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GCSE level Physics exam revision notes:
Electromagnetism
Electromagnetic effects: 12.5
How does a microphone work?
Conversion of sound wave energy to an electrical signal
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INDEX physics notes:
electromagnetic induction, generator applications
12.5 How does a
microphone work? - the structure and function of its parts
A microphone works due to electromagnetic
induction
A microphone
converts sound energy into electrical energy.
A microphone works in the opposite
way to a
loudspeaker.
A loudspeaker converts
electrical energy into sound energy.
The diagram above illustrate the principle of a
microphone e.g. for a vocalist or a telephone mouthpiece.
A microphone converts the energy of
the pressure variation of sound waves into an
electrical energy signal in an ac current.
The oscillation of the sound waves
vibrates the diaphragm which induces an oscillation in the electrical
circuit - the electrical signal.
The electrical signal could in turn
be used to re-generate sound in a
loudspeaker.
A microphone behaves like a loudspeaker in reverse -
The right-hand side of the diagram was borrowed from my loudspeaker
diagram!
The coil of wire surrounds
one pole of a permanent magnet, which is itself surrounded by
the other pole of the magnet - look for the
N and
S
on the diagram above.
The coil, in which the current
is induced, is connected to the flexible diaphragm cone cover made of
thin plastic or metal.
If the coil moves in the
magnetic field, a p.d. is induced in the coil.
This is no different in principle
to the working of a generator - a coil moves through a magnetic
field,
but there is no rotation, it is a
'to and fro' vibration effect.
The initial movement comes
from the sound waves hitting the flexible diaphragm.
The sound wave vibrations cause
the diaphragm to vibrate and move in resonance with the sound wave.
In turn, the coil
automatically moves at the same frequencies as the sound wave
inducing a varying current in the coil.
Also, the louder the
sound, the bigger displacement of the coil.
Therefore the movement of the
coil generates (induces) an electrical current signal that can be used to reproduce
both the frequencies and relative volumes (loudness) of the sound
waves in a loudspeaker system or a recording device
like a tape recorder.
See also
Sound waves nature and properties explained
INDEX physics notes:
electromagnetic induction, generators applications
Key points about electromagneti c
effects - how a microphone works - function of components
Information
sources for Doc Brown's key points: IGCSE-GCSE physics are based on
textbooks & syllabus-specifications for students taking the UK AQA, Edexcel,
OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE
physics 9-1 level science examinations
Here's a syllabus-aligned summary on
how a microphone works, tailored for GCSE/IGCSE
Physics students across WJEC, CCEA, CIE, AQA, Edexcel,
and OCR exam boards:
How a Microphone Works – Summary Notes
What Is a Microphone?
- A microphone is a transducer:
it converts sound energy (vibrations in air) into
electrical energy (a signal).
- It uses the generator effect (electromagnetic
induction) to produce a varying potential difference.
Structure of a Moving-Coil Microphone (Dynamic Microphone)
| Component |
Function |
| Diaphragm |
Thin, flexible membrane that vibrates when
struck by sound waves |
| Coil of Wire |
Attached to the diaphragm;
moves with it |
| Permanent Magnet |
Provides a magnetic field through which the
coil moves |
| Output Wires |
Carry the induced current to an amplifier
or recording device |
How a Microphone Works – Step-by-Step
- Sound waves hit the diaphragm, causing it to
vibrate.
- The diaphragm is attached to a coil of wire,
which moves back and forth in the magnetic field.
- As the coil moves, it cuts through magnetic field lines,
inducing a potential difference (Faraday’s Law).
- This produces an alternating current (a.c.) in
the coil that matches the frequency and amplitude
of the sound wave.
- The electrical signal is then amplified or recorded.
Energy Transfer of a microphone
- Input: Sound energy (vibrations in air)
- Output: Electrical energy (alternating current)
The microphone is often a required
practical
- Connect a microphone to an oscilloscope or amplifier.
- Speak or play sound near the diaphragm.
- Observe the waveform produced
- it mirrors the
sound wave.
Typical Exam Board Requirements about
how a microphone works
|
Key Focus Areas |
| Generator effect, microphone structure (HT only) |
| Sound to electrical energy, electromagnetic induction |
| Microphones versus loudspeakers, generator effect |
| Diaphragm movement, induced current |
| Sound wave conversion, electromagnetic principles |
| Moving-coil microphone, generator effect, waveform
interpretation |
Student Tips about how a microphone
works
What to Memorise about how a
microphone works
- Definition of a microphone as a transducer
- Function of each component
- How sound waves induce current via the
generator effect
- Energy transfer: sound → electrical
Common Misconceptions about how a
microphone works
- “Microphones use the motor effect” They use the
generator effect
- “Current is constant” It’s alternating,
matching the sound wave
- “Magnet moves” Usually the coil and diaphragm
move, not the magnet
Practical Tips for how a
microphone works
- Label diagrams clearly: diaphragm, coil,
magnet, motion,
field
lines
- Be able to sketch the output waveform and
relate it to sound frequency/amplitude
- Link to loudspeakers as the reverse process
Keywords, phrases and learning objectives
on the microphone
From a given diagram be able to describe and explain
how does a microphone work.
Know that a microphone converts a sound wave
of energy to an electrical energy signals.
Be able to explain the structure and function of
components such as the magnet, diaphragm
and coil.
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INDEX physics notes:
electromagnetic induction, generators applications
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