Dynamic, planar, balanced armature, electrostatic: headphone drivers explained

The common principle: a current, a magnetic field, a movement

Almost all drivers rely on the same law of physics: a conductor carrying a current and placed in a magnetic field experiences a force. The music signal is a varying current; the force varies with it and sets a diaphragm in motion, and the diaphragm pushes the air. The technologies differ in the shape of the conductor, the shape of the diaphragm and the way the force is applied to it.

The dynamic driver

It is a loudspeaker in miniature. A coil of copper wire, glued to the centre of a conical or domed diaphragm, sits in the field of a ring magnet. The current moves the coil back and forth, and the coil drives the diaphragm.

Its strengths. Simple, robust, inexpensive and efficient: it produces a lot of level from little energy, which suits battery-powered devices. Its diaphragm has a long excursion, hence powerful bass. From £20 headphones to £450 studio references, it equips the overwhelming majority of the market.

Its limits. The force is applied at the centre of the diaphragm; at high frequencies, the edges no longer follow perfectly, and the diaphragm deforms. Manufacturers respond with stiff, light materials (beryllium, biocellulose, carbon coatings), whose real effect depends above all on the quality of the design.

Common sizes: 6 to 12 mm in earbuds, 30 to 50 mm in headphones.

The planar magnetic driver

Here, the diaphragm is an extremely thin flat film on which the conductor is printed in a serpentine pattern. It is stretched between two rows of magnets. The force acts on the whole surface at once, rather than at a single point.

Its strengths. The diaphragm moves as one, like a piston: distortion is very low, the bass reaches very deep without rolling off, even on open-back headphones, and transients (the attack of a note) are crisp. The impedance stays constant across all frequencies, which makes the amplifier's job easier.

Its limits. The rows of magnets are heavy: many planar headphones exceed 400 grams. Their efficiency is often modest, which calls for an amplifier capable of supplying current. Finally, almost all of them are open-back, and they rarely cost less than £130.

The balanced armature

Invented for hearing aids, it fits in a metal casing a few millimetres across. A tiny metal reed, the armature, is surrounded by a coil and balanced between two magnets. The current makes it pivot; a rod transmits this movement to a stiff diaphragm.

Its strengths. Very sensitive, very precise in the mids and treble, and so small that several can be fitted into a single earpiece. It isolates well, because its casing is sealed.

Its limits. A single armature struggles to cover the whole spectrum and lacks weight in the bass. High-end in-ears therefore combine several, each assigned to a frequency band by a filter. Their impedance varies widely with frequency, which makes them sensitive to the quality of the headphone output.

Hybrid models pair a dynamic driver for the bass with armatures for the rest: a solution that has become common from around £40.

The electrostatic driver

The principle changes: no more magnet or coil. An ultra-thin diaphragm, lighter than the air it moves, carries a permanent electric charge. It is suspended between two metal grids to which the music signal is applied at several hundred volts. Attracted by one and repelled by the other, it moves perfectly uniformly.

Its strengths. The lowest distortion and the finest detail known to date.

Its limits. It needs a dedicated high-voltage amplifier, incompatible with other headphones. The whole system runs into thousands of pounds, dislikes dust and humidity, and never leaves the living room. It is a technology for enthusiasts.

Bone conduction

Sound does not reach the inner ear through the eardrum alone: the bones of the skull also carry vibrations to it. That is why you hear your own voice as deeper than it is on a recording. Bone conduction headphones place small vibrating pads on the cheekbones, just in front of the ear, and leave the ear canal completely open.

Its strengths. You hear your surroundings with no attenuation at all: it is the safety-first choice for running beside a road or walking in town. It also suits some forms of conductive hearing loss, where the eardrum or the ossicles do not work properly.

Its limits. The bass is weak, the sound leaks at high volume, and the vibrations tickle some users. See the bone conduction headphones buying guide.

Side by side

What to conclude

The technology sets a potential and some constraints, not a result. A well-designed, well-tuned dynamic driver outperforms a mediocre planar, and measurements regularly confirm it. What you hear depends first on the frequency response the manufacturer was aiming for, the acoustics of the earcup and the fit on your head.

The right approach is therefore to start from the use, not the technology: if you listen in a quiet room, want the best possible reproduction and accept the weight, an open-back planar is worth trying; for everything else, a good dynamic will do better, more simply and for less money.

Sources and references

1. IEC 60268-7 — Sound system equipment: headphones and earphones
2. Borwick (ed.) — Loudspeaker and Headphone Handbook, 3rd edition, Focal Press, 2001