Voice Coil
Converts a precise current waveform into proportional linear motion — driving a speaker cone, focusing a camera lens, or sweeping a hard disk head across platter tracks — with bandwidths into the tens of kilohertz and forces from millinewtons to hundreds of newtons.
A multi-turn coil hangs in the radial gap of a permanent-magnet motor, supported by a spider and surround. Current through the coil interacts with the radial magnetic field to produce an axial Lorentz force F = B·L·I, where B is the gap flux density and L the total wire length in the gap. Reversing the current reverses the direction; modulating amplitude varies force and hence acceleration of the moving mass.
In plain terms
Like a piston in a magnetic cannon — the coil is the projectile, the permanent magnet is the barrel, and the current is the powder charge whose direction and magnitude tell the piston exactly where to go.
Why designers use it
- Direct electromagnetic force can support short-stroke positioning or sound reproduction in an appropriate mechanical design.
- Within a specified operating range, force can be controlled by coil current; real actuators still have mechanical and thermal limits.
- A direct-drive arrangement can avoid gearbox backlash, but does not eliminate every source of friction or wear.
Best for
- Loudspeakers
- Camera autofocus
- HDD heads
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- When the required stroke or force exceeds the selected actuator's documented range.
- When continuous holding force would exceed its allowable current or thermal limits. Calculate the steady-state heating rather than assuming a universal time to failure.
Common mistakes
- Driving with a voltage source rather than a current source — at high frequencies, coil inductance reduces current and rolls off response.
- Pushing past the linear excursion limit (X-max) — the coil leaves the uniform-flux region, force becomes non-linear, and harmonic distortion explodes.
Where you will find it
- A KEF LS50 Meta bookshelf speaker uses an Uni-Q coaxial voice-coil driver: a 25 mm aluminium-dome tweeter sits at the throat of a 130 mm magnesium-aluminium cone whose own voice coil is driven by a current-mode amplifier — both coils share the same magnetic motor system but operate in independent gaps.
- An Apple iPhone 15 Pro main camera autofocus uses a tiny voice-coil motor (VCM) supplied by Mitsumi or Alps: a closed-loop driver IC reads a Hall sensor on the lens carrier and modulates VCM current at 200 Hz update rate to hold focus to ±5 µm against the iPhone's optical-image-stabilisation gimbal.
- A Western Digital Ultrastar HC570 22 TB HDD's head-stack assembly is positioned by a rotary voice-coil actuator: a single rectangular coil sits in a flat gap between two arc-shaped magnets, swinging the read/write heads across the platter at 30 ms full-stroke seek with sub-nanometre fine-positioning during track-follow.
A short history
The voice coil—a cylindrical coil of wire suspended in a fixed radial magnetic field so that current creates an axial force—was invented by Alexander Graham Bell as part of his 1876 telephone receiver patent, where it vibrated a diaphragm to reproduce speech. Chester Rice and Edward Kellogg at GE developed the direct-radiator cone loudspeaker driven by a voice coil in 1925, the basis of all modern dynamic loudspeakers. Voice coil actuators (VCAs) were later adapted for disk drive read/write head positioning in the 1960s and for precision linear actuation in semiconductor wafer stages, autofocus lens assemblies, and audio speaker systems.