Balanced armature drivers: how they work and why IEMs stack them

Inside a balanced armature

The motor is built around the armature, a thin strip of magnetically soft metal. Part of it passes through a small coil, and its free end sits in a narrow gap between two permanent magnets, held exactly midway, where the pull from each side cancels. That balance gives the driver its name.

When the music signal flows through the coil, it magnetizes the armature, which is drawn toward one magnet and pushed away from the other. The tip rocks by a tiny amount, and a drive pin carries that motion to a light, stiff diaphragm. The whole assembly sits in a sealed metal case a few millimeters long, with one port for the sound.

Miniature armature drivers were developed for hearing aids, which need a receiver small enough to hide in the ear and frugal enough to run from a button cell. Both traits carried over. An armature is very efficient, since its motor drives only a tiny moving part, and it's self-contained, so it can sit anywhere in a shell and be piped to the nozzle through a tube.

Why in-ears carry several of them

A single armature can cover most of the audible band, and some in-ears use just one. Each unit's output is shaped by its own resonances and by the tube it plays through, though, so it tends to be strongest in one region. Makers build variants for each job, larger units for the bass, others for the midrange and small ones for the treble, and give each its share of the band.

The division is part electrical and part acoustic. A passive network of small capacitors and resistors splits the signal between the drivers. The plumbing then shapes each output: the length and width of each driver's tube, and any damper in it (a mesh or fiber plug that acts as an acoustic resistor). The tubes end in the nozzle's sound bores, and the outputs combine in your ear canal.

The Kiwi Ears Orchestra Lite shows the layout at $249: eight armatures on a 3-way crossover, with two ultra tweeters, four midrange units and two for the bass. Its review notes what that count costs in the fit, a nozzle made extra thick by the crossover and the number of drivers inside.

How an armature makes bass in a sealed ear

In open air an armature can barely make bass, because its diaphragm is tiny and hardly moves. An IEM changes the job. Sealed in the ear canal, the driver only has to change the pressure in a closed space of a cubic centimeter or two, and in a space that small the pressure follows the diaphragm's movement closely, so the output can stay level deep into the bass.

The condition is the seal. An armature is sealed in its own case and needs no vent behind it, so an all-armature shell can be closed completely. The Kiwi Ears Belle, Cadenza II and Orchestra II review credits the ten-armature Orchestra II's non-vented resin shells with better isolation. A leak at the ear tip lets the bass escape, and an armature has little movement in reserve to make it up. If an armature IEM sounds thin, check the fit before you judge the driver.

Beyond that, the amount of bass is a tuning decision. The Orchestra Lite's bass rolls off in the sub-bass, while the Orchestra II's review hears clear low bass once a proper dongle drives it.

What is a hybrid IEM?

A hybrid puts more than one driver type in one shell. The usual pairing is a dynamic driver for the bass with armatures above it, so each works where it's strongest. The Simgot EM6L does this at $109, with one dynamic and four balanced armatures on a hybrid 3-way crossover, tuned toward the Harman 2019 in-ear target. Its review finds the sub-bass and treble mostly on target and some extra mid-bass energy that can muffle vocals.

Hybrid only means mixed types, and the partner isn't always an armature. The KBEAR KB02 pairs a 10 mm beryllium-plated dynamic driver with a bone conduction driver that KBEAR tuned for the lowest frequencies.

The hard part of any hybrid is the handover. Where two drivers share a band their outputs have to add up smoothly, and a poor match leaves a step in the response or a dip where they overlap.

Output impedance and multi-armature sets

An armature's coil is strongly inductive, so its impedance rises with frequency, and a crossover adds peaks and dips of its own. The impedance of a multi-armature IEM can swing a long way across the band. The Orchestra II's review gives its impedance as a range, 15 to 30 ohms, where most spec sheets print one number.

That swing meets the source's output impedance. The two form a voltage divider that takes more signal away where the IEM's impedance is low, so an uneven curve turns into an uneven response, and the same IEM can sound different on two sources with different output impedances.

The fix is the rule of eighths applied to the lowest point of the curve instead of the nominal figure. With the source's output impedance under one-eighth of the IEM's minimum, the response moves by no more than about 1 dB anywhere in the band, however far the curve swings. For most multi-armature sets that means an ohm or two at most. The impedance explainer covers the voltage divider in full.

What a driver count tells you

On its own, very little about the sound. Several armatures sharing a band can give more headroom there, but every extra crossover region is another place where two outputs have to sum in phase. More drivers also mean more tubes, more dampers and more assembly work, all of which has to be matched from one shell to the next.

The Orchestra II's review praises its ten armatures for staying coherent, with no audible overlap between them and no obvious dips, which is what a high count has to achieve to be worth having. What reaches you is the frequency response at your eardrum, set by the tuning, the integration and your fit. The best IEMs guide ranks in-ears by score, whatever the shell holds, and the IEM explainer covers fit and ear tips.