📖 In-Depth Theory
How Microphones Work
A MICROPHONE converts sound energy into electrical energy using ELECTROMAGNETIC INDUCTION — the reverse of a loudspeaker.
DYNAMIC MICROPHONE (most common):
COMPONENTS: diaphragm, attached coil, permanent magnet.
OPERATION:
1. Sound waves hit the diaphragm → diaphragm vibrates.
2. Coil attached to diaphragm moves back and forth in the permanent magnet's field.
3. Moving coil cuts magnetic field lines → GENERATOR EFFECT → induced emf.
4. Induced current alternates at the SAME FREQUENCY as the sound wave.
5. This AC signal is sent to amplifiers and recording equipment.
MICROPHONE vs LOUDSPEAKER:
Loudspeaker: AC in → coil vibrates → sound (motor effect).
Microphone: sound → coil vibrates → AC out (generator effect).
They are both electromagnetic transducers — one is the reverse of the other.
Types of Microphones
CONDENSER (CAPACITOR) MICROPHONE:
Uses a thin conductive diaphragm very close to a fixed backplate (forming a capacitor).
Sound waves vibrate diaphragm → capacitance changes → voltage changes → electrical signal.
Requires power supply (phantom power from the mixer).
Very sensitive, wide frequency response — used in recording studios.
RIBBON MICROPHONE:
A thin metal ribbon suspended between magnets.
Sound waves vibrate the ribbon → ribbon cuts field lines → induced emf.
Warm, natural sound — used in broadcasting and music recording.
ELECTRET MICROPHONE:
Small, cheap condenser microphone with permanently charged material.
Used in phones, computers, headsets.
FACTORS AFFECTING MICROPHONE QUALITY:
Frequency response: how evenly it responds across all audible frequencies.
Sensitivity: how much output voltage for a given sound level.
Noise floor: self-generated noise — lower is better.
Directional pattern: omnidirectional, cardioid, bidirectional.
Signal Processing
MICROPHONE OUTPUT:
Very small AC voltage — millivolts level.
Must be amplified before use.
Pre-amplifier (preamp) boosts the signal.
DIGITAL RECORDING:
Analogue signal from microphone → analogue-to-digital converter (ADC).
Sampled at high rate (44,100 times per second for CD quality).
Each sample converted to a binary number.
Binary data stored on digital media.
SIGNAL CHAIN:
Sound source → microphone → preamp → ADC → digital processing → storage/transmission.
CONNECTION TO PHYSICS:
Microphones demonstrate that electromagnetic induction works in both directions:
Current in magnetic field → motion (motor effect) = loudspeaker.
Motion in magnetic field → current (generator effect) = microphone.
Einstein's principle of symmetry — if one works, the reverse also works.
⚠️ Common Mistake
A microphone uses the GENERATOR EFFECT (motion → current), not the motor effect. A loudspeaker uses the MOTOR EFFECT (current → motion). They are exact reverses of each other. The frequency of the induced AC from a microphone equals the frequency of the sound waves hitting the diaphragm.