Dynamic drivers: how the moving-coil headphone driver works

How a moving coil makes sound

The motor has three parts. A permanent magnet, with iron parts that concentrate its field in a narrow ring-shaped gap. A voice coil of fine wire that sits in that gap. And a diaphragm glued to the coil and held at its edge by a flexible surround.

The music reaches the coil as an alternating current. A wire carrying current through a magnetic field feels a force, so the coil is pushed one way and then the other in step with the signal. The diaphragm moves with it and presses on the air in front. "Dynamic" is short for electrodynamic, and the same design is also called moving-coil. A cone loudspeaker works the same way at a larger size.

A parts list on a spec sheet maps straight onto that motor. The Sivga Anser review gives the full set: a 50 mm titanium-plated diaphragm with a carbon-fiber polymer dome, a 24.5 mm neodymium magnet, and a voice coil of copper-clad aluminum, which the review credits with pairing copper's conductivity with aluminum's lower weight.

The suspension sets a resonance in the bass

The surround behaves like a spring, and the diaphragm and coil are the mass sitting on it. Any mass on a spring has a natural frequency where it moves most freely, and in a headphone driver that resonance falls in the bass. Designers choose where it sits and how sharp it is, using vents, felt and mesh behind the driver and the volume of air inside the cup. The Anser's driver frame has a multi-hole design that, its review explains, controls the airflow inside the earcup.

The resonance shows up electrically too. A coil moving through a magnetic field generates a voltage of its own that opposes the drive signal, and that opposition is strongest at resonance, where the coil moves most. So a dynamic headphone's impedance rises to a peak in the bass, which can sit well above the nominal figure printed on the box.

That peak matters when the source has a high output impedance. The headphone then receives relatively more voltage at the peak than elsewhere, and the bass around it swells. The impedance explainer covers how to check a source for it.

Why the treble depends on the diaphragm

The coil pushes the diaphragm at one ring, where the two are glued. At low frequencies the whole diaphragm moves as a single stiff piece. As the frequency rises, the areas farthest from the coil start to lag and flex in patterns of their own, called breakup modes. Each can add a peak, a dip or a little ringing, mostly in the treble.

Diaphragm materials exist to manage this. A stiffer, lighter or better-damped diaphragm moves those modes higher or makes them milder, and a shaped dome resists flexing where a flat film would bend. The Meze 105 Silva shows how much engineering can go into it: a W-shaped dome of carbon-fiber-reinforced cellulose, surrounded by a titanium-coated torus with angled grooves cut to control how it flexes, all wrapped in a copper-zinc alloy ring to absorb vibration.

HIFIMAN's topology diaphragm, used in the RE2000 PRO Silver in-ear, is a nanoparticle coating applied to the diaphragm's surface, which its review says gives HIFIMAN more control over how the driver behaves. Read names like these as a description of what the maker set out to control. The review is where you find out what came of it.

One driver type, two very different tunings

Two 50 mm dynamic open-backs reviewed here sound nothing alike. The OLLO Audio X1 is a studio headphone with a PU and PET composite membrane, and its review calls it as neutral as headphones get. Each unit is individually calibrated to the same flat curve, typically within ±1 dB.

The Meze 105 Silva is tuned for enjoyment. Its review hears a warm balance with a slightly forward mid-bass, a small step back in the lower mids and noticeable extra sparkle in the treble. Both headphones share the motor and the driver size. The diaphragm, the damping and the tuning of the cup around it are where they part ways.

Do dynamic headphones need an amplifier?

Some do and many don't, because dynamic drivers span the whole range from very easy loads to demanding ones. The Meze 105 Silva is 42 ohms with 112 dB sensitivity, comes with a USB-C dongle that has its own DAC and amp, and its review found a phone gives it plenty of volume. The RE2000 PRO Silver is 60 ohms, which its review calls fairly high for an IEM, with 103 dB sensitivity. Power is no problem there, though the reviewer suggests at least a dedicated dongle over a phone.

At the demanding end, some studio and hi-fi models are wound to several hundred ohms and want more voltage than a phone can supply. The headphone power calculator takes a pair's impedance and sensitivity and works out the voltage and power your listening level needs.

Dynamic, planar or balanced armature

A planar magnetic driver spreads its drive force across a flat film instead of applying it at one ring, and its impedance barely changes with frequency. A balanced armature is a tiny sealed motor built for in-ears, often fitted several to a shell. Many IEMs are hybrids, with a dynamic driver for the bass and armatures above it.

Knowing the driver type tells you something about how a headphone will behave with an amplifier. It tells you much less about how it will sound, because a 50 mm dynamic can be tuned flat enough for a mixing desk or warm enough for a late-night album. The sensitivity and impedance tell you what to plug a pair into, and the review's account of the tuning tells you whether you'll like it.