The motor of a loudspeaker
A loudspeaker converts an electrical signal into cone motion with a voice-coil motor: a coil of wire sits in a radial magnetic gap, and current through the coil produces a force that drives the cone. The governing quantity is the force factor, Bl — the gap flux density B times the length l of coil wire in the field.
F = Bl · i. The force on the cone is the product of gap flux density, coil length in the gap, and current. The magnet sets B — so magnet choice directly sets how loud the driver plays for a given input.
More gap flux means more force per amp: higher sensitivity, better control of the cone (damping), and more headroom before the coil leaves the gap. That flux is the magnet's entire contribution.
The magnetic circuit
The magnet doesn't act alone — it drives a steel circuit that concentrates flux into a narrow gap where the coil lives:
- Magnet — the flux source (ceramic ring or neodymium slug).
- Front plate (top plate) and back plate / yoke — soft steel that carries flux from the magnet to the gap.
- Pole piece — completes the circuit and defines the inner wall of the gap.
- The gap — where front plate and pole piece face each other, and where the voice coil sits in a radial field.
Gap flux density depends on magnet strength and on the steel not saturating and the gap being tight. A strong magnet feeding a saturated or leaky circuit wastes flux — the circuit is as much a part of the design as the magnet. Steel geometry and magnetization direction together set what reaches the coil.
Ferrite vs neodymium motors
| Ferrite (ceramic ring) | Neodymium (inner slug) | |
|---|---|---|
| Position | Large ring outside the gap | Small slug inside the pole |
| Size / weight | Large, heavy | Small, light — a fraction of the mass |
| Cost | Very low, stable supply | Higher, rare-earth exposed |
| Flux density | Adequate; big volume needed | High flux in a compact motor |
| Stray field | Large external field | Contained; better near screens/electronics |
| Typical use | Budget & pro woofers, subwoofers | Compact, high-performance, portable, automotive |
Ferrite dominates where size and weight don't matter and cost does. Neodymium wins where the motor must be small and light — headphones, portable speakers, line arrays, car door drivers — trading material cost for a dramatic mass reduction. Both are stocked as rings and discs/slugs; the material comparison lays out the trade in full.
What magnet choice changes
- Sensitivity (SPL/W) — higher gap flux raises Bl and loudness for a given input.
- Motor weight — the single biggest reason to choose neodymium; it can cut motor mass by more than half, decisive for portable and overhead installs.
- Cone control (damping) — a stronger motor tightens electrical damping (lower Qes), shaping low-frequency response and transient accuracy.
- Thermal behavior — the magnet's steel circuit is also the coil's heat sink; motor design affects power handling and thermal compression.
- Cost & supply — ferrite insulates a product from rare-earth price swings; neodymium buys performance density at a premium.
Gap, linearity & shorting rings
- Underhung vs overhung coils — how coil height relates to gap height trades efficiency against linear excursion (Xmax); the magnet must supply enough flux over the working gap length either way.
- Gap tolerance — a tighter gap means higher flux density but less mechanical clearance for the coil. Plate flatness and pole concentricity directly affect flux and buzz-free travel.
- Shorting rings (Faraday rings) — a conductive ring on the pole reduces flux modulation and inductance for lower distortion; it's a circuit addition, not a magnet change, but it interacts with the field the magnet supplies.
Grade, heat & coating
- Grade: ceramic Y30–Y35 for ring motors; N40–N48 NdFeB where compact high flux is the goal (grades chart).
- Temperature: voice coils dump heat straight into the motor. High-power drivers can push a magnet past standard NdFeB's class — specify a temperature grade (H/SH) sized to real coil temperatures, per the temperature guide. Ferrite's stability is part of why it persists in high-power woofers.
- Coating: NdFeB slugs are sealed inside the motor but still need corrosion protection against humidity and handling; ferrite needs none.
- Assembly: magnets are bonded into the steel circuit — adhesive selection and cure matter for long-term gap integrity (bonding guide).
Design & spec pitfalls
| Mistake | Consequence | Fix |
|---|---|---|
| Chasing magnet strength, ignoring saturation | Extra flux wasted in saturated steel | Balance magnet to plate/pole cross-section |
| NdFeB grade too low for coil heat | Irreversible flux loss, output drops in use | Temperature-class grade sized to coil temp |
| Loose gap tolerance | Low flux density, coil rub / buzz | Tight plate flatness and pole concentricity |
| Ignoring stray field (neo) | Interference with nearby screens/sensors | Shielded motor structure or ferrite where field escape matters |
| Spec by grade alone | Two motors, same grade, different output | Specify gap flux density (or Bl) as the requirement |
Specifying a speaker magnet
For a loudspeaker magnet, alongside the RFQ checklist:
- Circuit target: required gap flux density (or Bl) — the real performance spec, not just a grade.
- Form & envelope: ring (OD/ID/height) or slug (dia/height), and the steel circuit it feeds.
- Material choice: ferrite vs NdFeB driven by weight, cost, and stray-field constraints.
- Thermal: expected coil/motor temperature setting the grade and temperature class.
- Tolerances: dimensional and magnetic consistency lot-to-lot for repeatable sensitivity.
- Assembly: bonding method and any magnetization-after-assembly requirement.