4.2.8Magnetism and Magnetic Fields
Microphones, Loudspeakers and Headphones
5 min read 4 parts Lesson 13 of 13 Unit quiz
Section 1 of 4Moving-Coil Microphone
A microphone converts the pressure variations of a sound wave into an electrical signal.
1
Sound waves reach the microphone and the pressure changes make a thin diaphragm vibrate.
2
The diaphragm is attached to a coil of wire that sits in the field of a permanent magnet, so the coil moves with it.
3
As the coil moves back and forth it cuts through the magnetic field lines of the magnet.
4
By electromagnetic induction an alternating potential difference is induced across the coil.
5
The alternating signal follows the pattern of the sound wave, so a louder sound gives a larger signal.
Section 2 of 4Loudspeakers and Headphones
A loudspeaker does the reverse job of a microphone.
1
An alternating current from the amplifier flows through a coil that is placed in the field of a permanent magnet.
2
The current in the coil creates its own magnetic field, which interacts with the field of the magnet.
3
By the motor effect a force acts on the coil, and Fleming's left-hand rule gives its direction.
4
As the current alternates, the force reverses direction repeatedly, so the coil and the paper cone attached to it oscillate.
5
The vibrating cone pushes on the air to create longitudinal sound waves with the same frequency as the current.
Section 3 of 4Comparing the Devices
Comparing the Devices
Microphone
- Input is sound, output is an electrical signal, and the principle is electromagnetic induction.
Loudspeaker
- Input is an electrical signal, output is sound, and the principle is the motor effect.
Headphones
- Work like tiny loudspeakers held close to the ear, using the same coil and magnet arrangement.
Frequency
- The frequency of the sound produced matches the frequency of the alternating current in the coil.
| Device | Energy change | Effect used |
|---|---|---|
| Moving-coil microphone | Sound to electrical | Electromagnetic induction |
| Loudspeaker | Electrical to sound | Motor effect |
Section 4 of 4Exam Checklist & Tips
4.2.8 – Key Points for the Exam
- Diaphragm A thin sheet that vibrates when sound waves hit it.
- Permanent magnet A magnet that keeps its magnetism and provides a constant magnetic field.
- Motor effect A force on a current-carrying conductor in a magnetic field.
- Electromagnetic induction The induction of a potential difference when a conductor cuts through magnetic field lines.
Common Mistakes
- Swapping the two devices; a microphone uses induction and a loudspeaker uses the motor effect.
- Forgetting to say that the current in a loudspeaker is alternating, which makes the coil oscillate.
- Saying that the diaphragm creates the electrical signal rather than the coil moving in the magnetic field.
- Writing that the sound travels as a transverse wave; the cone produces longitudinal sound waves.