Voice coil

Last updated
A three-inch diameter dual voice coil from a subwoofer driver Loudspeaker voice coil, three inch diameter.jpg
A three-inch diameter dual voice coil from a subwoofer driver

A voice coil (consisting of a former, collar, and winding) is the coil of wire attached to the apex of a loudspeaker cone. 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 voice coil linear motors, such as those used to move the heads inside hard disk drives, which produce a larger force and move a longer distance but work on the same principle.

Contents

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 without being damped too much by inertia), voice coils are usually made as light weight as possible, making them delicate. Passing too much current 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 distortion and possibly mechanical damage.

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. 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 100 mm diameter voice coil, with a 12 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 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 the dustcap draws in cool air and expels hot 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 150°C, or even 180°C. For professional loudspeakers, advanced thermoset composite materials are 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 but inconveniently 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 eddy currents within the material, which further increase the temperature, hindering long-term survival. In 1955 DuPont developed Kapton, a polyimide plastic 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 most hi-fi 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 glassfibre. It withstands brutal physical stresses and operating temperatures up to 300°C, while its stiffness helps maintain the speaker's 'cold' frequency response.

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 coil. While rather delicate in a manufacturing environment, aluminium wire has 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 occasionally used, allowing easier winding, along with a useful reduction in coil mass compared to copper.

Anodized aluminium flat wire may be used, providing an insulating oxide layer more resistant to dielectric breakdown than enamel coatings on other voice coil wire. 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 adhesives which bond the wire to the bobbin, or the bobbin to the spider and coil.

Voice coils can be used for applications other than loudspeakers, where time force linearity and long strokes are needed. Some environments like vacuum or space require specific attention during conception, in order to evacuate coil losses. Several specific methods can be used to facilitate thermal drain.

Overhung and underhung coils

Overhung and underhung voice coils. Light grey is soft iron, dark grey is permanent magnetic material and the coil is in red. Voice coil geometry.svg
Overhung and underhung voice coils. Light grey is soft iron, dark grey is permanent magnetic material and the coil is in red.

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 listed below.

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 strong magnetic field produced by permanent rare-earth 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.

See also

Related Research Articles

Electromagnetic coil

An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil, spiral or helix. Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in devices such as electric motors, generators, inductors, electromagnets, transformers, and sensor coils. Either an electric current is passed through the wire of the coil to generate a magnetic field, or conversely an external time-varying magnetic field through the interior of the coil generates an EMF (voltage) in the conductor.

Wire Single, usually cylindrical, flexible strand or rod of metal

A wire is a single usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads or electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Wire gauges come in various standard sizes, as expressed in terms of a gauge number. The term 'wire' is also used more loosely to refer to a bundle of such strands, as in "multistranded wire", which is more correctly termed a wire rope in mechanics, or a cable in electricity.

Loudspeaker Electroacoustic transducer that converts an electrical audio signal into a corresponding sound

A loudspeaker is an electroacoustic transducer; a device which converts an electrical audio signal into a corresponding sound. The most widely used type of speaker is the dynamic speaker. The sound source must be amplified or strengthened with an audio power amplifier before the signal is sent to the speaker.

Electric motor Machine powered by electricity that converts electrical energy into mechanical energy (rotation)

An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. Electric motors can be powered by direct current (DC) sources, such as from batteries, motor vehicles or rectifiers, or by alternating current (AC) sources, such as a power grid, inverters or electrical generators. An electric generator is mechanically identical to an electric motor, but operates with a reversed flow of power, converting mechanical energy into electrical energy.

Electromagnet Type of magnet in which the magnetic field is produced by the flow of electric current

An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the hole, denoting the centre of the coil. The magnetic field disappears when the current is turned off. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the magnetic core concentrates the magnetic flux and makes a more powerful magnet.

Tweeter

A tweeter or treble speaker is a special type of loudspeaker that is designed to produce high audio frequencies, typically from around 2 kHz to 20 kHz. Specialty tweeters can deliver high frequencies up to 100 kHz. The name is derived from the high pitched sounds made by some birds (Tweets), especially in contrast to the low woofs made by many dogs, after which low-frequency drivers are named (woofers).

Woofer

A woofer or bass speaker is a technical term for a loudspeaker driver designed to produce low frequency sounds, typically from 50 Hz up to 1000 Hz. The name is from the onomatopoeic English word for a dog's bark, "woof". The most common design for a woofer is the electrodynamic driver, which typically uses a stiff paper cone, driven by a voice coil surrounded by a magnetic field.

Superconducting magnet

A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire has no electrical resistance and therefore can conduct much larger electric currents than ordinary wire, creating intense magnetic fields. Superconducting magnets can produce greater magnetic fields than all but the strongest non-superconducting electromagnets and can be cheaper to operate because no energy is dissipated as heat in the windings. They are used in MRI machines in hospitals, and in scientific equipment such as NMR spectrometers, mass spectrometers, fusion reactors and particle accelerators. They are also used for levitation, guidance and propulsion in a magnetic levitation (maglev) railway system being constructed in Japan.

Ferrofluid

Ferrofluid is a liquid that is attracted to the poles of a magnet.

In the field of acoustics, a diaphragm is a transducer intended to inter-convert mechanical vibrations to sounds, or vice versa. It is commonly constructed of a thin membrane or sheet of various materials, suspended at its edges. The varying air pressure of sound waves imparts mechanical vibrations to the diaphragm which can then be converted to some other type of signal; examples of this type of diaphragm are found in microphones and the human eardrum. Conversely a diaphragm vibrated by a source of energy beats against the air, creating sound waves. Examples of this type of diaphragm are loudspeaker cones and earphone diaphragms and are found in air horns.

Thiele/Small parameters are a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published in specification sheets by driver manufacturers so that designers have a guide in selecting off-the-shelf drivers for loudspeaker designs. Using these parameters, a loudspeaker designer may simulate the position, velocity and acceleration of the diaphragm, the input impedance and the sound output of a system comprising a loudspeaker and enclosure. Many of the parameters are strictly defined only at the resonant frequency, but the approach is generally applicable in the frequency range where the diaphragm motion is largely pistonic, i.e. when the entire cone moves in and out as a unit without cone breakup.

Magnepan

Magnepan is a private high-end audio loudspeaker manufacturer in White Bear Lake, Minnesota, United States. Their loudspeaker technology was conceived and implemented by engineer Jim Winey in 1969.

Field coil Electromagnet used to generate a magnetic field in an electro-magnetic machine

A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.

The chief electrical characteristic of a dynamic loudspeaker's driver is its electrical impedance as a function of frequency. It can be visualized by plotting it as a graph, called the impedance curve.

Magnet wire

Magnet wire or enameled wire is a copper or aluminium wire coated with a very thin layer of insulation. It is used in the construction of transformers, inductors, motors, generators, speakers, hard disk head actuators, electromagnets, electric guitar pickups and other applications that require tight coils of insulated wire.

Transformer types Types of electrical transformer

A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.

Moving iron speaker

The moving iron speaker was the earliest type of electric loudspeaker. They are still used today in some miniature speakers where small size and low cost are more important than sound quality. A moving iron speaker consists of a ferrous-metal diaphragm or reed, a permanent magnet and a coil of insulated wire. The coil is wound around the permanent magnet to form a solenoid. When an audio signal is applied to the coil, the strength of the magnetic field varies, and the springy diaphragm or reed moves in response to the varying force on it. The moving iron loudspeaker Bell telephone receiver was of this form. Large units had a paper cone attached to a ferrous metal reed.

Electrodynamic speaker driver

A speaker driver is an individual loudspeaker transducer that converts an electrical audio signal to sound waves. While the term is sometimes used interchangeably with the term loudspeaker (speaker), it is usually applied to specialized transducers which reproduce only a portion of the audible frequency range. For high fidelity reproduction of sound, multiple loudspeakers are often mounted in the same enclosure, each reproducing a different part of the audible frequency range. In this case the individual speakers are referred to as drivers and the entire unit is called a loudspeaker. Drivers made for reproducing high audio frequencies are called tweeters, those for middle frequencies are called mid-range drivers, and those for low frequencies are called woofers, while those for very low bass range are subwoofers. Less common types of drivers are supertweeters and rotary woofers.

Altec Lansing Duplex Line of loudspeakers

DUPLEX was the trade name given by Altec Lansing to its line of coaxial loudspeakers, beginning with the first model 601 in 1943. However, the name was most commonly associated with the subsequent model 604 which was a seminal loudspeaker that became a milestone in loudspeaker development. Well over a dozen different models carried the Duplex name over a near 50-year period. The vast majority consisted of a high frequency (HF) compression driver mounted to the back of a large diameter paper cone low frequency (LF) driver. However, there were also a few models with small diameter LF cones and direct radiator tweeters.

In a loudspeaker, power compression or thermal compression is a loss of efficiency observed as the voice coil heats up under operation, increasing the DC resistance of the voice coil and decreasing the effective available power of the audio amplifier. A loudspeaker that becomes hot from use may not produce as much sound pressure level as when it is cold. The problem is much greater for hard-driven professional concert systems than it is for loudspeakers in the home, where it is rarely seen. Two main pathways exist to mitigate the problem: to design a way for the voice coil to dissipate more heat during operation, and to design a more efficient transducer that generates less heat for a given sound output level.

References