7.3.3 Microphones (HT)
The generator effect in microphones
Definition
Generator effect
The generator effect is the inducing of a potential difference across a conductor when it moves relative to a magnetic field.
Definition
Moving-coil microphone
A moving-coil microphone is a device in which sound waves make a coil move in a magnetic field, inducing a varying potential difference across the coil.
- If the conductor is part of a complete circuit, the induced potential difference produces an induced current.
- A microphone uses this to convert the pressure variations in a sound wave into variations in current in a circuit.
How a moving-coil microphone works
- A diaphragm is a thin, flexible sheet that the sound wave pushes on.
- A coil of wire is attached to the diaphragm.
- A permanent magnet produces a magnetic field around the coil.
- The coil is connected to an electrical circuit.
- The pressure variations in the sound wave make the diaphragm vibrate backwards and forwards.
- Because the coil is attached, it moves with the diaphragm, so it moves relative to the magnetic field.
- This movement induces a potential difference across the coil by the generator effect.
- In a complete circuit this produces a current, and as the coil's speed and direction change, the potential difference and current vary.
- When the coil reverses direction, the induced potential difference reverses, so the signal is alternating and its variations match the sound wave's pressure variations.
Key Idea
A microphone does not produce a steady current; the sound's pressure variations cause changing coil movement, which produces matching variations in the induced current.
How loudness and pitch affect the signal
- A louder sound gives larger pressure variations, so the coil moves more and the induced potential difference varies more.
- A higher-frequency sound makes the coil vibrate more often, so the electrical signal varies more frequently.
Common Mistake
- Do not confuse a microphone with a loudspeaker: a microphone uses the generator effect to turn sound into an electrical signal.
- Movement induces a potential difference across the coil; a current flows only in a complete circuit.
Exam technique
- Give a linked sequence: pressure variations vibrate the diaphragm, coil moves in a magnetic field, generator effect induces a potential difference, varying current in a complete circuit, variations match the sound wave.
- Use the terms diaphragm, coil, magnetic field, generator effect, induced potential difference and varying current.
Self review
- What makes the diaphragm of a microphone vibrate?
- Which component moves with the diaphragm?
- Why is a potential difference induced across the coil?
- When does the induced potential difference produce a current?
- How do the variations in the current relate to the original sound wave?
