Why do speakers have magnets?
A speaker’s magnet doesn’t make sound — it makes force. Here is how a magnet and a coil of wire turn your amplifier’s signal into moving air, and why the magnet’s size matters far less than almost everyone thinks.
A speaker has a magnet because sound needs force. The magnet creates a strong magnetic field around the voice coil. When your amplifier pushes current through that coil, the current and the field between them produce a push or a pull. The coil is glued to the cone, so the cone moves, and the moving cone moves air. That is sound.
The 30-second explanation
A conventional moving-coil speaker — which is almost every speaker you have ever owned — does four things in order.
- Your amplifier sends changing electrical current through the voice coil, a cylinder of fine wire glued to the back of the cone.
- The magnet and the steel around it flood the narrow gap the coil sits in with a strong, steady magnetic field.
- Current in the coil and field in the gap together produce a force that drives the coil out or pulls it back in.
- The coil drags the cone with it, the cone pushes air, and the air reaches your ears as sound.
When the music signal reverses, the force reverses with it. So the cone tracks the waveform your amplifier sent — out, in, out, in, thousands of times a second.
Big slow movements push a lot of air at a low rate: bass. Tiny fast movements: treble. The magnet itself never moves. Its whole job is to hold the field steady so the coil has something constant to react against.
What are the magnet and voice coil actually doing?
The voice coil is the moving part. It is a coil of very fine wire, and the music passes through it as current. The magnet is the still part. It, and the steel around it, supply the field.
Here is the thing most explanations get wrong. This is not two magnets attracting and repelling. The useful force comes from current flowing through wire that happens to be sitting inside a magnetic field. Put a current-carrying wire in a field and the wire gets shoved sideways. That is the entire principle, and a loudspeaker motor is just a very carefully arranged version of it.
Two things decide how hard that shove is: how strong the field is where the coil sits, and how much coil wire is actually inside it. Manufacturers roll both into a single published number called the force factor, printed on datasheets as Bl and measured in tesla-metres. It is the most useful number on the sheet, and we will use it throughout this page.
A watch out: that second character is a lower-case L, for the length of wire. Depending on the font it can look like BI, BL or even B1. SB Audience datasheets print it as BI. It has nothing to do with current.
It isn’t really the magnet. It’s the gap.
This is the part almost every explanation of this question leaves out, and it is the part that actually matters.
A magnet sitting on a bench radiates its field in a wide, soft, useless loop. On its own it would do very little for a loudspeaker. What makes a driver work is the steel around the magnet — the back plate, the pole piece running up the centre, and the top plate forming a ring around it. Steel carries magnetic flux far more willingly than air, so the magnet’s field is collected, bent through ninety degrees and squeezed into one small annular slot: the magnetic gap.
That gap is typically two to twelve millimetres tall and under a millimetre wide on each side of the coil. Everything the magnet does, it does there. Flux that escapes anywhere else is wasted — and in a conventional ferrite ring motor, a great deal of it does escape.
So the number that matters is not the mass of the magnet. It is the flux density in the gap, measured in tesla. A heavy magnet feeding a sloppy magnetic circuit can put less flux in the gap than a small one feeding a well-designed circuit.
It is also why the coil has to stay in that gap. Push the cone far enough and the coil starts to leave the strongest part of the field, the force stops tracking the signal, and you hear it as distortion. Keeping the motor’s strength constant as the coil moves is where driver engineers spend most of their effort — Klippel treats force factor as a value that changes with coil position rather than a constant. Copper caps and extended copper sleeves, which you will see listed on good datasheets, tidy up the same magnetic circuit from the other direction.
Does a bigger magnet make a better speaker?
No — and we can prove it with two tweeters from the same family.
| Specification | TW29R-B | TW29RN-B (neodymium) |
|---|---|---|
| Magnet weight | 0.22 kg | 0.10 kg |
| Gap flux density | 1.0 T | 1.5 T |
| Force factor, Bl | 2.0 T·m | 3.1 T·m |
| Sensitivity | 92 dB | 96 dB |
| Net weight | 0.53 kg | 0.40 kg |
Same 29 mm ring dome, same voice coil, same 2.5 mm gap, same nominal impedance. The neodymium version carries less than half the magnet mass and SB Acoustics still specifies 50% more flux in the gap, half again the force factor, and four decibels more output.
So the visible size or weight of a magnet tells you very little. What matters is the material, the magnetic circuit it feeds, and the coil geometry sitting in the result.
So how strong is a speaker motor, actually?
Almost nobody answers this with a number, so here are some. Every figure is from the manufacturer’s published datasheet, and they span a 29 mm dome tweeter to a 21-inch subwoofer. Most are drivers we supply — ask us about any specific model, as a few are OEM-only.
| Driver | Type | Magnet | Gap flux | Force factor Bl | Gap |
|---|---|---|---|---|---|
| Satori TW29R-B | 29 mm ring dome | 0.22 kg | 1.0 T | 2.0 T·m | 2.5 mm |
| Satori TW29RN-B | 29 mm ring dome | 0.10 kg neodymium | 1.5 T | 3.1 T·m | 2.5 mm |
| Satori TW29BN-B | 29 mm beryllium dome | 0.11 kg neodymium | 1.5 T | 3.1 T·m | 2.5 mm |
| SB17NAC35-8 | 6" midwoofer | 0.54 kg | 1.0 T | 6.2 T·m | 5 mm |
| Satori MW16P-8 | 6.5" midwoofer | 0.13 kg neodymium | 1.17 T | 6.3 T·m | 5 mm |
| NERO-8MWN400D | 8" pro midbass | Neodymium | 0.85 T | 11.9 T·m | 8 mm |
| BIANCO-15MW500 | 15" pro woofer | Ferrite | 1.13 T | 18.8 T·m | 10 mm |
| NERO-21SW1100D | 21" pro subwoofer | Ferrite | 1.12 T | 30 T·m | 12 mm |
Two things jump out of that table, and both contradict what you will read elsewhere.
- Gap flux barely changes. From the smallest tweeter to the largest subwoofer, it sits in a narrow band between 0.85 and 1.5 T. The saturation limit of steel puts a ceiling on it and everybody bumps into the same ceiling.
- Force factor changes enormously — by a factor of fifteen. Since the field is roughly fixed, that spread comes from coil diameter and how much wire is immersed in the field. A big driver’s motor is not brute-forcing a stronger field. It is threading far more wire through a field of much the same strength.
Two rows there are worth a second look. The NERO-21SW1100D is ferrite, despite sitting in SB Audience’s NERO line — in that range the neodymium models carry an N in the part code. And the compression drivers go higher still: our ferrite ROSSO-65CD-T reaches 1.6 T in its gap, and the neodymium ROSSO-65CDN-T reaches 1.9 T, about as high as a practical loudspeaker motor gets.
Why do some speakers use neodymium?
Four magnet materials have ever mattered in loudspeakers. Here is how they compare on remanence (how much field the material holds), energy product (how much magnetic work it stores per unit volume) and Curie temperature (where it stops being a magnet at all). Figures are from the comparison table in Arnold Magnetic Technologies’ TECHNotes TN 0205, converted from gauss.
| Material | Remanence | Energy product | Curie temp |
|---|---|---|---|
| Ferrite (ceramic) | 0.345 – 0.42 T | 2.7 – 4.2 MGOe | 450 °C |
| Alnico (cast) | 0.55 – 1.35 T | 1.4 – 10.5 MGOe | 840 °C |
| Samarium cobalt | 0.88 – 1.13 T | 18 – 32 MGOe | 750 – 825 °C |
| Neodymium (NdFeB) | 1.07 – 1.40 T | 27 – 52 MGOe | 310 – 365 °C |
Look at the energy product column. Neodymium stores roughly ten times as much magnetic energy per unit volume as ferrite. That is the whole story: the designer can reach the same field from a fraction of the material, and spend the difference on a smaller, lighter, shallower motor instead.
| Consideration | Ferrite | Neodymium |
|---|---|---|
| Cost | Cheap, and still the default | Considerably more per kilogram |
| Size and weight | Larger and heavier for the same field | Much smaller and lighter |
| Heat | Very tolerant | More sensitive |
| Best suited to | Anything where weight is not critical | Compact tweeters, headphones, car audio, touring rigs |
One caveat vendors rarely mention: neodymium does not guarantee a stronger field. Our neodymium NERO-8MWN400D runs 0.85 T — lower than the ferrite BIANCO-15MW500 at 1.13 T. Neodymium’s advantage is field per kilogram, not field outright. A designer can spend that advantage on a lighter driver instead of a stronger one, and often does.
It does not automatically sound better either. It buys the designer freedom, not quality.
Heat is where ferrite still wins
Neodymium’s weakness is temperature, and it is a real one. Its Curie temperature is by far the lowest of the four, but the practical working limit is far lower still — standard grades are rated to about 80 °C before losses become permanent, and a voice coil in hard use can pass 180 °C.
Worse, the two materials move in opposite directions. Neodymium’s resistance to demagnetisation falls as it heats, while ferrite’s actually rises. Arnold puts the coefficients at +0.27%/°C for sintered ferrite against −0.50 to −0.65%/°C for sintered neodymium. That single asymmetry is why a great deal of professional touring equipment still ships with ferrite motors, decades after neodymium was supposed to have replaced them.
There is a second reason to care about heat. A loudspeaker is an appallingly inefficient machine — Klippel puts it bluntly: “in small, direct radiating loudspeakers almost 100% of the electrical power will heat up the voice coil”. As the coil heats its resistance climbs, it draws less current for the same voltage, and the driver quietly gets less sensitive. Turn it up hard enough for long enough and you get progressively less for your extra power. That is thermal compression, and it is why cooling and coil construction matter as much as the number on the magnet.
Do all speakers have magnets?
No. Almost every conventional speaker does, but the magnet is one answer to “how do I make a controllable force?”, not the only one.
| Speaker type | Uses a magnet? |
|---|---|
| Cone or dynamic driver | Yes |
| Dome tweeter | Yes |
| Compression driver | Yes |
| Planar magnetic | Yes |
| Ribbon | Usually yes |
| Balanced armature (in-ear) | Yes |
| Electrostatic panel | No |
| Piezoelectric | No |
Electrostatic panels use electrical attraction and repulsion on a very light charged diaphragm. Piezoelectric elements simply change shape when a voltage is applied. Neither needs a magnet — and neither, it is fair to say, will move much air at 30 Hz. Planar magnetics and ribbons look nothing like a cone driver but still rely on permanent magnets, and so do the balanced-armature transducers in most in-ear monitors.
The first proper loudspeaker didn’t use a permanent magnet either
The moving-coil driver came out of Chester Rice and Edward Kellogg’s work at General Electric. The application was filed on 20 April 1925 and granted on 2 April 1929 as US patent 1,707,570. One detail is worth getting right, because it is usually got wrong: that patent names Rice alone as inventor, and cites a separate joint Rice–Kellogg application alongside it.
And it did not use a permanent magnet. The field comes from a DC-fed coil — an electromagnet, though that is our word and not the patent’s: “The inner pole piece 9 is surrounded by the coil 11 through which a steady direct current may be supplied for producing a strong magnetic field in the annular air gap 8.”
That was not a preference. Permanent magnets strong enough for the job did not exist at a sensible price in 1925, so early speakers used a field coil, which in a valve radio doubled conveniently as the power supply’s smoothing choke. Field coils stayed dominant until alnico became widely available after the Second World War; most manufacturers moved to ferrite in the 1960s, and neodymium arrived in the 1980s.
What actually makes one speaker better than another?
The magnet is one component in a system that has to work as a whole. Cone material, suspension, basket rigidity, coil construction, how well the motor behaves as the coil moves, the cabinet it goes into and the crossover in front of it all decide what you actually hear.
Which is why “big magnet” and “neodymium” are not shortcuts for sound quality, and neither is a single impressive force factor. If you want one number to start from, start with force factor — then ask what the driver had to give up to get it.
Common questions about speaker magnets
What does the magnet do in a speaker?
It creates the steady magnetic field the motor works against. Current from your amplifier flows through the voice coil, and the interaction between that current and the field produces the force that moves the coil and the cone.
How does a speaker magnet make sound?
It doesn’t, on its own. It supplies the field. Current in the voice coil creates force inside that field, the force moves the cone, and the moving cone moves air. The air is what you hear.
Does a bigger speaker magnet mean better sound?
No. Our Satori TW29RN-B puts 1.5 T in its gap from a 0.10 kg neodymium magnet, while the TW29R-B manages 1.0 T from 0.22 kg. The smaller magnet wins because the material and the magnetic circuit are better, not because it is bigger.
Can a speaker work without a magnet?
Yes, if it makes force another way — electrostatic and piezoelectric speakers both do. A conventional moving-coil driver does need a magnetic field, but that field can come from an electromagnet instead of a permanent magnet, which is exactly how field-coil loudspeakers work.
Why is neodymium used in speakers?
Because it stores around ten times as much magnetic energy per unit volume as ferrite, so it delivers the same field from far less material. That means lighter drivers, smaller motors and shallower cabinets. It does not automatically mean better sound, and it tolerates heat considerably worse.
Do speaker magnets wear out?
Not from age in normal use. The realistic risks are heat and physical shock. Engineers writing in audioXpress put the onset of trouble for professional neodymium motors at around 130 °C, and report concert systems measuring 5 to 6 dB down by the end of a season. There is no single universal number — it depends on the magnet grade and the motor design — but ferrite tolerates heat considerably better.
Will a speaker magnet damage my phone, cards or TV?
A finished speaker in a cabinet is very unlikely to — the magnetic circuit is designed to keep flux in the gap, and a flat panel has no electron beam to deflect the way an old CRT television did. Phones are not entirely immune: Apple documents that a magnet held near an iPhone can temporarily disable the lens-position sensors used for stabilisation and autofocus. That is interference, not damage. Keep bare drivers and loose magnets away from magnetic-stripe cards, and if you have an implanted cardiac device, follow your manufacturer’s instructions — the FDA’s advice for consumer electronics with strong magnets is to keep them at least six inches (15 cm) away.
Choosing a driver, not just understanding one
Every driver figure on this page came off the manufacturer’s published datasheet — SB Acoustics and Satori for hi-fi, SB Audience for professional systems, all supplied by us in Singapore and Malaysia. If you are working out which motor suits a project, or want the full datasheet for anything named above, we are happy to go through it with you.
Sources: driver specifications from SB Acoustics and SB Audience datasheets. Magnet material properties from Arnold Magnetic Technologies TECHNotes TN 0205 and Understanding and Using Reversible Temperature Coefficients. Motor behaviour from Klippel’s Loudspeaker Nonlinearities and Green Speaker Design (Part 2). History from US patent 1,707,570. Diagrams credited in their captions.