7.2.4 Loudspeakers and headphones (HT)
Converting an electrical signal into sound
Definition
Motor effect
The motor effect is the force experienced by a current-carrying conductor when it is placed in a magnetic field.
- Loudspeakers and headphones use the motor effect to convert a varying current into sound.
- The current passes through a coil in the field of a permanent magnet, so a force acts on the coil.
- As the current varies, the force varies, so the coil moves backwards and forwards.
Key Idea
A loudspeaker converts variations in current into variations in force, then into movement of a cone or diaphragm, and finally into pressure variations in the air.
How a moving-coil loudspeaker works
- A permanent magnet produces a magnetic field.
- A coil of wire sits within this field, with a cone attached to it.
- A varying current passes through the coil, and the motor effect produces a force on it.
- When the size of the current changes, the size of the force changes, and when the direction of the current reverses, the direction of the force reverses.
- The changing force makes the coil, and the attached cone, move backwards and forwards.
- The moving cone pushes the surrounding air, producing pressure variations that travel as a sound wave.
- Faster variations in current give a higher-frequency sound, and larger variations give larger cone movements and a louder sound.
How moving-coil headphones work
- Headphones work in the same way as a loudspeaker, but use a much smaller coil and a small diaphragm instead of a large cone.
- A varying current in the coil, which is in the field of a permanent magnet, produces a changing force through the motor effect.
- The coil and diaphragm move backwards and forwards, creating pressure variations in the air near the ear that form the sound wave the listener hears.
Example
Question: Explain how a moving-coil loudspeaker produces sound from a varying electrical current.
Answer: The current passes through a coil in the field of a permanent magnet, so the motor effect produces a force on it. As the size and direction of the current vary, the force varies, making the coil and attached cone move backwards and forwards. The moving cone produces pressure variations in the air, which travel as a sound wave.
Common Mistake
- Do not say only that the current makes the cone vibrate; explain that a current-carrying coil in a magnetic field feels a motor-effect force.
- Sound is not an electrical current travelling through the air; the current moves the cone or diaphragm, and that movement produces pressure variations.
Exam technique
- Give a complete chain: current in the coil, coil in a magnetic field, motor-effect force, varying current gives a varying force, coil and cone move, pressure variations, sound wave.
- Use the phrase pressure variations, rather than stopping at the cone vibrating.
Self review
- What force acts on a current-carrying coil placed in a magnetic field?
- Why does the force on the coil vary?
- What happens to the force when the current reverses direction?
- How does the movement of the coil make the loudspeaker cone move?
- How does a moving cone or diaphragm produce a sound wave?
- How is the operation of moving-coil headphones similar to that of a loudspeaker?
