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Physics · Magnetism and electromagnetism

Loudspeakers and headphones

Explain how loudspeakers and headphones use the motor effect to convert alternating current into sound (Higher tier only; Physics only).

  • Higher
  • 6 key terms
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Teacher resources

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Student handouts

The same files the students see, to print or hand out.

Warm-up

Answer each one, then check.

  1. 1

    What is sound?

    Show answerHide answer

    A longitudinal wave

  2. 2

    What is frequency?

    Show answerHide answer

    Number of waves per second

  3. 3

    What is a.c.?

    Show answerHide answer

    Alternating current

  4. 4

    What is the motor effect?

    Show answerHide answer

    Force on a current-carrying wire in a magnetic field

  5. 5

    What is a diaphragm?

    Show answerHide answer

    A thin flexible cone or membrane

Learning Objectives

  1. 1Describe the structure of a moving-coil loudspeaker.
  2. 2Explain how a loudspeaker converts electrical signals into sound.
  3. 3Explain how the frequency and loudness of the sound depend on the a.c.

LOUDSPEAKERS

In a loudspeaker an alternating current flows through a coil in a magnetic field. The motor effect makes the coil, and the cone attached to it, move back and forth at the same frequency as the current, producing sound waves.

Loudspeakers and headphones use the motor effect; microphones (next lessons) use the generator effect.

How a Loudspeaker Makes Sound

Follow the steps.

  1. 1 a.c. flows

    An alternating current from the amplifier flows through the coil.
  2. 2 The coil is in a field

    The permanent magnet provides a magnetic field.
  3. 3 Motor effect

    A force acts on the coil, changing direction as the current changes direction.
  4. 4 The cone vibrates

    The coil moves the cone back and forth at the same frequency.
  5. 5 Sound waves

    The vibrating cone makes longitudinal sound waves in the air.

Changing the Sound

What controls the sound?

  • Higher frequency a.c.

    Sound: Higher pitch

  • Larger a.c. amplitude

    Sound: Louder sound (larger force, greater vibration)

  • Zero current

    Sound: No sound

Headphones

Explain how headphones work in the same way.

Show the solutionHide the solution
  1. 1 Small coil A small coil and magnet inside
  2. 2 a.c. signal Changing current moves the coil
  3. 3 Diaphragm A small diaphragm vibrates and creates sound

AnswerHeadphones are tiny loudspeakers.

Speaker Soak

Write a chain of events from the amplifier to the sound reaching your ear. Then explain why the cone vibrates at the same frequency as the current.

1. Follow the chain.

A good answer shows: a.c. → coil → motor effect → cone vibrates → sound. The force reverses each time the current reverses.

Can I...?

  1. 1State the parts.
  2. 2Name the effect used.
  3. 3Explain the coil movement.
  4. 4Explain the cone.
  5. 5Link frequency and pitch.
  6. 6Link amplitude and loudness.
  7. 7Describe headphones.
  8. 8Give the energy transfer.

Summary & Exam Focus

  • Alternating current flows through a coil near a magnet.
  • The motor effect gives a force that changes direction with the current.
  • The coil and cone vibrate at the same frequency.
  • The cone makes sound waves in the air.

Exam focus

Explain why the cone in a loudspeaker vibrates. (3 marks) (3 marks)

Alternating current in the coil, motor effect.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Loudspeaker
A device that uses the motor effect to convert electrical signals into sound.
Cone
The part of the loudspeaker that vibrates to make sound.
Alternating current
A current that repeatedly changes direction.
Frequency
The number of vibrations each second.
Amplitude
The size of the vibration.
Diaphragm
A thin vibrating membrane.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question State 2 marks Easier

What type of current flows through the coil of a loudspeaker? Describe how the cone moves.

Mark scheme — 2 marks available

  • Alternating current — 1 mark
  • Cone vibrates back and forth — 1 mark

Model answer

An alternating current. The cone moves back and forth (vibrates).

2. Exam question Explain 4 marks Easier

Explain how a loudspeaker produces sound.

Mark scheme — 4 marks available

  • a.c. in a coil in a magnetic field — 1 mark
  • Motor effect force — 1 mark
  • Force changes direction so cone vibrates — 1 mark
  • Sound waves produced — 1 mark

Model answer

An alternating current flows in the coil, which is in the field of a permanent magnet. The motor effect gives a force on the coil that reverses as the current reverses. The coil and cone vibrate at the frequency of the current. The cone produces longitudinal sound waves.

3. Exam question Explain 2 marks Easier

The frequency of the alternating current is increased. State the effect on the sound.

Mark scheme — 2 marks available

  • Higher pitch / higher frequency — 1 mark
  • Cone vibrates faster — 1 mark

Model answer

The pitch of the sound rises because the cone vibrates faster.

4. Exam question Explain 2 marks Easier

The amplitude of the current is increased. State the effect on the sound and explain.

Mark scheme — 2 marks available

  • Louder — 1 mark
  • Larger force or larger vibration — 1 mark

Model answer

The sound is louder because a larger current gives a larger force so the cone vibrates with a larger amplitude.

5. Exam question Explain 3 marks Easier

Explain why the current in a loudspeaker must be alternating rather than direct.

Mark scheme — 3 marks available

  • d.c. gives a steady force in one direction — 1 mark
  • Cone only moves once/stays displaced — 1 mark
  • a.c. reverses the force so the cone vibrates — 1 mark

Model answer

A direct current would give a force in one direction, so the cone would only move once and stay there. An alternating current reverses the force so the cone moves back and forth and produces sound.

6. Multiple choice 1 mark Easier

A loudspeaker uses the...

  1. A generator effect
  2. B motor effect Correct
  3. C greenhouse effect
  4. D Doppler effect

Why: Force on a coil in a field.

7. Multiple choice 1 mark Core

The current in a loudspeaker coil is...

  1. A direct
  2. B zero
  3. C alternating Correct
  4. D static

Why: It reverses.

8. Multiple choice 1 mark Core

Higher frequency current gives a sound of...

  1. A lower pitch
  2. B louder only
  3. C no sound
  4. D higher pitch Correct

Why: Faster vibrations.

9. Multiple choice 1 mark Core

The vibrating cone produces...

  1. A longitudinal sound waves Correct
  2. B transverse waves
  3. C light
  4. D static

Why: Sound.

10. Multiple choice 1 mark Stretch

Larger current amplitude gives a...

  1. A quieter sound
  2. B louder sound Correct
  3. C lower pitch
  4. D no change

Why: Bigger force.