Voice coil

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A voice coil (also known as Bobbin, Collar and Winding) 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.

1 Operation
2 Design considerations
3 Overhung & underhung coils
4 Other uses for the term
5 Online references

[edit] 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.

[edit] Design considerations

Overhung & underhung voice coils.

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 (see ohmic heating). 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.

The power handling of a voice coil in a typical loudspeaker is dictated by a complex set of factors, including the type of enclosure the speaker is installed within, and whether it is of horn-loaded, sealed or vented configuration. Due to the varying forces exerted on the driver's cone by air pressure at different frequencies and the fact that the voice coil is an inductor, the electrical impedance of the voice coil can change dramatically over the operating frequency range. Since power dissipated is the product of current and voltage (see Ohm's law), the impedance (Z) of the coil will determine the current flow at a given frequency. At system resonance in a sealed box, the coil impedance will be higher than nominal, because there is less mechanical resistance to cone movement at this frequency. In a vented system, this Z_max occurs at twice the vent tuning frequency; the corollary to this is that Z_min occurs at the vent tuning frequency. This happens to be the frequency at which the greatest mechanical resistance to movement exists, so the electrical impedance of the coil is lowest and thus subject to the effects of heating due to increased electrical power dissipation.

Numerous factors affect voice coil heating and while mechanical resistance affects the Z of a coil, many drivers used in low frequency reproduction utilize forced air cooling, using the cone motion to force air through the voice coil gap within the pole piece assembly. With increased excursion, more air can flow across the voice coil, providing additional cooling and increasing the electrical power handling.

Mechanical power handling of the voice coil is related to the coil's position within the magnetic field as well as the length of the gap within the magnetic motor assembly. Recent developments in loudspeaker technology have resulted in the "underhung" voice coil (see below), one in which the coil overlaps the pole piece by a certain number of millimeters, providing linear electromotive force over its specified operating excursion. This specification is commonly referred to as X_max and is the maximum linear travel of the coil (in one direction) and represents the operating limits of the driver where objectionable distortion is not produced. Once the speaker is overdriven mechanically, the voice coil can actually partially leave the gap, at which point, it's not bathed in the magnetic field and is now "coasting". When this happens, it is no longer following the electrical input waveform, and severe distortion results. Other mechanical limitations, such as the suspension components, the length of the gap, height of the magnet backplates, etc., will also impose other mechanical restrictions that can result in damage when one or more of these limits is reached.

Voice coil cooling occurs by several means: conversion of electrical energy to mechanical energy (conversion efficiency), forced air cooling and blackbody radiation. The diameter of the coil also determines, to a degree, its power handling capacity. Modern voice coil technology employs high temperature dielectric materials to withstand operating temperatures up to 500ºF in professional sound reinforcement installations. One manufacturer uses anodized aluminium flat wire, which is effectively insulated against shorting between turns of the coil, but which is not subject to dielectric breakdown as is the case with various enamel coatings used on most voice coils. One of the leading failures of voice coils is breakdown of the enamel coating of the windings due to excessive heating, which causes the coil to "fracture", producing a rupture in the windings as the adhesive qualities of the enamel are lost and the coil literally becomes unwound in portions of its length. This is the most common voice coil failure mode. Manufacturers meet the challenges of high power dissipation through the use of advanced materials in the voice coil former, some using aluminum and Kapton laminates, in conjunction with high temperature adhesives and dielectric insulators.

[edit] Overhung & underhung coils

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 given in the table below.

Overhung coil		Underhung coil
•	Coil height is greater than the gap's height.	•	Gap's height is greater than the coil's height.
•	This method attempts to keep the number of windings within the magnetic field (or flux) constant over the coil's normal excursion range.	•	This method attempts to keep the magnetic flux that the coil experiences, constant over the coil's normal excursion range.
•	Greater sensitivity, hence for a given SPL less power is needed.^‡	•	Tends to have less sensitivity, hence for a given SPL more power is needed or the magnet has to be more powerful to improve sensitivity. Either way, the overall cost increases.^‡
•	The coil's travel is less linear over the normal excursion range.	•	The coil's travel is more linear over the normal excursion range, hence many high-end drivers^‡ use the underhung coil design.
•	Softer compression or non-linearity as the coil approaches excursion limits.	•	Harder compression or non-linearity as the coil approaches excursion limits.

Considering the second point above, both methods try to achieve the same thing: A linear force on the coil, which means that the coil's motion is a linear function of the applied signal and hence the driver reproduces the applied signal more faithfully.

‡ – SPL relates specifically to loudspeaker drivers. All the comments are otherwise applicable to other devices (see below) that use the same underlying principle as a loudspeaker's motor.

[edit] Other uses for the term

Nowadays the term voice coil 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 lightweight 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.