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

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

Nearly 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 center of a cone- or dome-shaped diaphragm, sits in the field of a ring magnet. The current moves the coil back and forth, and the coil drives the diaphragm.

Strengths. Simple, rugged, 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 $500 studio references, it powers the vast majority of the market.

Limitations. The force is applied at the center of the diaphragm; at high frequencies, the edges no longer follow perfectly, and the diaphragm flexes. Manufacturers respond with stiff, light materials (beryllium, biocellulose, carbon coatings), whose real-world effect depends mostly 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 with the conductor printed on it in a serpentine pattern. It is stretched between two arrays of magnets. The force acts on the whole surface at once, rather than at a single point.

Strengths. The diaphragm moves as one piece, like a piston: distortion is very low, the bass extends 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.

Limitations. The magnet arrays are heavy: many planar headphones top 400 grams. Their efficiency is often modest, which calls for an amplifier that can deliver current. Finally, almost all of them are open-back, and they rarely cost less than $150.

The balanced armature

Invented for hearing aids, it fits in a metal housing a few millimeters across. A tiny metal reed, the armature, is surrounded by a coil and balanced between two magnets. The current makes it pivot; a rod transfers that motion to a stiff diaphragm.

Strengths. Very sensitive, very precise in the mids and treble, and so small that several can fit inside a single earpiece. It isolates well, because its housing is sealed.

Limitations. A single armature struggles to cover the whole spectrum and lacks weight in the bass. High-end in-ear monitors therefore combine several, each assigned to a frequency band by a crossover 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: an approach that has become common from $50 up.

The electrostatic driver

The principle changes: no more magnet, no more coil. An ultrathin diaphragm, lighter than the air it moves, carries a permanent electrical 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 with perfect uniformity.

Strengths. The lowest distortion and the finest detail known to date.

Limitations. It needs a special high-voltage amplifier that is incompatible with other headphones. The whole setup runs into the thousands of dollars, is vulnerable to dust and humidity, and never leaves the living room. This is a technology for enthusiasts.

Bone conduction

Sound doesn't reach the inner ear only through the eardrum: the bones of the skull carry vibrations to it as well. That's why you hear your own voice as deeper than it sounds on a recording. Bone conduction headphones place small vibrating transducers on the cheekbones, just in front of the ear, and leave the ear canal completely open.

Strengths. You hear your surroundings with no attenuation at all: it's the safe choice for running along a road or walking in the city. It also suits certain kinds of conductive hearing loss, in which the eardrum or the ossicles don't work properly.

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

Side by side

The bottom line

The technology sets a potential and a set of 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 ear cup and the fit on your head.

So the right approach is to start from the use, not from the technology: if you listen in a quiet room, want the best possible reproduction and accept the weight, an open-back planar is worth a try; 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