Consisting of a winding, a collar and a bobbin, a voice coil is a kind of electric coil. It attaches to the apex of a loudspeaker cone, where its purpose is to help it magnify sound.
To work, the loudspeaker system runs a current through the voice coil, which creates a magnetic field. Inside this magnetic field, the voice coil attracts a permanent magnet permanently attached to a loudspeaker cone. In response to this attractive, the magnet moves, bringing the cone and its audio waveform with it. Once it applies its audio waveform to the coil, the cone reproduces sound pressure waves that correspond to the original input signal.
When designing voice coils, manufacturers seek to make sure they are as lightweight as possible, with the lowest mass possible (too much inertia makes it hard to accurately reproduce high frequency sounds). Often, they encase them in housings to create actuating (movement controlling) parts that help protect them. Nevertheless, because voice coils must be so lightweight and small, they are quite delicate, and users must take care to not pass more power through them than for what they’re designed. Pairing excessive input power with low frequencies risks causing the coil to knock around and become distorted.
The windings, or electromagnetic coil portion, of voice coils are made from a conductive metal like aluminum. Manufacturers can customize voice coils in a number of ways, including by: size, housing type, speed, force, etc. To vary their properties and the areas in which the coils can work, manufacturers customize wire type. For example, ribbon-wire (flattened wire) creates a coil with a high packing density in magnetic gaps. A coil made from round wire, on the other hand, has a low packing density. To immerse voice coils in a ferrofluid, manufacturers can surface seal the bobbin and collar materials. This helps cool the coil by conducting heat away from it and into the magnet structure.
Voice coils come in two main varieties: overhung and underhung. The difference between the two is in how they’re immersed into the magnetic field.
Overhung voice coils are taller than the magnetic gap. These coils feature a higher mass, and have correspondingly low to medium sensitivity. If and when the coil exceeds its limits, it displays soft nonlinearity. (Nonlinearity is when the output signal strength does not change in direct proportion to the input signal strength. Too much nonlinearity compromises the quality of the loudspeaker.)
Underhung voice coils are shorter than the magnetic gap. With a low coil mass, they display a medium to high level of sensitivity. For that reason, they’re mostly installed in high-end speakers. Unlike overhung voice coils, if and when they exceed their limits, they respond with hard nonlinearity.
To learn more about voice coils, check out our manufacturing association pages, which are great resources when you’re researching about the industry. When you’re ready to make a purchase, visit the pages of one or more of those many exceptional coil suppliers with whom we partner.