Electromagnetic induction - EdexcelLoudspeakers and headphones - Higher

Headphones, which contain small loudspeakers, use the reverse effect to microphones - the motor effect. In these devices, variations in an electric current cause variations in the magnetic field produced by an electromagnet.

This causes a cone to move, which creates pressure variations in the air and forms sound waves

Alternating current supplied to the loudspeaker creates sound waves in the following way:

1. a current in the coil creates a magnetic field
2. the magnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
3. the current is made to flow in the opposite direction
4. the direction of the magnetic field reverses
5. the force on the cone now pulls it back in
6. repeatedly alternating the current direction makes the cone vibrate in and out
7. the cone vibrations cause pressure variations in the air - which are sound waves

To make a loudspeaker cone vibrate correctly, the elastic current must vary in the same way as the desired sound.