voice coil
A coil of wire attached to the apex of the cone of a loudspeaker that provides the motive force to the cone by the reaction of a magnetic field to the current passing through it.
voice coil: the electromagnet that moves a speaker cone
A voice coil is a tightly wound coil of insulated copper wire, typically 25 to 50 millimeters in diameter, bonded to the apex of a loudspeaker cone. When an alternating audio signal passes through it, the coil sits within a permanent magnetic field generated by a ferrite or neodymium magnet. The interaction between the current in the coil and this magnetic field produces a linear force that drives the cone back and forth, creating sound waves. The name comes from its historical use in telephone receivers and early radio speakers, where it literally converted electrical voice signals into mechanical motion.
Voice coils are wound on cylindrical formers, usually made from kapton or fiberglass-reinforced plastic, which provide both mechanical support and thermal stability. Wire gauge ranges from about 0.25 to 1.5 millimeters depending on speaker power rating; high-power speakers require thicker wire to handle greater current without overheating. The number of turns varies, but typical designs have between 50 and 500 turns. Both single-layer and multi-layer windings exist, with multi-layer designs used in professional or high-fidelity applications where impedance matching requires precise resistance values, typically 4, 8, or 16 ohms.
The voice coil is suspended in the magnetic gap by a combination of the surround (the flexible ring at the cone's outer edge) and a smaller spider element closer to the apex. This suspension must keep the coil centered within the gap while allowing free movement; if the coil scrapes against the magnet structure, friction distorts the audio and generates heat. The coil's excursion, or maximum displacement, ranges from a few millimeters in compact speakers to 20 millimeters or more in subwoofers designed for deep bass.
Thermal limits and failure modes
Heat is the primary enemy of voice coil longevity. Voice coils can reach 100 degrees Celsius or higher under continuous operation at high power; adhesives bonding the wire to the former can soften, and the wire's insulation breaks down. Many professional speakers include voice coil temperature sensors or thermal fuses. In extreme cases, heat causes the coil to delaminate from the former, or the wire to fracture entirely. Moisture ingress into the winding also causes failure by promoting corrosion of the copper and degradation of the insulation.
Voice coil replacement is possible but labor-intensive, requiring careful removal of the cone assembly, unwinding and discarding the old coil, and hand-winding a new one with identical turn count and wire gauge. For this reason, voice coil design and material selection are critical engineering decisions; manufacturers balance cone stiffness, coil mass, thermal capacity, and impedance to achieve target frequency response and power handling within cost constraints. Subwoofer voice coils often use 4-ply or higher insulation and copper wire with higher purity to survive the extreme excursion and heat cycles associated with bass reproduction.