What is voice coil?

A voice coil (consisting of a former, collar, and winding) is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it. The term is also used for voice coil linear motors, such as those used to move the heads inside hard disk drives, which produce a larger force and move a longer distance but work on the same principle.

Design considerations

Because the moving parts of the speaker must be of low mass (to accurately reproduce high-frequency sounds without being damped too much by inertia), voice coils are usually made as light weight as possible, making them delicate. Passing too much current through the coil can cause it to overheat (see ohmic heating). Voice coils wound with flattened wire, called ribbon-wire, provide a higher packing density in the magnetic gap than coils with round wire. Some coils are made with surface-sealed bobbin and collar materials so they may be immersed in a ferrofluid which assists in cooling the coil, by conducting heat away from the coil and into the magnet structure. Excessive input power at low frequencies can cause the coil to move beyond its normal limits, causing distortion and possibly mechanical damage.

Power handling is related to the heat resistance of the wire insulation, adhesive, and bobbin material, and may be influenced by the coil’s position within the magnetic gap. The majority of loudspeakers use ‘overhung’ voice coils, with windings that are taller than the height of the magnetic gap. In this topology, a portion of the coil remains within the gap at all times. The power handling is limited by the amount of heat that can be tolerated, and the amount that can be removed from the voice coil. Some magnet designs include aluminium heat-sink rings above and below the magnet gap, to improve conduction cooling, significantly improving power handling. If all other conditions remain constant, the area of the voice coil windings is proportional to the power handling of the coil. Thus a 100 mm diameter voice coil, with a 12 mm winding height has similar power handling to a 50 mm diameter voice coil with a 24 mm winding height.

The actual wire employed in voice coil winding is almost always copper, with an electrical insulation coating, and in some cases, an adhesive overcoat. Copper wire provides an easily manufactured, general purpose voice coil, at a reasonable cost. Where maximum sensitivity or extended high frequency response is required from a loudspeaker, aluminium wire may be substituted, to reduce the moving mass of the coil. While rather delicate in a manufacturing environment, aluminium wire has about one third of the mass of the equivalent gauge of copper wire, and has about two-thirds of the electrical conductivity. Copper-clad aluminium wire is occasionally used, allowing easier winding, along with a useful reduction in coil mass compared to copper.

flat wire may be used, providing an insulating oxide layer more resistant to dielectric breakdown than enamel coatings on other voice coil wire. This creates lightweight, low-inductance voice coils, ideally suited to use in small, extended range speakers. The principal power limitation on such coils is the thermal softening point of the adhesives which bond the wire to the bobbin, or the bobbin to the spider and coil.

Voice coils can be used for applications other than loudspeakers, where time force linearity and long strokes are needed. Some environments like vacuum or space require specific attention during conception, in order to evacuate coil losses. Several specific methods can be used to facilitate thermal drain.

Overhung and underhung coil

The image above shows two ways in which the voice coil is immersed in the magnetic field. The most common method is the overhung design where the height of the voice coil is greater than the magnetic gap’s height. The underhung design which is used mostly in high-end speakers has the coil’s height smaller than the gap’s. The differences, advantages and disadvantages of both methods are listed below.

Overhung coilCoil height is greater than the gap’s height.This method keeps the number of windings within the magnetic field (or flux) constant over the coil’s normal excursion range.Higher coil mass, sensitivity low to medium.‡Soft non-linearity as the coil exceeds limits.

Underhung coilGap’s height is greater than the coil’s height.This method keeps the magnetic flux that the coil experiences, constant over the coil’s normal excursion range.Low coil mass, sensitivity medium to high.‡Hard non-linearity as the coil exceeds limits.

Both topologies attempt the same goal: linear force acting on the coil, for a driver that reproduces the applied signal faithfully.

Coil Size Matter

A smaller speaker like a 6″ speaker, often has a smaller voice coil than a larger speaker like the 8” version of the same speaker. Why does that matter? Well, the larger you make the voice coil, the more power it can handle—but at the same time, it also generates more heat. Smaller coils can be a little more resonant, but larger coils tend to have better control.

Some speakers have to have larger coils. Subwoofers like the MTX 12” 150-Watt Vented Powered Subwoofer need a larger magnetic gap, so they have bigger coils and commensurately bigger heat and power. As the gap gets bigger, so too must the magnet. A tweeter, which only produces high frequencies, can get away with a much smaller coil.

The Sound of Experience

Having a well-engineered coil helps your speaker’s power handling, sound reproduction, and long-term durability. MTX has been doing this for over 40 years—since 1971!—and all that experience has gone into making speakers that are well-engineered, solidly-built and excellent value. That expertise means something.

We can help set you up with the perfect speaker for your customers. Check out the rest of Petra.com to see the huge variety of MTX speakers we carry, from car audio to in-wall audio to home theater. See the newest products and help your customers hear the difference with Petra and MTX.