson on measuring the speed of sounds in air and solids.

Here's a quick summary.

The speed of sounds in air is about 340 metres per second, and it can be measured using an echo method.

You measure the distance to a wall, and the time taken between a sound being made, and the echo being heard.

And then speed is double the distance to the wall because the sound's got to go there and back, divided by the time taken.

And the speed of sound in a metal rod can also be measured by hitting the rod and measuring the frequency of the sound, and the length of the rod.

The wavelength of the waves is double the length, and the wave speed is the frequency times the wavelength.

And finally a result is accurate if it's close to the true or accepted value.