Voice coil

In some applications, such as the operation of servo valves, electronic focus adjustment on digital cameras, these are known as voice coil motors (VCM).

By applying an audio waveform to the voice coil, the cone will reproduce the sound pressure waves, corresponding to the original input signal.

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.

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.

If the coil is overdriven it may leave the gap, generating significant distortion and losing the heat-sinking benefit of the steel, heating rapidly.

Many hi-fi, and almost all professional low frequency loudspeakers (woofers) include vents in the magnet system to provide forced-air cooling of the voice coil.

For professional loudspeakers, advanced thermoset composite materials are available to improve voice coil survival under severe simultaneous thermal (<300°C) and mechanical stresses.

Motion of the aluminium bobbin in the magnetic gap creates eddy currents within the material, which further increase the temperature, hindering long-term survival.

As successful as these dark brown plastic films were for most hi-fi voice coils, they also had some less attractive properties, principally their cost, and an unfortunate tendency to soften when hot.

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.

The term "voice coil" has been generalized and refers to any galvanometer-like mechanism that uses a solenoid to move an object back-and-forth within a magnetic field.