Lesson video

Hello, my name is Mr. Fairhurst and this lesson is about the oscilloscope.

In this lesson, we're going to look at the oscilloscope and find out how to interpret and explain the sound waves that they can show on the screen.

When we're looking at how an oscilloscope works, we need to use some key terms. The word oscilloscope is quite an interesting one, but we'll see where that comes from in a little while.

The oscilloscope shows displacement-time graphs of the longitudinal waves.

And to understand what's going on, we need to understand the period and the frequency of a wave and what that tells us about the oscillations that's involved, and we need to think about the loudness of the sound wave and how that is shown on an oscilloscope graph.

So, this lesson's divided into two halves.

Before we think about the oscilloscope graphs in the second half, we're gonna have to work out, or really have a think about how the oscilloscope works and what's going on.

And that's going to help us to interpret these graphs that it's showing later on.

So let's make a quick start and have a look at the oscilloscope.

When it's connected to a microphone, an oscilloscope can show us a graph of what the sound looks like.

On this particular oscilloscope, it's showing the sound of a pure note, a single note.

Most sounds that we have are much more complex than this, as we'll see later on.

But to start with, we're gonna think about what makes a single note like this one.

Now, the word oscilloscope is an interesting one.

Why is it called in oscilloscope? Well, a wave is caused by vibration, which is also known as an oscillation.

An oscillation is anything that goes backwards and forwards and vibrates.

So the first part of the word oscilloscope comes from that word, oscillation.

The second part comes from scope.

And scope means to observe.

It's a Greek word which means to observe, and it's used in things like a microscope that's used to observe very small things on a micro scale, or a telescope that's used to observe things a long way away.

And if you put those two words together, oscillation and scope, it makes up the word oscilloscope, which means a device that allows us to observe oscillations or waves.

It allows us to see waves and we can make a graph of the waves.

So, we're going to have a look now at how the oscilloscope works.

In a moment, I'm going to show you a video that illustrates the first point.

But what you can see here at the bottom of the screen is a microphone connected to an oscilloscope.

And when we make a sound, we make a little thin diaphragm, it's like a piece of paper almost, that sort of thickness, in the end of the microphone vibrate up and down.

Modern microphones use a different technique, but this is okay for us at the moment.

And the sound wave will make that diaphragm move backwards and forwards just like the loudspeaker cone moves backwards and forwards to make a sound in the first place.

And that diaphragm is connected to an electric circuit, and it creates a potential difference or a voltage in an electrical circuit that vibrates in time to the sound wave.

And as that diaphragm vibrates in and out, the potential difference, the voltage, goes from positive to negative, and that makes the dot on the screen move up and down.

Now, we're gonna watch a short video to see what we mean by that.

So, on this video, we're gonna have a look to see how the oscilloscope will show the display the sound wave that we input.

Now, the oscilloscope we've got is on the right-hand side, and as you can see at the moment, it's just got a tiny dot in the centre, and this is because it's got no signal coming into it whatsoever.

Now, on the left of the screen, we've got a signal generator.

And the job of the signal generator is to create the same signal for the oscilloscope that the oscilloscope would receive if it was getting a sound wave through a microphone.

So, what I'm going to do now is I'm going to play the video and show you what happens when we start putting a signal through the oscilloscope.

So first of all, we're gonna turn on the signal generator and we're going to put a signal through of about one hertz.

That's one vibration per second.

And as you can see on the oscilloscope screen, that dot has started to move up and down once a second.

If we turn the frequency up to two hertz, two vibrations a second, the dot on the screen is going up and down twice every second.

And then if we make it go to three times per second, the dot will increase its speed accordingly.

And the faster we make that dot go, the faster the line on the oscilloscope goes up and down.

And as we can see now, the dot is starting to drag a little bit, and we can see that line that's following and it's becoming longer and longer as the frequency's increased.

And if we increase the frequency right up to 55 times per second, then what we can see is we get a steady line.

The dot is going up and down so quickly that our eyes just see it as a single straight line.

And what we've seen here is that, when we input a sound wave into an oscilloscope, the effect of the sound wave makes the dot on the screen go up and down.

What it does not do is make the dot go from left to right.

That's to do with a different function that we have to add later.

Okay, let's check what you've understood about that video.

Have a really careful read through this question and have a go at it, and just pause the video whilst you do so.

Okay, what do you think? Which oscilloscope shows the dot when a sound wave moves part of the microphone in the same way but backwards? The top picture shows it moving forwards.

Which diagram shows it moving backwards? And the correct answer is C because when the diaphragm moves forwards it goes up above the line and when it goes backwards it goes down below the line.

Remember, we're talking about longitudinal waves and forwards motion goes up, backwards motion goes down on the graph.

And the oscilloscope graph is just the same as a normal graph for longitudinal waves.

But how do we get that, sort of, length to the wave? What we've got on the oscilloscope is something called a time-base.

This is a control on the oscilloscope that makes a dot move from left to right at a steady speed.

And it goes left to right, then starts again the left and moves to the right again.

And changing the time-base will change the time that it takes for that dot to move from the left to the right.

So in other words, it's adding a time scale to your graph.

Again, we're going to watch a video just to see how this works.

Here we can see a dot moving across the screen of an oscilloscope.

And now we've turned off what we call the time-base, which is this control up here which controls the speed at which that dot moves across the screen.

One thing you will notice is that the oscilloscope is not connected to anything else, so the control of the speed of that dot moving across the screen is controlled entirely by the oscilloscope.

Now, if we turn that back on, that dot is making one pass of the screen every second.

And we can make it go faster, we can slow it down.

And when we speed it up again, what you'll notice is that it starts to form a straight line because our eye is not able to see that dot move completely on its own.

And it's reached the stage now where we've got a complete straight line.

The dot is actually moving very, very quickly from left to right across the screen, starting back at the left-hand side and moving quickly across the screen again.

And there we are.

We've got the dot back, we've turned the time-base off and we've got that dot just waiting there with no signal.

Okay, just to check you've understood what was said on the video about the time-base.

On this oscilloscope, the time base is set for 0.

1 seconds per division.

How long does it take the dot to move across the whole screen? If you want to pause the video whilst you think about this, please do so.

Okay, what do you think? If it's 0.

1 second per division.

If you look horizontally across the screen, there's eight divisions, so that gives us a time of 0.

8 seconds in total.

So the correct answer is C.

If you've got that one, well done.

Now, what we're seeing on the oscilloscope is a displacement-time graph.

What it's showing is how the displacement of a sound wave changes over time as it passes a single point, which is the point at the microphone.

So what essentially is happening is the vertical scale is showing the displacement of the sound wave and the time-base, the horizontal scale, is showing the time along there.

So we're drawing here on the oscilloscope a displacement-time graph.

We can see how the two mechanisms that we've seen about how the oscilloscope works so far are combined to make that wave and to make that displacement-time graph.

What we've got here is an oscilloscope which is displaying the signal that is been fed into it from the signal generator, which is effectively a sound wave, or the same as a sound wave, with a frequency of 1.

00 hertz, in other words, one vibration per second.

And we see that as a dot going up and down on the display of the oscilloscope.

It's going up and down and not across because the time-base of the oscilloscope is turned off.

So let's turn the time-base of the oscilloscope on.

And again, we've got it set so the dot is going across the screen once every second.

What happens, then, if we add in the original signal from the signal generator? What's gonna happen to that dot? And what happens, of course, is that, as well as moving across with the time-base, it also is moved up and down.

Speeding it up, we get that continuous line that we're familiar with from other videos in this lesson.

And if we increase the frequency on the signal generator, the vibrations going up and down increase, but the dot is travelling across the screen in the same length of time, so we end up with more waves on the screen.

Okay, so it's your turn now.

What you can see in front of you are a set of statements that describe how the oscilloscope works.

The first one's given to you, but all of the others are jumbled up, and your task is to sort those into the right order that clearly describe how the oscilloscope works.

Just pause the video whilst you do that.

Okay, how did you get on? Here's the first statement that you were given.

Inside a microphone is a thin diaphragm that can move.

A sound wave can move it backwards or forwards.

And a positive potential difference is made when it moves one way and a negative potential difference is made when it moves the other way, and this makes the dot on the oscilloscope move up and down.

So in other words, the sound wave is pushing a very thin diaphragm up and down at the top of the microphone.

That's creating a voltage which is moving the dot on the screen up and down.

But that doesn't move it across the screen, does it? So, the next statement is number six.

The time-base changes how quickly the dot moves across the screen.

When it's off, the sound wave can only move the dot up or down, but when it's on, the dot moves across the screen at a steady speed.

And changing the time-base changes the scale on the time axis.

So in other words, we can change the scale on the time axis, so as the dot is moving up and down, we can change the time-base so we can see clearly how quickly it's moving up and down, and we end up with a displacement-time graph.

Now that you've found out how an oscilloscope works, what we need to do next is to have a look at the graphs that the oscilloscope shows and see if we can interpret what they tell us about sound waves.

One thing we know about a sound wave is that the bigger the vibration, the louder the sound is going to be, and that in turn gives it a bigger amplitude.

And what that means is that, at the microphone, a louder sound will cause the diaphragm to vibrate in and out more, a greater distance, and in turn, that will produce a bigger potential difference, or a voltage, inside the oscilloscope.

And what that does is that moves the dot up and down on the screen a greater distance.

So if we compare these two graphs, the one on the right represents a louder sound because it's going up and down with a bigger amplitude.

We can see that the dot on the screen's being displaced up and down, further from its mean position.

Just to check you've got that, have a look at these three graphs at the bottom, A, B and C, and compare them to the one at the top.

You're gonna have to look carefully to get this right, but which one or which ones of those on the bottom are showing a louder sound than the one at the top? Just pause the video whilst you have a look.

Okay, well, the obvious correct answer is C because that wave is higher than the one on the top, and it also goes down lower, so its amplitude is definitely bigger.

If we look at A, A goes higher, but at the top, the crests of the wave are higher, but the troughs at the bottom are also higher up on the screen.

And if you compare it closely, it's got actually the same amplitude as the top wave.

So that's not the right answer.

If we look at B, that wave from top to bottom is actually a little bit bigger than the one at the top.

So, although the mean position isn't on the horizontal axis anymore, it's a little bit below, that still has a higher amplitude.

So B also is a louder sound.

What about higher pitched sounds that are a little bit more squeaky? The higher the pitched sound, the higher its frequency.

(high voice) The higher pitch the sound is, the higher its frequency (low voice) and the lower pitch the sound is, the lower its frequency.

This means that higher pitched sounds cause the diaphragm of a microphone to vibrate more times each second.

So that's one sound we've just represented by an oscilloscope.

One with a higher frequency will produce more waves in the same amount of time.

Remember, the time-base takes the dot across the screen in the same amount of time unless we change it.

So, each of these has got the same time-base.

The one on the right has got a higher pitched sound with more waves in the same amount of time.

That's just what I've just said.

Now, Izzy, when you look at her voice, so if you look at your voice, you get a much more complicated sound than the ones we've seen so far.

And the reason that that's the case is because Izzy doesn't just talk in one pure note.

She has lots of frequencies in her voice.

And the voice goes up and down in loudness as well.

It gets louder or quieter depending on what she's saying, and that allows us to make out the words that she's saying and for her to speak to us and for us to hear her.

Okay, it's over to you again.

What does that line, XY, on our displacement-time graph on the oscilloscope represent? Just pause the video whilst you read the answers and make your choice.

Okay, so if you remember, this is a displacement-time graph, so it's not the wave length of the sound because the horizontal axis is a time and not a distance, so you can't have a wavelength.

It's actually the time for the microphone to vibrate once.

So it goes in and out, and we get that one full vibration which matches the vibrations of the sound wave.

What about this question, then? What's the best reason why an oscilloscope shows more waves when the frequency of a sound is higher? Again, pause the video whilst you make your choice.

And the correct answer to this one is because the period of each wave is shorter.

It takes less time for one complete wave to form, so more waves can fit on in the time it takes the time-base to move the dot from one side of the screen to the other side of the screen.

Well done if you've got those two questions right.

Okay, so we've covered what we need to cover.

It's your turn to have a have a go at some questions to practise.

So just pause the video whilst you have a go at these questions.

How did you get on? Let's just look at some answers.

So the first part, you were asked to draw a graph with a louder sound and with the same frequency.

So the frequency will change the number of waves on the screen, so we need the same number of waves on the screen.

But we also need a sound that's louder, so we need it to have a bigger amplitude.

So that would look something like this.

So, the peaks and the troughs of the wave need to be above those two, sort of, lines closest to the horizontal axis.

In part B, you were asked to draw a graph with a louder sound and also a higher frequency.

So this time we need to draw the the sound louder, like we did in the last one, so it's got a bigger amplitude, but with a higher frequency, we need to put more waves on the same graph.

So that's an example of a wave that's louder because it's taller with a higher amplitude and it's got more waves in the same amount of time, so it's got a higher frequency.

And for part C, we want to quieter sound and one with a lower frequency.

So a quieter sound would have a smaller amplitude and a lower frequency would fit fewer waves in the same amount of time.

So something like this would be a good answer.

This has got two waves rather than three and it's got a smaller amplitude.

Okay, just one more question.

Could you explain why the oscilloscope of Izzy's voice shows such a complex voice? Sorry, such a complex shape.

Just pause the video whilst you write down why it does that.

Okay, what have you written? Why is that a complex shape? And if you remember what we said before, when somebody's talking, their voice doesn't just stay at one volume, it goes up and down in volume, so the graph's going to go up and down, and also they're going to use lots of different frequencies of sounds to form the words.

So when Izzy's speaking, her voice is using lots of different frequencies and the loudness of her voice is changing, which allows us to understand what she's saying.

Well, that's taken us to the end of this session on this lesson on oscilloscopes.

Let's just summarise what we've learned.

An oscilloscope makes a displacement-time graph of a sound wave.

Okay, there's the axis on our oscilloscope.

The higher the frequency, the more waves fit on the screen.

The louder the sound, the greater the height or the amplitude of the wave.

And many sounds contain lots of different frequencies and varying loudness to give us all the sounds, the voices, the music, the bird song and so on that we can listen to.

So the oscilloscope helps us to visualise longitudinal waves like sound waves much more clearly.

Hopefully, you found that lesson useful and worthwhile.