Lesson video

Hello, my name's Dr.

George, and this lesson is called Applications of motor and generator effects.

And is part of the unit Electromagnetism.

The outcome for this lesson is, I can explain how a loudspeaker and a microphone work.

And here are the key words.

I'm not going to read out these definitions now.

I'll introduce them as we go along, but this slide is here in case you want to come back and check the meanings anytime.

There are three parts to the lesson, electronic music, loudspeaker and microphone.

Let's get started.

You can use an oscilloscope with a microphone to make an electronic representation of a sound wave.

So the oscilloscope here has a screen, the microphone plugs into it, and if you speak or make noise into the microphone, you can see the sound wave represented on the screen.

The screen itself usually doesn't show what's on the axis, but what we have shown vertically is a potential difference, typically in volts, and horizontally, we have the time typically shown in milliseconds.

Now the louder a sound, the greater is the amplitude of the sound wave.

A way a sound affects a microphone is to make a small cone, called a diaphragm, near the front of the microphone, move.

So when a sound wave arrives, the air particles are vibrating and they hit the diaphragm and they make it vibrate.

And a louder sound hits harder.

So it makes the diaphragm vibrate in and out further, it travels a further distance as it vibrates, and that produces a larger potential difference.

And so the oscilloscope detects that and displays that as a wave with a larger amplitude, as you can see on the right.

So that would represent a louder sound.

Now, which of the oscilloscope traces represents the loudest sound? With these short questions, I'll wait five seconds and if you need to pause for longer, then do and press play when you're ready.

The correct answer is C.

It's showing the largest amplitude.

So that's volume.

What about pitch of a sound? How high or low it is? The higher the pitch, the greater the frequency.

And so a higher pitched sound arriving at a microphone will make its diaphragm vibrate with higher frequency, so more times per second.

So on the right we have the oscilloscope trace.

What you would see if a higher pitched sound reaches the microphone.

The higher the frequency, the more complete vibrations there are in the same amount of time.

And so the shorter the time period, that's the time for one complete vibration.

Now Izzy is singing into the microphone.

And if you look at the oscilloscope screen, the trace of the sound doesn't look like the other sound waves that we've been seeing.

As you can see, it's more complicated, it's not a simple wave, and that's because Izzy's voice isn't a single pure note.

The pitch of her voice changes as she sings and so does the loudness.

So we get this more complicated trace.

And even if you do play a single note on a musical instrument, for example, you usually won't get a very clean looking wave on the screen because the sound is actually a mixture of some different frequencies, and that's what makes different musical instruments sound different even when they're playing the same note.

Now which of the statements correctly explains why an oscilloscope trace shows more waves when the pitch of a sound is higher? Take as long as you need, press pause, and press play when you're ready.

The correct answer is B.

Because the time period of each wave is shorter.

When the pitch is higher, the frequency is higher and each vibration takes less time.

Now I'd like you to make a copy of the oscilloscope trace of the sound wave shown here.

It's best if you use squared paper for this, and then sketch a trace that shows a louder sound with the same pitch, and then a louder sound with a higher pitch, and then a third one with a quieter sound with a lower pitch.

So pause the video for as long as you need while you do this and then press play when you're ready.

Now remember, what's actually shown horizontally on the screen is time.

So what you actually see here are not wavelengths, but they're representations of complete vibrations taking up a certain amount of time.

So here are some example answers.

On the top left we have the original sound, sound X, and in one we have a louder sound with the same pitch, same pitch, same frequency, same time period.

So each vibration fills the same amount of time across the screen horizontally.

Louder sound has a larger aptitude.

In two, we have a louder sound here it's been given the same amplitude as one and a higher pitch.

So we have more complete vibrations happening in the time shown across the screen.

And then in three, we have a sound that's quieter and lower pitched compared with X.

Quieter, lower amplitude.

So less distance vertically on the screen and lower pitch.

We have a longer time for each vibration.

We have a lower frequency.

So well done if you've got those.

Now let's move on to the second part of this lesson, loudspeaker.

So this is one of the applications of electromagnetic effects.

Here we have a picture of the inside of a simple loudspeaker of a type called a moving-coil loudspeaker, and it can convert an electronic copy of a sound wave that's sent to it into an actual sound wave.

So this part is called the speaker cone.

It could be made of cardboard and it's a moving part of the speaker.

Here we have a coil of wire that's insulated, and here we have a magnet.

It's a permanent magnet in this picture.

And the coil is connected to the input signal that could be coming from some sort of sound system.

It could be coming from a computer that tells the loud speaker what sound to play.

And that signal is going to change direction.

So it's an alternating current, it's AC, and the symbol here represents that.

And whenever current flows through the coil, the coil becomes an electromagnet.

That's what an electromagnet is.

It's a coil of wire and current flows through it.

There's a magnetic field around it, and it actually behaves like a bar magnet.

So when the coil has a current and it's an electromagnet, then there's a force between the coil and the permanent magnet because they are two magnets now that are close to each other.

And when the current flows one way, the coil will experience an upwards force.

That will be when the electromagnet and the magnet have like poles facing each other so they repel.

And when the coil experiences an upward force, it takes the cone upwards with it.

They move upwards.

And when the current flows the other way, we'll now have unlike poles facing between the electromagnet and permanent magnet, and there'll be attraction.

So the coil will be attracted downwards and it will move downwards taking the cone with it because they're attached to each other.

So what we have is a cone and coil that are moving up and down in time to the electronic signal like this.

Now, how could you make a coil and speaker cone move up and down more quickly? Would you increase the potential difference across the coil, change the direction of potential difference across the coil more quickly, more often, or increase the number of turns on the coil? Press pause and press play when you have your answer ready.

The correct answer is B.

You would change the direction of the potential difference more quickly, more times per second, because each time you change the direction of potential difference, the current changes direction and that makes the coil and speaker cone change direction.

Now the size of potential difference across a coil does make a difference.

It determines how far the coil and speaker cone move.

Here we have one volt across the coil, and if we increase it to three volts, the coil experiences a larger force and that makes it and the cone move further.

So how would the coil and cone of a speaker move if the coil received the signal shown here? So if this was a potential difference across the coil over a period of time, what would happen to the coil and the cone? Press pause and press play when you've chosen your answer.

The correct answer is A.

Now all three options begin with forwards.

So from that we can see that a positive potential difference here must be causing a forwards motion.

And the initial motion is caused by a relatively large potential difference.

So that's going to make the speaker cone move a relatively long way in one direction.

Then we have a negative potential difference.

It's got a smaller value, so we're going to have backwards motion, but not so far, and it happens for a shorter time.

And finally, we have positive potential difference again, so forwards movement, and it's quite a small potential difference.

So it moves a short way, but for a longer time.

Well done if you got that.

Now I'd like you to try making a simple loudspeaker yourself, and you can test it using a signal generator.

A signal generator will produce an alternating voltage causing an alternating current in the coil, and it allows you to set the frequency of that alternating voltage.

So you'll be able to try out different frequencies with your speaker and see how they sound.

So first, cut out a circle of card, 15 centimetres in diameter, and then cut along one radius up to the centre and curve the card to form a cone shape.

And you can use a piece of tape to stick that together.

Then you are going to take a strip of card eight centimetres wide, and you're going to make it into a roll by wrapping it around a length of broom handle that will give you about the right diameter.

And when you've done that, stick it down and make a coil around the tube of card that you've made and tape the coil so that it sticks to that tube.

And then take the tube off the broom handle and tape the cone to one end of the tube.

And then hang the cone by a thread over a permanent magnet.

And now you have a simple loudspeaker.

You have the key parts.

And so connect the end of the coil to the signal generator and test it out.

Pause the video while you do that and press play when you're finished.

These are the sort of things you might have noticed.

Your loud speaker should make a low humming sound for frequencies around 100 hertz.

And as you increase the frequency, your loud speaker should make a higher pitched sound.

It gets squeakier.

You might have found that you needed quite a high potential difference set on your signal generator to get a reasonable volume because you need a strong enough attraction and repulsion between the magnet and the electromagnet to move this cone.

So you could increase the volume by increasing the potential difference, which increases the size of the vibrations.

By the way, the speaker works better if you have a stronger permanent magnet because then you have larger forces between the coil and the magnet.

I hope your loud speaker worked reasonably well, and I hope you enjoyed investigating it.

I hope you've realised by now that a loud speaker is making use of the motor effect when a current carrying conductor that's in a magnetic field experiences a force.

And now let's have a look at the microphone, which makes use of the generator effect.

So a type of microphone that we're going to look at is a moving-coil microphone.

And you'll notice that in this picture, what is shown as a microphone actually looks remarkably similar to a loudspeaker.

And it is, but what it does is converts a sound wave into an electronic copy of the wave.

That's the opposite from what the loudspeaker was doing, which was converting an electronic copy of a wave into sound.

It has a diaphragm, a cone, which looks very similar to a loudspeaker diaphragm.

It has a coil, it also has a permanent magnet, just like the loudspeaker.

When a sound wave hits the cone, it makes it vibrate backwards and forwards.

And that causes an electronic copy of the sound to be generated.

I'll show you how that works.

When the diaphragm in the microphone is pushed backwards, the coil moves through the magnetic field of the permanent magnet.

And when a conductor moves in a magnetic field, a potential difference is induced, is made to happen across it.

So we get a potential difference across the coil, and that's going to push a current around this circuit.

When the microphone diaphragm moves forwards, the coil moves the other way through the magnetic field and potential differences induced that's in the opposite direction.

So it pushes the current around in the opposite direction.

So when a sound wave hits the cone, it causes an induced potential, which causes an induced current.

But each time the cone changes direction, the current is going to change direction, and a sound wave will make the cone vibrate.

It will make it move backwards and forwards.

So we will get this current that keeps changing direction.

The more the diaphragm, the cone is pushed in or out, the greater the size of the induced potential difference.

And therefore, the greater the current.

And the electronic signal is generated by the microphone represents the movement of the diaphragm that was caused by the sound wave.

So we have this sort of electronic copy of the original sound wave.

Now, how does a high note affect the movement of a microphone diaphragm compared to a low note? Press pause and press play when you're ready.

And the correct answer is it moves it in and out at a greater frequency.

A high note, higher pitch, is a sound wave with higher frequency.

That means more vibrations per second, and that's going to make more vibrations per second happen to the diaphragm as well.

Now, the electronic signal generated by the microphone is much smaller, has a much smaller potential difference than the signal needed to move the cone of a loudspeaker.

So we can't just directly connect the microphone to a loudspeaker and speak into the microphone and expect the loudspeaker to produce a good volume of sound.

We're not really going to hear anything.

So what we need is an electronic device called an amplifier, and that does the job of increasing the potential difference across an electronic signal, but without changing the shape.

So now we can take the signal from a microphone and send it via an amplifier to a loudspeaker and hear the original sound coming out of the loudspeaker.

Which of these traces represents an amplified version of the original electronic signal shown at the top? Press pause and press play when you've decided.

The correct answer is A, because what it shows is the same shape, but with larger potential differences.

So it stretched the signal vertically in this picture.

In B, there hasn't actually been any amplification.

It's been stretched in time, so it's going to sound strange.

Every frequency will actually be lower and it's going to take longer.

And in C, it's been stretched in both ways.

So it will be louder, but it will also take longer and sound lower.

Now I'd like you to explain how a microphone produces an electronic signal of a sound wave.

So this is an opportunity to show what you've been learning today.

Press pause while you're writing and take as long as you need and press play when you're ready and I'll show you an example answer.

So let's take a look at a possible answer.

The sound wave moves the diaphragm of the microphone back and forth.

This moves a coil back and forth through the magnetic field of a magnet.

Potential difference is induced in the coil pushing a current around the circuit.

The induced current exactly matches the pattern of the sound wave.

And there are more details that you could have added there, but these are the most important points.

And now we've reached the end of the lesson.

So here's a summary.

A moving-coil microphone is made from a diaphragm, a cone connected to a coil of wire that is moved by a sound wave in relation to a fixed magnet.

Microphones use the generator effect to induce a potential difference across a coil to make an electronic copy of the sound.

A moving-coil loudspeaker is made from a cone connected to a coil of wire that moves in relation to a fixed magnet.

Loudspeakers use the motor effect to convert electrical signals into sound waves.

So well done for working through this lesson.

I hope you understood how the microphone and loudspeaker work, and perhaps you'll think about that next time you hear any electronic sound.

I hope to see you again in a future lesson.

Bye for now.