How Do Speakers Use Magnets?

Speakers use magnets to create a fixed magnetic field for the voice coil to push against. Current from the amplifier flowing through the coil generates its own field, and the two fields repel and attract as the current reverses. That force moves the coil, the cone attached to it moves the air, and the air movement is what you hear.

Without the permanent magnet there is nothing for the coil’s field to react with, so no force is produced and the cone stays still. The magnet is not an accessory to the design. It is half the motor.

Where the magnet sits

Turn a speaker driver around and the heavy cylinder at the back is the magnet assembly, and it is more than one part.

The permanent magnet itself is a ring or a disc. Around it sits a steel structure, usually a back plate with a central pole piece and a front top plate, whose job is to route the magnetic field into a narrow circular gap.

That gap is where the work happens. It is typically a millimeter or two wide, and the field across it is intense and, importantly, uniform. The voice coil hangs inside it, wound onto a cylindrical former glued to the cone.

Concentrating the field into that small volume is why the steel around the magnet matters as much as the magnet. A magnet with poorly designed pole pieces wastes most of its strength.

How the force is produced

A wire carrying current in a magnetic field experiences a force at right angles to both. That is the motor principle, and it applies here directly.

The voice coil is a long length of wire wound in a circle and sitting in a radial field, so the force on it acts along the axis of the cylinder, pushing it out of the gap or pulling it in. Reverse the current and the force reverses with it.

Audio is alternating current, so the coil is driven back and forth at the frequency of the signal. A 200 Hz tone moves it 200 times a second. Louder passages carry more current and move it further.

The cone is glued to the coil, so it goes wherever the coil goes, and the air in front of it is alternately compressed and rarefied. There is more on the full chain in our explainer on how speakers make sound.

What magnet strength actually changes

A stronger field in the gap means more force for the same current, which means more sound for the same power. That is sensitivity, quoted in dB at one watt measured at one meter.

The difference is real and worth money. A speaker with a heavy, well designed magnet assembly at 92 dB sensitivity plays twice as loud as an 89 dB one on the same amplifier, and four times as loud as an 86 dB one.

Magnet strength also affects damping. A strong field means the coil resists being moved by anything other than the signal, so the cone stops when the music stops instead of ringing on. This is part of why bass from a well built driver sounds tighter than bass from a cheap one at the same frequency response.

What it does not do is make a small speaker produce deep bass. That is a question of how much air the cone can displace, and no magnet changes the physical size of the cone.

Ferrite against neodymium

Ferrite, also called ceramic, is the large black ring on most conventional drivers. Cheap, stable, and heavy. A 6 inch woofer might carry a magnet weighing a pound or more.

Neodymium is a rare earth alloy with far higher field strength per unit of mass, so a neodymium magnet delivering the same performance is a fraction of the size and weight. It costs considerably more, and it loses magnetism at lower temperatures, which matters in professional drivers that run hot.

You will find neodymium in headphones, portable speakers, in-ear monitors and professional touring drivers, all places where weight is the constraint. Ferrite dominates home hi-fi and car audio, where a heavy magnet is not a problem and the cost saving is.

Neither is better as a sound quality matter. The design around the magnet decides that.

Magnetic shielding

An unshielded driver projects a field well beyond the cabinet, which used to be a genuine problem. Cathode ray tube televisions steer their electron beam magnetically, so a speaker placed close to one produced colored patches on the screen.

Manufacturers solved it with a second magnet fitted behind the first in opposing polarity, cancelling the stray field, or with a steel cup enclosing the assembly. Speakers sold for use beside a TV were marketed as magnetically shielded.

Flat panel displays are unaffected, so shielding has largely disappeared from consumer speakers. The residual field still deserves respect around magnetic media, mechanical watches, and older credit cards, and around anyone with a pacemaker, who should keep several inches away from a large unshielded driver.

Frequently asked questions

Why do speakers need magnets?

The permanent magnet supplies the fixed field that the voice coil pushes against. When current flows through the coil it creates its own magnetic field, and the interaction between the two produces the force that moves the cone. Remove the magnet and the coil still carries current but has nothing to react with, so no movement and no sound is produced.

Does a bigger magnet mean a better speaker?

Not on its own. A stronger field in the gap raises sensitivity and improves damping, both of which are genuinely useful, but the steel pole pieces routing that field matter as much as the magnet itself, and a large magnet with poor pole design wastes most of its strength. A big magnet also does nothing for bass extension, which depends on cone area and cabinet volume.

What kind of magnets are used in speakers?

Ferrite, also called ceramic, is the common choice for home and car speakers. It is inexpensive, thermally stable and heavy. Neodymium is used where weight matters, including headphones, portable speakers and professional touring drivers, because it produces a much stronger field for its size. Neodymium costs more and loses magnetism at a lower temperature than ferrite.

Can speaker magnets damage electronics or cards?

The stray field from an unshielded driver can affect magnetic stripe cards, mechanical watches and older magnetic media held close to it, and it used to distort the picture on cathode ray tube televisions. Flat panel screens are unaffected. Most modern speakers use shielded or opposed magnet designs anyway, and keeping a few inches of distance removes any practical risk.