Voice coil

A voice coil (consisting of a bobbin, collar and winding) is the coil of wire attached to the apex of the 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. The term is also used for similar actuators and are commonly used as the positioning actuator in the disk read-and-write head of computer hard disk drives.

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 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 as possible, which inevitably makes them delicate. Passing too much power through the coil can cause it to overheat (see ohmic heating). Voice coils wound with flattened wire (so-called "ribbon-wire voice coils") provide a better packing density in the magnetic gap than coils wound of round wire. Some coils are made with special materials so they may be immersed in a ferrofluid which assists in cooling the coil under high power conditions, by conducting heat away from the coil to the magnet structure. Excessive input power at low frequencies can cause the coil to move beyond its normal limits, causing knocking and distortion such as clipping.

Power handling is related to 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 sits within the gap at all times. The power handling is limited by the amount of heat that can be removed from the voice coil. Some magnet designs include aluminium rings above and below the magnet gap, to improve conduction cooling, significantly improving power handling. 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 6 mm winding height has similar power handling to a 50 mm diameter voice coil with a 24 mm winding height.

In 'underhung' voice coil designs (see below), the coil is shorter than the magnetic gap, a topology that provides consistent electromotive force over a range of motion, known as X_max. If the coil is overdriven it may leave the gap, and lose 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 are used to draw in and expel 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 other acoustic advantages.

In the earliest loudspeakers, voice coils were wound onto paper bobbins, which was appropriate for the modest power levels of the period. As more powerful amplifiers became available, alloy 1145 aluminium foil was substituted for paper bobbins, and the voice coils survived the increased power. Typical hi-fi loudspeaker voice coils employ materials to withstand operating temperatures up to 180°C. Advanced composite materials have recently become available to improve voice coil survival under severe simultaneous thermal (300°C) and mechanical stresses.

Aluminium was widely used in the speaker industry due to its low cost, ease of bonding, and structural strength. When higher power amplifiers emerged, especially in professional sound, the limitations of aluminium were exposed. It rather efficiently transfers heat from the voice coil into the adhesive bonds of the loudspeaker, thermally degrading or even burning them. Motion of the aluminium bobbin in the magnetic gap creates internal eddy-currents in the material, which further increase the temperature, not helpful to long-term survival. In 1955 DuPont developed Kapton, a polyimide film which did not suffer from aluminium's deficiencies, so Kapton, and later Kaneka Apical were widely adopted for voice coils. As successful as these dark brown plastic films were for many voice coils, they also had some less attractive properties, principally their cost, and an unfortunate tendency to soften when hot. Hisco P450, developed in 1992 to address the softening issue in professional speakers, is a thermoset composite of thin glassfibre cloth, impregnated with polyimide resin, combining the best characteristics of polyimide with the temperature resistance and stiffness of glass. It can withstand brutal physical stresses and temperatures up to 300°C.

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 soft-parts (cone-coil structure). While rather delicate in a manufacturing environment, aluminium wire is 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 also used, allowing easier winding, along with a useful reduction in coil mass compared to copper.

One manufacturer uses anodized aluminium flat wire, which is effectively insulated against shorting between turns of the coil, so is not subject to dielectric breakdown as is the case with various enamel coatings used on most voice coils. 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 adhesive that bonds the wire to the bobbin.

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 keeps the number of windings within the magnetic field (or flux) constant over the coil's normal excursion range.	•	This method keeps the magnetic flux that the coil experiences, constant over the coil's normal excursion range.
•	Higher coil mass, sensitivity medium to high.‡	•	Low coil mass, sensitivity low to medium.‡
•	Soft non-linearity as the coil exceeds limits.	•	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.

Other uses for the term

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. In particular, it is commonly used to refer to the coil of wire that moves the read-write 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. The voice coil is the motor part of the servo system that positions the heads: an electric control signal drives the voice coil and the resulting force quickly and accurately positions the heads.

References

* [http://www.consult-g2.com/papers/paper8/paper.html G2 Consulting] , A good paper on Voice Coil Actuators.
* "The Hisco Loudspeaker Voice Coil Handbook", 7th Edition, August 2001. Hisco, Anaheim, CA 92807. 714 777 2665.
* ALMA [http://www.almainternational.org/content/committees/standards/softparts/NP-003%20page%201%20of%201%20rev%202-4-04.pdf NP-003] Voice Coil Nomenclature
* ALMA [http://www.almainternational.org/content/committees/standards/softparts/DG-003%20rev%202-2-04.pdf DG-003] Dimensioning & Tolerancing Guidelines for Voice Coils
* ALMA [http://www.almainternational.org/content/committees/standards/softparts/MG-003%20rev%20DRAFT.pdf MG-003] Measurement Guidelines for Voice Coils