Voice coil
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A voice coil is the coil of wire attached to the apex of the moving cone of a loudspeaker. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it.
Operation
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 voice, music, etc.
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 with the lightest-weight wire possible. Because of this, passing too much current through the coil can cause it to overheat. Voice coils wound with flat wire (so-called flat-wound voice coils) are better able to dissipate heat than coils made of round wire. Modern coils may also use a ferrofluid in the gap between the coil and the magnet frame to focus the magnetic field and assist in cooling the voice coil under high power conditions; the ferrofluid conducts heat away from the voice coil to parts of the speaker that have both more thermal mass and are better-able to dissipate the heat.
Excessive current can also cause the voice coil to extend beyond its normal excursion limits, causing a thumping noise and distortion. In extreme cases the voice coil has been known to tear off the cone.
Other uses for the term
Nowadays the term has been generalized and refers to any coiled wire that is used to move an object back-and-forth within a magnetic field. In particular, it is commonly used to refer to the coil of wire that moves the read-write disk heads in a moving-head disk drive. In this application, a very light weight coil of wires is mounted within a very strong magnetic field produced by rare earth permanent magnets. By means of a servomechanism driving the voice coil, the heads of the disk drive can be positioned very quickly and accurately.
Online references
- [G2 Consulting], A good paper on Voice Coil Actuators.
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