By driving a current through the voice coil, a magnetic field is produced. This magnetic field causes the voice coil to react to the magnetic field from a permanent magnet fixed to the speaker's frame, thereby moving the cone of the speaker. By applying an audio waveform to the voice coil, the cone will reproduce the sound pressure waves, corresponding to the original input signal.
Design considerations
Because the moving parts of the speaker must be of low mass (to accurately reproduce high-frequency sounds), voice coils are usually made as light weight as possible, making them delicate. Passing too much power 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 knocking and distortion.
Power handling is related to the thermal tolerance 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 100mm diameter voice coil, with a 6mm winding height has similar power handling to a 50mm diameter voice coil with a 24mm winding height.
Overhung & underhung voice coils.
In 'underhung' voice coil designs (see below), the coil is shorter than the magnetic gap, a topology that provides consistent electromotive force over a limited range of motion, known as Xmax. 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. The pumping action of the cone and dustcap draws in and expels cool air. This method of cooling relies upon cone motion, so is ineffective at midrange or treble frequencies, although venting of midranges and tweeters does provide some acoustic advantages.
In the earliest loudspeakers, voice coils were wound onto paper bobbins, which was appropriate for modest power levels. As more powerful amplifiers became available, alloy 1145 aluminium foil was widely substituted for paper bobbins, and the voice coils survived increased power. Typical modern hi-fi loudspeaker voice coils employ materials which can withstand operating temperatures up to 150C, or even 180C. For professional loudspeakers, advanced thermoset composite materials are available to improve voice coil survival under severe simultaneous thermal (
