Microphones | Physics

📖 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.

📌 Key Note

Dynamic microphone: sound → diaphragm vibrates → coil in magnetic field → generator effect → AC signal at same frequency as sound. Reverse of loudspeaker. Condenser: capacitance change method. Output: small AC (millivolts) → needs amplification. Both microphone and loudspeaker are electromagnetic transducers.

🎯 Matching Activity — Microphone vs Loudspeaker

Match each device to the energy conversion and physical principle. — drag the symbols on the right to match the component names on the left.

Loudspeaker

Drop here

Dynamic microphone

Drop here

Both

Drop here

Condenser microphone

Drop here

Sound changes capacitance of diaphragm-backplate gap → voltage signal

Use a coil of wire moving in a permanent magnetic field

Sound → electrical energy — sound vibrates coil in magnetic field → generator effect → AC

Electrical energy → sound — AC current drives coil by motor effect → cone vibrates

⭐ Higher Tier Only

HT only — describe how a dynamic microphone uses the generator effect to convert sound to electrical signals. Compare with the motor effect in loudspeakers. Explain why the induced current has the same frequency as the sound.

🔬 Triple Science Only

Microphones (HT only, physics only) — not in Combined Science.

🎯 Test Yourself

Question 1 of 2

1. A dynamic microphone and a loudspeaker have similar construction. How does their operation differ?

2. What frequency is the AC output from a microphone when someone sings a note at 440 Hz?

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⚡ Microphones

How Microphones Work

Types of Microphones

Signal Processing

Mr. Badmus AI