The Moving-Coil Microphone: Sound into Electricity
Simulation Description
This simulation models how a moving-coil microphone converts sound waves into an alternating electrical signal — the reverse process of the loudspeaker — and sits directly within the AQA GCSE Physics topic of electromagnetic induction (Paper 2).
The scene shows a photorealistic cross-section of a moving-coil microphone rendered with metallic PBR materials: a chrome back plate, ceramic magnet body, central pole piece (labelled N), outer top-plate ring (labelled S), copper voice coil with visible winding rings, a paper-tan diaphragm cone, rubber surround, corrugated spider, and a six-strut steel basket. An output cable and jack plug extend below the body, reinforcing that the microphone produces an electrical signal.
Sound pressure waves arrive from the front as incoming cyan and blue rings — bright cyan for compression, dim blue for rarefaction. These are not decorative; they drive the physics. When a compression wavefront reaches the diaphragm it displaces it outward; rarefaction pulls it back. This causes the voice coil to oscillate in the radial magnetic field of the permanent magnet. The B-field arrows in the gap point radially outward from the N pole piece toward the S outer ring — the direction is now physically correct and consistent with the pole labels visible on the magnet.
Because the coil is moving through the B-field, an EMF is induced by Faraday’s law: ε = BLv. The critical physics displayed is that the EMF is proportional to the velocity of the coil, not its displacement — so the purple output waveform (bottom-left, always visible) peaks when the diaphragm passes through the centre of its oscillation at maximum speed, and drops to zero at the extremes of travel. The coil moves in both directions with each pressure cycle, so the output is alternating current — an AC voltage signal whose frequency and amplitude mirror the incoming sound. Amber current-direction arrows on the coil reverse with every half-cycle, verified by the Lorentz force derivation (F = qv × B with outward B and axial velocity). A purple EMF arrow on the coil axis flips direction in sync.
Students can drag the scene to any angle to inspect the magnetic gap, view the coil from the side, or look directly down the barrel of the diaphragm. The frequency slider (left) changes how fast the diaphragm oscillates — students can watch the output waveform change pitch. The amplitude slider changes how far the diaphragm travels — a louder sound produces a taller waveform and larger EMF. Toggling 🏷 Labels adds the five-step GCSE causal chain and the sound/B/ε/output badge. The audio output tone reflects the induced signal, rising in pitch and volume with larger EMF.
Suggested Class Activity
“Electricity from Sound” — Reverse Engineering the Microphone Suitable for: GCSE Physics Year 10/11 — Electromagnetic Induction and Microphones. 25–30 minutes.
Setup (2 min) Display the simulation on the board with labels off. Students have mini whiteboards or a printed blank diagram of the microphone cross-section.
Stage 1 — Prompt the prior knowledge link (4 min) Without showing the simulation running yet, ask: “We have already studied the loudspeaker. In the loudspeaker, electricity produces sound. What do you think a microphone does?” Take a few answers, then ask: “If it does the opposite, what must the microphone do to the coil and magnet that is different from the loudspeaker?” Target: in the loudspeaker, electricity forces the coil to move; in the microphone, something must move the coil to produce electricity.
Stage 2 — Watch and identify (5 min) Play the simulation at medium frequency and amplitude. Ask students to watch the diaphragm, the coil, and the output waveform strip at the bottom-left. Questions to narrate:
- What is moving?
- When does the output waveform peak? When is it zero?
- What happens to the waveform when you increase the amplitude slider?
Key target observation: the waveform peaks not at maximum displacement but when the diaphragm is moving fastest — this is ε = BLv in action, even before naming it.
Stage 3 — Labels on: name the stages (6 min) Toggle 🏷 Labels on. Work through the five step chips in order as a class:
- Sound wave arrives — link to the compression/rarefaction rings students can see approaching
- Diaphragm vibrates — the cone oscillates because air pressure alternates
- Coil moves in B-field — drag the scene to show the coil sitting in the magnetic gap; point to the red B-field arrows pointing outward from N to S
- EMF induced (Faraday) — ε = BLv; ask what happens to EMF if the coil moves faster (louder sound, higher frequency)
- AC signal output — the output alternates because the coil moves in both directions; write on whiteboards: “AC because…”
Stage 4 — Comparison task: microphone vs loudspeaker (6 min) Students draw a two-column comparison table on their whiteboards:
| Loudspeaker | Microphone |
|---|---|
| Input: electrical AC | Input: ? |
| Coil forced to move by… | Coil forced to move by… |
| Output: sound waves | Output: ? |
| Uses motor effect | Uses ? |
Ask pairs to fill in the microphone column using what they have just observed. Target: the microphone uses electromagnetic induction (Faraday’s law) rather than the motor effect, but the structure is identical.
Stage 5 — Written explanation (5 min) Students write a GCSE-style answer (4 sentences) to: “Explain how a moving-coil microphone converts sound into an electrical signal.”
Use the step chips as a writing frame when labels are on. A strong answer will include: sound wave → diaphragm vibrates → coil moves in magnetic field → change in flux / EMF induced → AC output whose frequency matches the sound.
Adaptation notes
- For lower-confidence learners: leave labels on throughout; use the badge as sentence starters; reduce frequency slider to 1–2 for very slow oscillation
- For higher ability: ask why the output is specifically alternating current and not DC; link to Lenz’s law — the coil’s return motion induces an opposing EMF, producing the negative half of the cycle
- For SEND learners: extra-slow mode and the reading ruler; mute the audio if the tone is distracting; the waveform strip gives a visual alternative to the audio channel so the physics is fully accessible without sound