This article provides an in-depth discussion of voice coils. You will learn:
This chapter will discuss the design features and functions of voice coils.
A voice coil is a winding of a wire, usually copper, aluminum, or copper-clad aluminum that is wrapped around a former (often called a bobbin) and then attached to the apex of a speaker cone. This voice coil results in the movement of the speaker cone by reacting to the magnetic field of the motor structure.
A magnetic field is created when a current is sent through a speaker or subwoofer’s voice coil. This magnetic field is created within the gap between the magnet structure and the voice coil. This reaction results in the movement of the coil. As in music, a waveform is added, and the cone will react by reproducing the music or audio signal.
In the voice coil actuator working principle, the current that carries the inductors’ interactions within a permanent magnetic field generates a force. This force is proportional to the product of the magnetic flux and current in the coil. The Lorenz equation of force depicts this:
F = B × I
Where F is the Force (N), B is the magnetic Flux density (Tesla), and I is the Current (Amps).
Throughout the stroke of the actuator, the generated force is typically constant, although it can minutely decrease at the start and end of the stroke.
The moving element of the actuator can either be the assembly of the coil or of the permanent magnetic field.
The voice coil actuators are mostly found in the moving coil type. This has a coil wound around a bobbin. It moves in and out of the permanent magnetic field with a steel housing.
The moving magnet design is another commonly used voice coil actuator. In this case, the magnet assembly moves while the coil is fixed. It usually comes with a permanent magnetic field that is attached to a shaft. The moving magnet design actuator also has end caps with bearings, so it’s typically supplied with an integrated bearing system.
The design features of voice coils include:
Due to the fact that a speaker must move freely and frequently, the components have to be lightweight. The lightweight components also help to produce accurate high-frequency sounds that are less damped by inertia. Voice coils must not only be delicate but strong enough to handle the abuse or force caused by distorted audio signals or very loud volumes.
The delicacy of voice coils requires a trade-off in strength. When one applies too much current or a distorted audio signal, the voice coil will be damaged over time. Therefore, there must be a careful balance between the construction and durability of a voice coil.
Power handling refers to the heat resistance of a voice coil’s components, such as the wire, the wiring insulation, the adhesives, and the material of the former. Some, if not all, subwoofers and speakers are designed with cooling features, which include heat sinks or pile piece vents.
These features help by removing heat from the voice coil and improving its power handling ability.
The way the voice coil sits within the magnetic gap also helps in the cooling process. A voice coil entails Ohmic heating, meaning the coil is heated when too much current passes in or through it. A flattened wire called a ribbon wire can be used to wind voice coils, and this helps by providing a higher packing density in the magnetic gaps compared to voice coils with round wires.
Some coils are made with a surface-sealed bobbin and with collar materials that can be immersed in ferrofluid. This ferrofluid helps in the cooling of the voice coil by conducting heat away from the voice coil and leading it into the magnetic structure.
When excessive power is applied at low frequencies, this may result in the voice coil moving beyond its normal limits, causing distortion or mechanical damage.
Due to its ability to effectively conduct current and stand against higher amounts of heat, copper wire is used in most speaker applications to wind the voice coils. Copper also allows for easily-made and dependable all-purpose voice coils at an affordable manufacturing price.
For a speaker to maintain a very high maximum sensitivity or a higher response to frequency, an all-aluminum wire can be used to lessen the overall moving mass contained by the voice coil. An aluminum wire is about a third of the mass compared to copper wire, but aluminum only has about 61% of the conductive power of copper.
In order to achieve similar functionality and power handling for an aluminum wire as in a copper wire, the cross section of the aluminum wire coil must be 56% larger than that of the copper coil. Both the aluminum wire and copper wire can be used for different applications depending on the overall design and use of the specific speaker. There are also pros and cons attached to each wire in different applications.
In traditional speakers, paper bobbins were used to wind voice coils, which was appropriate for their modest power levels. But as time went on, more powerful amplifiers were introduced, and alloy 1145 aluminum foil was used to replace the paper bobbins. The new material also helped the voice coils to have increased power survival. Nowadays, some hifi loudspeaker voice coils can withstand operating temperatures reaching up to 302 °F (150 °C) or even 356 °F (180 °C).
Professional speakers use advanced thermoset composite materials, which help to improve voice coil survival when placed under severe thermal and mechanical stresses, including heat signatures greater than 572 °F (300 °C). Aluminum is employed due to its low cost, its stronger structure, and its ability to easily bond.
With the rise of higher power amplifiers, the limitations of aluminum metal have become clearly exposed. It efficiently transfers heat from the voice coil into the adhesive bonds of the loudspeaker, but does so in a way that leads to thermal degradation or burning of the voice coils.
The motion of aluminum bobbins in the magnetic gap within the material has been known to create eddy currents, which increase temperature and hinder the long term survival of the voice coils. A polyimide plastic film was once developed, and this did not suffer from the deficiencies of the aluminum. But these also had less attractive properties, including their cost and tendency to soften when heated.
To cover for the softening issue in professional speakers, Hisco P450, a thermoset composite of a thin glass fiber cloth impregnated with polyimide resin, was developed to combine the best elements of polyimide with the temperature resistance and stiffness of glass fiber. Hisco P450 can likely withstand brutal operating temperatures of up to 572 °F (300 °C) and physical stresses, all while its stiffness helps to maintain the speaker’s cold frequency response.
The real wire in voice coils’ winding is at some point always copper with an electrical insulation coating or in some cases an adhesive overcoat. Anodized aluminum flat wire can also be used. Anodization acts by providing an insulating oxide layer that is more resistant to dielectric breakdown than the enamel coating on other voice coils’ wire.
This act leads to the creation of lightweight voice coils, which also have low inductance and are ideally suited for use in small extended range speakers. Such speakers have a principal power limitation due to the thermal softening point of the adhesives, which bond the wire to the bobbin or bond the bobbin to the spider and the voice coil.
Impedance is the resistance of current as it passes or tends to pass through the voice coil. The voice coil impedance and the current are inversely proportional; this means that the lower the impedance, the more current passes through the voice coil.
As an example, a 2 ohm voice coil will have less resistance as compared to a 4 ohm voice coil. This later translates into how much power your amplifier can transfer into your speaker or subwoofer.
Speakers or other elements containing voice coils may come in a number of different configurations. This means that the resistance or the impedance of the voice coils can vary depending on how the equipment is configured. For example, speakers from the same manufacturer may come with a single voice configuration but with varying impedances.
Some with dual voice configurations may also have varying impedances. Of both types, single voice coil equipment is easier to wire because those with dual or multiple voice coils often have more wiring options, thus making them a little more versatile when comparing the capabilities to handle power of the subwoofer or speaker to an amplifier.
For a component containing a voice coil such as a speaker to accurately reproduce a sound source, the voice coil must remain suspended within the magnetic gap between the center pole piece and the top plate.
The voice coil must be able to move freely but not so far as to lose the interaction between the permanent magnet of the speaker and the varying magnetic field created by the voice coil current.
The physics of the motor arrangement requires that the voice coil stays in the middle of the magnetic gap, but it still must be able to move up and down with a limited range of motion. The former refers to a rigid cylinder around which the voice coil is wrapped, thus maintaining the circular shape of the loop. This is combined with the inside ring of the diaphragm, which is centered by the driver surround or the spider suspension system.
Materials used for the former can vary and are usually chosen to address needs in power handling. Paper is usually common in lower-rating drivers, and materials like Kapton or Nomex are used for higher power loudspeakers. The choice of material can also impact the tonal nature of the output from the driver, which plays a meaningful role in larger drivers like subwoofers.
To ensure the free movement of the voice coil in the gap over time, it is imperative that the gap remain free of other obstacles. This is why the dust cap plays an important function. But as with the former, different materials can lend tonal characteristics to the output of the sound produced by the driver, which is significant to a broad range of drivers.
Materials can range from paper to rubber to rigid plastics. Besides the material of the dust cap, the shape of the dust cap also influences the tonal character of the driver. There is a considerable difference between simple domes, domes with secondary concentric cones, and corrugated discs.
Variations in outputs at various frequencies are influenced by choice of dust cap shape. This is within the pass band of the driver’s frequency response curve.
This is a coil made up of wire, usually copper or aluminum, connected to the terminal of the driver. Its duty is to conduct and transfer current, which is driven by the source signal from an amplifier stage contained by the overall sound system. The current running through this wire creates a magnetic field.
According to physics, two magnets held together interact by mutually applying a force on each other. This theory also works in voice coils. Since a part of any speaker driver is a permanent magnet, the magnetic field induced from the current around the voice coil wire then interacts with the field of the permanent magnet. This leads to a force being exerted between the magnet and the voice coil, similarly to that of two magnets exerting a force on each other.
The word “voice coil” has been normalized, and now it refers to any galvanometer-like mechanism which uses a solenoid to move an object back and forth within a magnetic field. In general, it is used to note a coil of wire that moves the read-write heads in the moving head disk drive.
In this use, a very lightweight coil of wires is mounted within a magnetic field which is strong because it is produced by permanent rare earth magnets. This voice coil is in the motor part of the servo system, and it does the job of positioning the heads. An electric control signal drives the voice coil, and the force that results will lead to quick and accurate positioning of the heads.
The specifications for the voice coil include force constant, the linear stroke, peak force, torque constant, and peak torque. The force constant refers to the force that the voice coil actuators develop per ampere turn of coil excitation. This same act also applies to voice coil motors. It is specified in pounds per ampere turn or Newtons per amp.
The maximum and continuous force developed by a linear voice coil actuator or a voice coil motor is referred to as the peak force. Torque constant in relation to the voice coils refers to the torque which a voice coil motor develops per ampere of coil excitation. This torque constant is usually highlighted in pounds per amp or in any other unit that is equivalent to this.
Angular stroke refers to the maximum angle of displacement found in rotary voice coils actuators and rotary voice coil motors. Peak torque is another important issue to take into consideration when selecting a rotary voice coil device. The time the voice coil current takes to reach 63% of its final value is called the electrical time constant, and this is recorded as the actuator is subjected to a step input voltage.
Designs of voice coils aim to achieve linear force acting on the coil and to have a driver that produces the applied signal in a faithful manner. The basic types of voice coil products are linear and rotary, and the designs are either overhung or underhung.
Linear voice coils are able to provide a precise linear motion over short distances, whereas rotary voice coils are precise with circular motion over short angles.
This is the most common design, in which the voice coils inserted in the magnetic field have a height greater than the magnetic gap’s height. This method keeps the number of windings contained in the magnetic field or magnetic flux constant over the normal excursion range of the coil. They have a high coil mass and a sensitivity range from low to medium.
The coil will exhibit soft non-linearity as it exceeds its limits. In this type, a portion of the voice coil protrudes from the top of the magnetic gap in the motor structure, and the remainder is contained inside the gap. This type of voice coil allows for greater cone movement and also cools the voice coil because a fraction of it is not contained in the gap. The issue of added mass is a disadvantage when it comes to overhung coils. This is because the larger coil winding tops up weight to the voice coils’ weight, thus making it more difficult to move along with the magnetic force. This fact results in speakers or audio outputs with lower sensitivity.
This type of design is mostly used in high end speakers, and the voice coil’s height is smaller than the magnetic gap height. This type of method keeps the magnetic field or the magnetic flux at a constant over the normal excursion range of the voice coil. Underhung voice coils have a low coil mass, and their sensitivity ranges from medium to high. They also contain hard non-linearity as the voice coil exceeds its functional limits.
The different considerations for voice coils can be categorized as configuration considerations and wire considerations.
The configuration considerations include:
The outside face of the blank is coated with a thin film of B-staged thermoset adhesive, taking into consideration the desired strength, the tolerance in temperature, and the mechanical and electrical properties. These blanks are usually obtained already pre-coated from a converter, which refers to a firm that coats, slits, stocks, and distributes voice coil winding materials.
These bobbins might contain spiral or butt joints; there is usually later a slit or gap in that joint. The butt gap is usually around 1/64” (40 mm) to 1/32” (80 mm) wide depending on the diameter of the voice coil. The gap contained will expand during high-temperature operation of the voice coil. To improve the roundness of the bobbins, spiral bobbins are used where tight gaps are required.
At first sight, non-electrically-conductive bobbin substrates do not need a gap since overlapping will not lead to the creation of a shorted turn. Inductive effects such as high eddy currents, distortion, rocking, and heating of the voice coil are a product from the shorted turns with electrically conductive materials. To avoid distortion and increasing damping, superior speakers have aluminum bobbins designed with techniques that reduce eddy currents, such as copper caps and shorting rings. Nevertheless, most bobbins are used to accommodate thermal expansion of the voice coil.
In some speakers, holes are punched into the bobbin between the top stack of the voice coil and the joint of the neck. This style is used to improve the cooling effect in the system, but it has the disadvantage of reducing the air cavity pressure behind the dust cap. This process has some other advantageous effects, which include mass reduction and dampening torsion bobbin resonances.
Too-great air velocity passing through the vents can lead to loud whistling from turbulence. This can also result from the partial blockage of the vents on large excursions. Bucking may result if the vent is too large, possibly compromising the structural integrity of the bobbin.
Collars are one or more turns of a band of material located between the coil stack and the neck of the joint. The collar is essentially used to properly dress the lead out wires from the coil, but these lead out wires may also be glued directly to the bobbin. Secondary working elements of the collar include enhancing the adhesion and maintaining roundness.
Furthermore, other purposes of the collar may include the added wall strength of the bobbin, the insulation against temperature between the bobbin and the cone, and use as a correction factor for the proper fitting of bobbins which may be too small for the stock cone I.D.
Bobbins made from aluminum are widely used due to their high thermal transfer, but a possible con may be the excessive heating of the neck joint. This is due not only from their thermal conductivity but also from the self-heating effects resulting from the eddy currents. The selection of the proper adhesives should be considered if high heat is anticipated at the joint of the neck.
| Wire Insulation Coatings | ||
|---|---|---|
| Insulation Type | U.L. Temperature Class | NEMA Specs |
| Polyurethane | 105°C | MW-2 |
| Cellulose Acetate | 130°C | MW-75 |
| Polyurethane | 130°C | MW-75 |
| Polyurethane + Nylon | 130°C | MW-28 |
| Epoxy-Acrylic Resin | 130°C to 155°C | |
| Solderable Polyester | 180°C | MW-26 |
| Modified Polyester | 200°C | MW-74 |
| Polyimide | 220°C | MW-16 |
| Table 1: Wire Insulation Coating Standards And Temperature Ratings | ||
Along the width of the bobbin, the eddy currents are not uniform due to the divergence of the current that is induced at each end of the sheet. A tipping or canting force is then induced on the voice coil due to this effect, and this force is proportional to the velocity of the cone.
The wire considerations of voice coils include:
Magnet wire is available in a wide range of sizes, wire insulation coatings, and cross-sectional profiles. The insulation layer of the magnet wire is called the base coat; it is available with an adhesive bond as the top or outer bond coat. This magnetic wire is usually pre-coated using an adhesive which is later reactivated during the winding. Another method is that the magnetic wire can be wet wound when the top coat adhesive is applied to the non-adhesive coated wire.
This happens during winding. Copper wire is usually used for the speakers, but in some cases aluminum- and copper-clad aluminum wire are used. Due to the fact that copper is more conductive than aluminum, replacing aluminum with copper will require a larger diameter wire for the same conductivity. Copper also offers better strength and is easier to solder compared to aluminum.
Special fluxes or mechanical connection techniques are needed for joining load out wires to aluminum voice coils. An advantage of aluminum is that it has lower mass and greater conductivity per unit weight compared to copper. Aluminum is also prone to work, which means it hardens at relatively high operating temperatures. Hardening results in brittleness and wire failure.
These vary in type depending on the type of speaker being produced. Some of the differences include variations in gauge or fabric strands woven in with the wire. Flat conductors’ strips alongside phosphor, bronze or beryllium alloy are usually used in high-powered tweeters and compression drivers.
Options for the outlook of the wires can be selected for the load out wires, and these may include wire spacing and using a collar between the stack of the coil and the joint of the neck.
Most voice coils use round wire, while speakers with higher performance sometimes use wire that is flattened and wound on edge. The advantages included from this include a greater density of the wire in the gap and less inductance, as a single layer of flat wire contains less inductance compared to two or more layers of wire that is round. This wire can be flattened to different aspect ratios all depending on the required measurements. 4:1 and 5:1 are the most common aspect ratios.
Flat wire provides many advantages compared to round wire coils, and the wire can be flattened and coated with an insulation for a higher performance. This act needs full magnet wire production capability. Square wire is also used as it also provides higher wire density in the magnetic gap and at the same time avoids the problematic single layer flat wire.
This chapter will discuss the applications and benefits of voice coils.
Besides being used for loudspeakers, voice coil actuators are used in applications that require focusing, in oscillatory systems, in mirror tilting, and in the miniature position control.
The benefits of voice coils include their simple design and easy construction process. They have a low hysteresis, and they are relatively small in size to achieve a more efficient design footprint. Other advantages include its high accelerations and a lack of what is called cogging or commutation.
Voice coils, also referred to as non-commutated DC linear actuators, are a type of direct drive linear motor. They consist of a permanent magnetic field assembly and a coil assembly. A current flows across the coil assembly and interacts with the permanent magnetic field. This leads to a force vector which is perpendicular to the current direction being produced.
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