A sound can be represented by an electronic signal changing in time to match the sound wave.
Loudspeakers use the motor effect to convert electrical signals into sound waves.
A moving–coil loudspeaker is made from a cone connected to a coil of wire that moves in relation to a fixed magnet.
Microphones use the generator effect to convert sound waves into electrical signals.
A moving–coil microphone is made from a diaphragm connected to a coil of wire that moves in relation to a fixed magnet.

Common misconception

It is common for pupils to hold the misunderstanding that stationary magnetic fields can attract or repel electric charge or to conflate magnetic and electrostatic effects.

Provide opportunities for pupils to explain applications of the motor effect and the generator effect in order to consolidate a scientific understanding.

Keywords

Oscilloscope - A piece of equipment that can display a representation of a sound wave.
Motor effect - The effect of causing movement between a magnet and an electromagnet.
Moving-coil loudspeaker - A loudspeaker that produces sound using the motor effect.
Generator effect - The effect of inducing a current by the relative movement of a circuit and a magnetic field.
Moving-coil microphone - A microphone that produces an electric signal from sound using the generator effect.

Loudspeakers can be made by wrapping a coil of insulated wire around a paper tube attached to a paper cone, connecting the coil to a signal generator and placing the coil over a strong magnet. Details can be found with an internet search: ‘how to make a paper loudspeaker’.

Teacher tip

Equipment

Making a loudspeaker: A4 sheet of thin card, short length of broom handle, single core insulated wire, strong bar magnet, thread, sticky tape, access to a signal generator and leads.

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

The abbreviation ‘p.d.’ stands for an electrical quantity. What is the full name of this quantity and what is its unit?

positive difference, measured in amps

potential difference, measured in amps

positive difference, measured in volts

Correct answer: potential difference, measured in volts

Q2.

Each of the diagrams shows a wave, with distance along the horizontal axis. All of the diagrams have the same scale. Which two of these waves have the same amplitude?

Correct Answer: An image in a quiz

Q3.

An object makes a sound. Which of the following must the object be doing?

rotating

Correct answer: vibrating

changing state

changing its temperature

moving from one place to another

Q4.

Which of the following are correct definitions of the frequency of a wave?

the distance between the wave’s centre line and a peak

the distance between the wave’s centre line and a trough

Correct answer: the number of wavelengths created per second

the time taken for one wavelength to be created

Correct answer: the number of wavelengths that pass a point each second

Q5.

Which of the following statements are correct?

Correct answer: The pitch of a sound is how low or high it is.

The pitch of a sound is how loud or quiet it is.

The amplitude of a sound wave sets the pitch of the sound.

Correct answer: The frequency of a sound wave sets the pitch of the sound.

Q6.

When a loudspeaker makes a sound, a part of it called the cone moves backwards and forwards. How does the motion of the cone change if the loudspeaker now makes a louder sound with the same pitch?

Correct answer: The cone moves faster.

Correct answer: The cone moves a greater distance backwards and forwards.

The cone moves backwards and forwards more times per second.

The cone takes less time to move backwards and forwards once.

Exit quiz

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6 Questions

Q1.

Match each of the following devices to a description of what it does.

Correct Answer:oscilloscope,displays a representation of a sound wave

displays a representation of a sound wave

Correct Answer:loudspeaker,produces sound from an electronic signal

produces sound from an electronic signal

Correct Answer:microphone,produces an electronic signal from sound

produces an electronic signal from sound

Q2.

Each of the diagrams shows an oscilloscope trace of a sound wave. All of the diagrams have the same scale. Which of the sound waves have the same pitch?

Correct Answer: An image in a quiz

Q3.

The diagram shows the coil, cone and permanent magnet of a loudspeaker. The coil is connected to an AC power supply. Which of the following statements are correct?

Correct answer: When there is a current in the coil, it becomes an electromagnet.

When current flows in the coil, the coil is attracted to the permanent magnet.

The lower the frequency of the alternating voltage, the louder the sound.

Correct answer: The lower the p.d. across the coil, the quieter the sound.

Q4.

Sort the following statements into the correct order to describe the operation of a moving–coil microphone.

1 - A sound wave hits the diaphragm (cone).

2 - This makes the diaphragm (cone) vibrate.

3 - This causes an alternating p.d. to be induced across the coil.

4 - This causes a current in the circuit attached to the coil.

5 - The pattern of varying current matches the pattern of varying sound.

Q5.

In which of the following ways does an amplifier change an electronic signal?

Correct answer: It increases the signal’s p.d. without changing its frequency.

It increases the signal’s frequency without changing its p.d.

It increases the signal’s p.d. and increases its frequency.

It increases the signal’s p.d. and decreases its frequency.

Q6.

Which of the following statements are correct?

Correct answer: A loudspeaker uses the motor effect.

A microphone uses the motor effect.

A loudspeaker uses the generator effect.

Correct answer: A microphone uses the generator effect.

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