Chapter 8 of 8

All 8 chapters
  1. 1Magnetic fields, readunderstood
  2. 2Fields around wires and coils, readunderstood
  3. 3The motor effect, readunderstood
  4. 4Electric motors, readunderstood
  5. 5Electromagnetic induction, readunderstood
  6. 6Generators and dynamos, readunderstood
  7. 7Transformers, readunderstood
  8. 8Speakers and microphones, readunderstood

S4.13.8 Chapter 8

Speakers and microphones

A loudspeaker makes sound from a wire. A microphone makes a signal in a wire from sound. What is inside them, and how do they work?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how a loudspeaker turns current into sound
  • Explain how a microphone turns sound into current
  • Find the magnet and coil inside an old broken speaker

A loudspeaker and a microphone are almost the same machine. Each has a magnet and a coil that can move. In the loudspeaker, a current makes the coil move. In the microphone, the coil is moved and a current is made.

We cut a loudspeaker through the middle. A circular magnet makes a field across a narrow gap. The field runs outwards from the centre pole to the outer ring. A light coil of wire sits in the gap, wound on a short tube. The tube is fixed to a paper cone.

A loudspeaker cut through the middle. A hatched magnet at the bottom, a centre pole labelled N and an outer ring labelled S on each side. A coil sits in the gap between the centre pole and the ring. On the left side of the coil, two crosses show a current into the page. On the right side, two dots show a current out of the page. Arrows show the field running outwards across the gap. A cone rises from the coil to a wide opening, with an arrow pointing up. Arrows labelled sound point up from the cone. Labels: N, S, S, magnet, coil, cone, sound.NSSsoundconemagnetcoil
The coil sits in a field that points outwards, left on the left and right on the right. The current goes round the coil, so the forces on both sides push the same way.

Check the direction with the left hand rule. On the left side of the coil the field points left and the current goes into the page. On the right side the field points right and the current comes out. Both forces point up, so the coil and cone are pushed up.

The amplifier sends the speaker an alternating current. It reverses many times a second. When the current reverses, the force on the coil reverses, and the cone moves down. So the cone moves up and down, or in and out, with the same frequency as the current.

Draw it first

The cone moves in and out with the current

  1. Two pictures of a loudspeaker cut through the middle. Step 1: the current goes into the page on the left of the coil and out on the right, the cone is pushed up, and arrows labelled sound point up. Step 2: the current is reversed, so it comes out of the page on the left and goes in on the right, the cone is pushed down, and arrows labelled sound point up. Labels: N, S, S, magnet, coil, cone, sound.NSSsoundconemagnetcoil

    1 The current goes one way round the coil. The forces push the coil and the cone out. The cone pushes the air in front of it, and makes a region of higher pressure.

  2. Two pictures of a loudspeaker cut through the middle. Step 1: the current goes into the page on the left of the coil and out on the right, the cone is pushed up, and arrows labelled sound point up. Step 2: the current is reversed, so it comes out of the page on the left and goes in on the right, the cone is pushed down, and arrows labelled sound point up. Labels: N, S, S, magnet, coil, cone, sound.NSSsoundconemagnetcoil

    2 The current reverses. The forces reverse and pull the cone back. The air in front of it is thinned out, a region of lower pressure. The pattern runs out as a sound wave.

The moving cone pushes and pulls the air. That makes a sound wave, with the same frequency as the current. The pitch of the note is the frequency of the current. A current of 400 Hz makes a note one octave above a current of 200 Hz.

A graph with the axes current (A) and time (ms). Time runs from 0 to 10 ms and current from minus 0.5 to 0.5 A. A solid wave labelled A has 2 cycles in 10 ms. A dashed wave labelled B, the same height, has 4 cycles in 10 ms. Both start at zero and rise. Dots on the time axis mark 2.5, 5, 7.5 and 10 ms, where both waves are at zero.AB510−0.5−0.250.250.50time (ms)current (A)
Current A makes a low note, 200 Hz. Current B makes a note twice as high, 400 Hz. Both are equally loud.

The loudness depends on the size of the current. A bigger current gives a bigger force, so the cone moves further. A bigger movement pushes more air and gives a louder sound. The pitch does not change.

A graph with the axes current (A) and time (ms). Time runs from 0 to 10 ms and current from minus 0.5 to 0.5 A. A solid wave labelled A has a height of 0.25 A and 2 cycles in 10 ms. A dashed wave labelled B has a height of 0.5 A and 2 cycles in 10 ms. Dots on the time axis mark 2.5, 5, 7.5 and 10 ms, where both waves are at zero.AB2.557.510−0.5−0.250.250.50time (ms)current (A)
The same note, 200 Hz. Current A is quiet. Current B is twice the size, so the cone moves twice as far and the sound is louder.

Worked example

A speaker coil has 2.5 m of wire in a field of 0.8 T. A current of 0.5 A flows. What is the force on the coil?

  1. Use F = B × I × L, with B = 0.8 T, I = 0.5 A and L = 2.5 m.
  2. F = 0.8 × 0.5 × 2.5 = 1, so F = 1 N.

Answer: 1 N

Worked example

A speaker makes a note of 250 Hz. How long does the cone take to move out and back once?

  1. The cone makes one in and out movement for each cycle of the current.
  2. The frequency is 250 Hz, so there are 250 cycles each second.
  3. The time for one cycle is 1 ÷ 250 = 0.004 s.

Answer: 0.004 s

Now turn it round. In a microphone, a thin diaphragm moves when sound hits it. A coil is fixed to the diaphragm, in the field of a magnet. As the coil moves, the field through it changes, and a voltage is induced. This is electromagnetic induction.

A moving coil microphone cut through the middle. A hatched magnet at the bottom, a centre pole labelled N and an outer ring labelled S on each side. A coil sits in the gap. On the left side, two crosses show a current into the page. On the right side, two dots show a current out of the page. A diaphragm rises from the coil, with an arrow pointing down. Arrows labelled sound point down towards the diaphragm. Labels: N, S, S, magnet, coil, diaphragm, sound.NSSsounddiaphragmmagnetcoil
Sound pushes the diaphragm down. The coil moves down in the field, and a current is induced in the coil.

The diaphragm moves back and forth in step with the sound wave, so the induced voltage alternates in step with it. A higher pitch gives a higher frequency of voltage. A louder sound moves the diaphragm further and gives a bigger voltage.

The two machines are linked. A speaker used backwards, with sound moving its cone, works as a poor microphone. Many microphones work this way. Others work in different ways, and the tiny ones in phones are built on a chip. All of them turn the sound wave into a changing voltage.

A table with the columns Device and Uses. Rows: loudspeaker, motor effect. microphone, induction.DeviceUsesloudspeakermotor effectmicrophoneinduction
A loudspeaker turns a current into sound. A microphone turns sound into a current.

Try it

Find the magnet and the coil in an old speaker

You need: an old broken loudspeaker that is not joined to anything, a steel pin or paper clip, a torch cell, two short pieces of wire.

Be safe: Use only a speaker that is already out of its device, or from a battery toy with the batteries taken out. Never open anything that has been joined to mains electricity. The magnet is strong and can pinch fingers. Keep it away from phones and bank cards. The metal frame can have sharp edges. Do not cut the cone.

  1. Look at the back of the speaker. You should see a round metal block. Hold a steel pin near it. It is pulled hard towards the block: that is the magnet.
  2. Look at the front. Push very gently on the cone with a finger and let go. It springs back. In the middle of the cone is a small dome. The coil is under it, wound on a short tube that sits in the gap of the magnet.
  3. Find the two thin wires that run from the cone to the terminals: they lead to the coil. If the cone is already torn, you may see the tube and its wire.
  4. Use the two short wires to join the speaker’s two terminals to the torch cell for one second. The cone moves a little.
  5. Swap the wires on the cell and touch again. The cone moves the other way.

What you will see: The pin is pulled by the magnet. The cone moves when a current goes round the coil, because the coil is in the field of the magnet. A current the other way moves it the other way. This is the motor effect at work. The same coil and magnet in a microphone would make a current when the cone is moved.

If you cannot try it, this is what happens.
A table with the columns Cell and Cone. Rows: one way, moves. reversed, other way.CellConeone waymovesreversedother way

A wrong idea many people have

A steady current in a loudspeaker makes a steady sound.

Why it feels right: A current makes the cone move, so a current that flows all the time seems to give a sound all the time.

The test that breaks it: Touch a cell to the coil of an old speaker. You hear one click, and the cone moves out or in and stays there. Nothing more happens while the current is steady. Take the cell away and you hear another click as the cone springs back.

What is true: A steady current pushes the cone out or in and holds it. Sound needs the cone to move back and forth. The current must keep changing, so the sound comes from an alternating current.

A wrong idea many people have

A louder sound has a higher pitch.

Why it feels right: Loud sounds seem sharper and more piercing than quiet ones, so people often mix up the two.

The test that breaks it: Turn the volume up on a radio, with the tuning the same. The sound gets louder, but the note stays at the same pitch. On the graphs, the louder note is a taller wave with the same number of cycles.

What is true: Loudness is the size of the vibration, the height of the wave. Pitch is the frequency, the number of cycles each second. They are different things.

A wrong idea many people have

A loudspeaker and a microphone are two quite different machines.

Why it feels right: One makes sound and the other picks it up, so they seem to do opposite jobs with different parts.

The test that breaks it: Look at the two pictures. Each has a magnet, a coil and a cone or a diaphragm. Connect a loudspeaker to a sensitive amplifier and speak into it. It works, badly, as a microphone.

What is true: They share the same parts. The loudspeaker uses the motor effect, and the microphone uses induction. They are the same idea run in opposite directions.

Key words

loudspeaker
a device that turns an alternating current into sound, by the force on a coil in a magnetic field.
microphone
a device that turns sound into an alternating voltage. A moving coil microphone uses induction.
cone
the paper part of a loudspeaker that moves with the coil and pushes the air.
diaphragm
the thin sheet of a microphone that sound moves.
pitch
how high or low a note is. It is the frequency of the sound.
loudness
how strong a sound seems. It depends on the size of the vibration.

Back to your guess

The answer is A: A coil in the field of a magnet: a current moves the coil and the cone makes sound.

A loudspeaker has a coil that can move in the field of a magnet. A current in the coil makes a force on it, by the motor effect. The coil moves a paper cone, and the cone pushes the air to make sound. A microphone is the same idea turned round.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What makes the cone of a loudspeaker move?

    Tap your answer.

    1. Not this one. The cone is made of paper, which a magnet does not pull. The force is on the coil that carries the current. Try another.

    2. Not this one. The heating is tiny and plays no part. The force on the coil from its current in the field moves the cone. Try another.

    3. Yes, that is right. The coil carries the current and sits in the field of the magnet. By the motor effect it is pushed, and it moves the cone with it.

    4. Not this one. Static charge plays no part. The force is on the coil, which carries a current in the magnet’s field. Try another.

    See the answer

    The force on the coil, which has a current in the field of a magnet.

    The coil carries the current and sits in the field of the magnet. By the motor effect it is pushed, and it moves the cone with it.

    • If you chose A: The cone is made of paper, which a magnet does not pull. The force is on the coil that carries the current.
    • If you chose B: The heating is tiny and plays no part. The force on the coil from its current in the field moves the cone.
    • If you chose D: Static charge plays no part. The force is on the coil, which carries a current in the magnet’s field.

  2. The current in a loudspeaker is changed to a frequency twice as high, and kept the same size. What happens to the sound?

    Tap your answer.

    1. Yes, that is right. The cone moves at the frequency of the current, so a higher frequency gives a higher pitch. The size of the current did not change, so the loudness did not change either.

    2. Not this one. The size of the current decides the loudness, and that did not change. Try another.

    3. Not this one. The size of the current has not changed, so the loudness stays the same. Try another.

    4. Not this one. The frequency of the current is the frequency of the sound, so the pitch changes. Try another.

    See the answer

    The pitch is higher, and the loudness is the same.

    The cone moves at the frequency of the current, so a higher frequency gives a higher pitch. The size of the current did not change, so the loudness did not change either.

    • If you chose B: The size of the current decides the loudness, and that did not change.
    • If you chose C: The size of the current has not changed, so the loudness stays the same.
    • If you chose D: The frequency of the current is the frequency of the sound, so the pitch changes.

  3. The current in a loudspeaker is made twice as big, at the same frequency. What happens?

    Tap your answer.

    1. Not this one. The pitch depends on the frequency, and the frequency did not change. Try another.

    2. Not this one. A bigger current gives a bigger force, so the cone moves further and the sound is louder, not quieter. Try another.

    3. Yes, that is right. A bigger current gives a bigger force on the coil, so the cone moves further and pushes more air. The frequency is the same, so the pitch is the same.

    See the answer

    The sound is louder, at the same pitch.

    A bigger current gives a bigger force on the coil, so the cone moves further and pushes more air. The frequency is the same, so the pitch is the same.

    • If you chose A: The pitch depends on the frequency, and the frequency did not change.
    • If you chose B: A bigger current gives a bigger force, so the cone moves further and the sound is louder, not quieter.

  4. How does a moving coil microphone make a signal from sound?

    Tap your answer.

    1. Yes, that is right. The diaphragm moves with the sound and carries the coil with it. The coil moves in the magnetic field, so a voltage is induced.

    2. Not this one. The heat from sound is tiny and plays no part. The movement of the coil in the field is what induces the voltage. Try another.

    3. Not this one. The magnet is fixed. It is the coil on the diaphragm that moves in the field. Try another.

    4. Not this one. There is no magnetic cone. The coil that moves in the field is what makes the voltage. Try another.

    See the answer

    Sound pushes the diaphragm, which moves a coil in a field and induces a voltage.

    The diaphragm moves with the sound and carries the coil with it. The coil moves in the magnetic field, so a voltage is induced.

    • If you chose B: The heat from sound is tiny and plays no part. The movement of the coil in the field is what induces the voltage.
    • If you chose C: The magnet is fixed. It is the coil on the diaphragm that moves in the field.
    • If you chose D: There is no magnetic cone. The coil that moves in the field is what makes the voltage.

  5. A steady direct current is joined to the coil of a loudspeaker. What happens to the cone?

    Tap your answer.

    1. Yes, that is right. A steady current gives a steady force on the coil, so the cone is held to one side. Sound needs the cone to move back and forth, which needs a changing current.

    2. Not this one. A steady current gives a steady force, so the cone does not vibrate. Only a changing current makes a vibration. Try another.

    3. Not this one. The force is real, so the cone moves to one side and stays there while the current flows. Try another.

    4. Not this one. A cell gives a steady current, with no rate of its own. A steady current gives a steady force, so the cone does not vibrate. Try another.

    See the answer

    It is pushed out or in, and held there, and makes no sound while the current is steady.

    A steady current gives a steady force on the coil, so the cone is held to one side. Sound needs the cone to move back and forth, which needs a changing current.

    • If you chose B: A steady current gives a steady force, so the cone does not vibrate. Only a changing current makes a vibration.
    • If you chose C: The force is real, so the cone moves to one side and stays there while the current flows.
    • If you chose D: A cell gives a steady current, with no rate of its own. A steady current gives a steady force, so the cone does not vibrate.

  6. Which pair is right?

    Tap your answer.

    1. Not this one. It is the other way round. A loudspeaker has a current and gets a force. A microphone has a movement and gets a voltage. Try another.

    2. Not this one. Only the loudspeaker does. The microphone is made to give a voltage, which is induction. Try another.

    3. Not this one. Only the microphone does. The loudspeaker gets a force from a current, which is the motor effect. Try another.

    4. Yes, that is right. In a loudspeaker a current in a field makes a force: the motor effect. In a microphone movement in a field makes a voltage: induction.

    See the answer

    A loudspeaker uses the motor effect. A microphone uses induction.

    In a loudspeaker a current in a field makes a force: the motor effect. In a microphone movement in a field makes a voltage: induction.

    • If you chose A: It is the other way round. A loudspeaker has a current and gets a force. A microphone has a movement and gets a voltage.
    • If you chose B: Only the loudspeaker does. The microphone is made to give a voltage, which is induction.
    • If you chose C: Only the microphone does. The loudspeaker gets a force from a current, which is the motor effect.

  7. You hold a steel pin near the back of an old speaker and it is pulled hard against a round metal block. What have you found?

    Tap your answer.

    1. Not this one. The coil is thin copper wire, which a pin is not pulled to. It is hidden in the gap of the magnet, under the dome of the cone. Try another.

    2. Not this one. The cone is paper, which is not magnetic. A pin is not pulled to it. Try another.

    3. Not this one. Those wires are copper, which is not magnetic. A pin is pulled only to the magnet. Try another.

    4. Yes, that is right. A steel pin is pulled only by a magnetic material or a magnet. The round metal block at the back of a speaker holds the magnet. The coil is copper wire and the cone is paper.

    See the answer

    The magnet.

    A steel pin is pulled only by a magnetic material or a magnet. The round metal block at the back of a speaker holds the magnet. The coil is copper wire and the cone is paper.

    • If you chose A: The coil is thin copper wire, which a pin is not pulled to. It is hidden in the gap of the magnet, under the dome of the cone.
    • If you chose B: The cone is paper, which is not magnetic. A pin is not pulled to it.
    • If you chose C: Those wires are copper, which is not magnetic. A pin is pulled only to the magnet.

  8. From an earlier chapter

    Which statement is true of an ideal transformer?

    Tap your answer.

    1. Not this one. It depends on the turns. A step-up transformer has a smaller current in the secondary. Go to S4.13.7. Try another.

    2. Yes, that is right. An ideal transformer wastes no energy, so the power out equals the power in: Vp × Ip = Vs × Is.

    3. Not this one. A transformer needs a changing current, so it needs AC. Go to S4.13.7. Try another.

    4. Not this one. No machine makes energy. An ideal transformer passes on all the power it gets. Go to S4.13.7. Try another.

    See the answer

    The power in the secondary equals the power in the primary.

    An ideal transformer wastes no energy, so the power out equals the power in: Vp × Ip = Vs × Is.

    • If you chose A: It depends on the turns. A step-up transformer has a smaller current in the secondary. Go to S4.13.7.
    • If you chose C: A transformer needs a changing current, so it needs AC. Go to S4.13.7.
    • If you chose D: No machine makes energy. An ideal transformer passes on all the power it gets. Go to S4.13.7.

  9. A speaker coil has 2 m of wire in a field of 0.6 T, with a current of 0.25 A. What is the force on the coil?

    See the answer

    0.3 N

    F = B × I × L = 0.6 × 0.25 × 2 = 0.3 N.

  10. A speaker makes a note of 500 Hz. How long does the cone take to move out and back once, in seconds?

    See the answer

    0.002 s

    The time for one cycle is 1 ÷ frequency = 1 ÷ 500 = 0.002 s.

Keep it

Copy this into your exercise book.

Loudspeaker: current in a coil in a field, force, cone moves, sound. Microphone: sound moves a coil in a field, voltage induced. Pitch is frequency. Loudness is the size of the current.

Draw: A speaker cut in half with a magnet, a coil and a cone, and arrows for the sound.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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