This article provides an in-depth discussion of voice coils. You will learn:
This chapter will cover the design characteristics and functions of voice coils.
A voice coil consists of a winding of wire, typically made of copper, aluminum, or copper-clad aluminum, wrapped around a former, also known as a bobbin. It is connected to the apex of a speaker cone. The interaction between the voice coil and the magnetic field of the motor structure causes the speaker cone to move.
When a current flows through a speaker or subwoofer’s voice coil, a magnetic field is generated within the gap between the magnet structure and the voice coil. This interaction causes the coil to move. When a waveform, such as music, is added, the cone responds by reproducing the music or audio signal.
In the working principle of a voice coil actuator, the current interacting with the inductors within a permanent magnetic field generates a force. This force is proportional to the product of the magnetic flux and the current in the coil. This relationship is described by the Lorentz equation of force.
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 generally consistent, though it may slightly decrease at the beginning and end of the stroke.
The moving element of the actuator can be either the coil assembly or the permanent magnetic field.
Voice coil actuators are typically of the moving coil type, featuring a coil wound around a bobbin. This assembly moves in and out of a permanent magnetic field enclosed by a steel housing.
Another commonly used voice coil actuator is the moving magnet design. In this setup, the magnet assembly moves while the coil remains stationary. The actuator typically features a permanent magnetic field attached to a shaft and includes end caps with bearings, often providing an integrated bearing system.
The design characteristics of voice coils include:
Since a speaker needs to move freely and frequently, its components must be lightweight. This reduces inertia, allowing for accurate reproduction of high-frequency sounds. Voice coils need to be both delicate and robust enough to withstand the stresses from distorted audio signals or very loud volumes.
The delicate nature of voice coils necessitates a trade-off with strength. Excessive current or distorted audio signals can damage the voice coil over time. Therefore, achieving a balance between construction quality and durability is essential.
Power handling refers to the heat resistance of various components in a voice coil, including the wire, insulation, adhesives, and former material. Many subwoofers and speakers are designed with cooling features, such as heat sinks or vented pole pieces, to manage heat dissipation.
These features help dissipate heat from the voice coil, thereby enhancing its power handling capacity.
The position of the voice coil within the magnetic gap also contributes to the cooling process. Since the voice coil experiences Ohmic heating—where it heats up due to excessive current—a ribbon wire, which is a flattened type of wire, can be used for winding the voice coil. This design improves packing density within the magnetic gaps compared to coils wound with round wires, enhancing heat dissipation.
Some coils are constructed with a surface-sealed bobbin and collar materials that can be immersed in ferrofluid. This ferrofluid aids in cooling the voice coil by conducting heat away from it and transferring it into the magnetic structure.
Applying excessive power at low frequencies can cause the voice coil to move beyond its normal limits, leading to distortion or mechanical damage.
Copper wire is commonly used for winding voice coils in speakers because it conducts current efficiently and can handle high temperatures. Additionally, copper enables the production of reliable, versatile voice coils at a cost-effective price.
To achieve very high maximum sensitivity or enhanced frequency response, an all-aluminum wire can be used in the voice coil to reduce the overall moving mass. Aluminum wire weighs about one-third as much as copper wire, but it has only around 61% of the conductivity of copper.
To match the functionality and power handling of a copper wire, an aluminum wire coil must have a cross-sectional area 56% larger than that of the copper coil. Both aluminum and copper wires have their respective advantages and drawbacks depending on the design and application of the speaker. The choice between the two often depends on the specific requirements of the application.
In traditional speakers, paper bobbins were used to wind voice coils, which were suitable for their lower power levels. As more powerful amplifiers were introduced, paper bobbins were replaced with alloy 1145 aluminum foil. This new material improved the voice coils' ability to handle higher power levels. Today, some high-fidelity loudspeaker voice coils can endure operating temperatures of up to 302°F (150°C) or even 356°F (180°C).
Professional speakers utilize advanced thermoset composite materials to enhance voice coil durability under extreme thermal and mechanical stresses, including temperatures exceeding 572°F (300°C). Aluminum is used for its low cost, strong structural properties, and ease of bonding.
With the advent of higher power amplifiers, the limitations of aluminum have become more apparent. While it efficiently transfers heat from the voice coil into the adhesive bonds of the loudspeaker, this process can lead to thermal degradation or burning of the voice coils.
The movement of aluminum bobbins within the magnetic gap can generate eddy currents, which increase temperature and compromise the long-term durability of the voice coils. Although polyimide plastic film was developed as an alternative and does not have the drawbacks of aluminum, it has its own limitations, such as higher cost and a tendency to soften when heated.
To address the softening issue in professional speakers, Hisco P450 was developed. This thermoset composite combines thin glass fiber cloth impregnated with polyimide resin, merging the benefits of polyimide with the temperature resistance and stiffness of glass fiber. Hisco P450 can endure harsh operating temperatures up to 572°F (300°C) and physical stresses, while its stiffness helps maintain the speaker's frequency response at lower temperatures.
In voice coil windings, the wire is typically copper with an electrical insulation coating or, in some cases, an adhesive overcoat. Anodized aluminum flat wire may also be used. Anodization provides an insulating oxide layer that is more resistant to dielectric breakdown compared to the enamel coating found on other voice coil wires.
This approach results in lightweight voice coils with low inductance, making them ideal for small, extended-range speakers. However, these speakers face a primary power limitation due to the thermal softening point of the adhesives used to bond the wire to the bobbin or to attach the bobbin to the spider and the voice coil.
Impedance refers to the resistance a voice coil presents to the current flowing through it. There is an inverse relationship between voice coil impedance and current: as impedance decreases, more current flows through the voice coil.
For example, a 2-ohm voice coil offers less resistance than a 4-ohm voice coil. This lower resistance allows the amplifier to transfer more power to the speaker or subwoofer.
Speakers and other components with voice coils can be configured in various ways, affecting their resistance or impedance. For instance, speakers from the same manufacturer may feature a single voice coil configuration but offer different impedance options.
Speakers with dual voice coil configurations can also have varying impedances. Single voice coil equipment is simpler to wire, whereas dual or multiple voice coil setups offer more wiring options, making them more versatile in matching the power handling capabilities of the subwoofer or speaker with the amplifier.
For a component containing a voice coil, such as a speaker, to accurately reproduce sound, the voice coil must remain properly suspended within the magnetic gap between the center pole piece and the top plate.
The voice coil must move freely within its range but not so far as to lose interaction with the permanent magnet of the speaker and the varying magnetic field created by the voice coil current.
The motor arrangement of a speaker requires that the voice coil remains centered within the magnetic gap while allowing limited up-and-down movement. This setup involves a rigid cylinder around which the voice coil is wound, maintaining its circular shape. The voice coil is also centered by the diaphragm's inner ring, supported by the driver surround or spider suspension system.
Materials used for the former can vary based on power handling requirements. For lower-rating drivers, paper is commonly used, while higher-power loudspeakers often utilize materials like Kapton or Nomex. The choice of material also affects the tonal quality of the driver’s output, which is particularly important for larger drivers such as subwoofers.
To ensure the voice coil can move freely within the gap over time, it's crucial that the gap remains free of obstacles. This is where the dust cap plays a vital role. Additionally, like the former, different materials used in the dust cap can influence the tonal characteristics of the sound produced by the driver, which is important for a wide range of drivers.
Dust caps can be made from materials such as paper, rubber, or rigid plastics. Beyond the material, the shape of the dust cap also affects the tonal character of the driver. Variations include simple domes, domes with secondary concentric cones, and corrugated discs, each contributing differently to the driver's sound output.
The choice of dust cap shape influences variations in output across different frequencies within the driver's pass band.
This coil, typically made of copper or aluminum wire and connected to the driver's terminal, conducts and transfers current driven by the amplifier's source signal within the sound system. The current flowing through the wire generates a magnetic field.
In physics, two magnets exert forces on each other through their magnetic fields. This principle also applies to voice coils. In a speaker driver, the permanent magnet's field interacts with the magnetic field induced by the current flowing through the voice coil wire. This interaction results in a force between the magnet and the voice coil, similar to the force between two magnets.
The term “voice coil” has become standardized to describe any galvanometer-like mechanism that uses a solenoid to move an object back and forth within a magnetic field. Generally, it refers to a coil of wire that controls the movement of read-write heads in a moving-head disk drive.
In this application, a lightweight coil of wire is mounted within a strong magnetic field generated by permanent rare earth magnets. This voice coil is part of the motor in the servo system, responsible for positioning the read-write heads. An electric control signal drives the voice coil, producing a force that enables precise and rapid positioning of the heads.
The specifications for a voice coil include the force constant, linear stroke, peak force, torque constant, and peak torque. The force constant indicates the force generated by the voice coil actuators per ampere-turn of coil excitation, a principle that also applies to voice coil motors. It is typically specified in pounds per ampere-turn or Newtons per ampere.
The maximum and continuous force generated by a linear voice coil actuator or voice coil motor is known as the peak force. The torque constant refers to the torque produced by a voice coil motor per ampere of coil excitation. This torque constant is typically expressed in pounds per ampere or other equivalent units.
Angular stroke refers to the maximum angle of displacement in rotary voice coil actuators and rotary voice coil motors. Peak torque is a crucial factor to consider when selecting a rotary voice coil device. The electrical time constant is the time it takes for the voice coil current to reach 63% of its final value, measured when the actuator is subjected to a step input voltage.
Voice coil designs focus on creating a consistent force on the coil and ensuring that the driver accurately reproduces the applied signal. The main types of voice coils are linear and rotary, and they come in two design configurations: overhung and underhung.
Linear voice coils provide precise linear motion over short distances, while rotary voice coils offer accurate circular motion over short angles.
This common design features voice coils with a height greater than the magnetic gap’s height. This configuration maintains a consistent number of windings within the magnetic field or magnetic flux throughout the coil's normal excursion range. These voice coils generally have a high mass and offer a sensitivity range from low to medium.
In this design, the voice coil exhibits soft non-linearity when it exceeds its limits. A portion of the coil extends beyond the top of the magnetic gap in the motor structure, while the remainder is contained within the gap. This configuration allows for greater cone movement and better cooling of the voice coil, as part of it is not confined within the gap. However, the added mass of overhung coils, due to the larger coil winding, increases the overall weight. This makes it more challenging for the coil to move with the magnetic force, leading to lower sensitivity in speakers or audio output.
This design, commonly used in high-end speakers, features a voice coil with a height smaller than the magnetic gap height. This configuration maintains a consistent magnetic field or magnetic flux throughout the voice coil's normal excursion range. Underhung voice coils have a low mass and offer sensitivity ranging from medium to high. They also exhibit hard non-linearity when the voice coil exceeds its functional limits.
Voice coil considerations can be categorized into configuration considerations and wire considerations.
Configuration considerations include:
The outside face of the blank is coated with a thin layer of B-staged thermoset adhesive, selected based on the required strength, temperature tolerance, and mechanical and electrical properties. These blanks are typically pre-coated by a converter, a company specializing in coating, slitting, stocking, and distributing voice coil winding materials.
These bobbins may have spiral or butt joints, often with a slit or gap in the joint. The butt gap typically ranges from about 1/64” (1.6 mm) to 1/32” (2 mm) wide, depending on the diameter of the voice coil. This gap can expand during high-temperature operation. To enhance the roundness of the bobbins, spiral bobbins are used when tight gaps are needed.
At first glance, non-electrically-conductive bobbin substrates don't require a gap, as overlapping does not create shorted turns. However, electrically conductive materials can cause issues like high eddy currents, distortion, rocking, and heating due to shorted turns. To minimize distortion and increase damping, high-quality speakers often use aluminum bobbins with techniques to reduce eddy currents, such as copper caps and shorting rings. Despite this, most bobbins are designed to accommodate the 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 design enhances cooling within the system but can reduce the air cavity pressure behind the dust cap. The process also offers additional benefits, such as reducing mass and damping torsion resonances in the bobbin.
Excessive air velocity through the vents can cause loud whistling due to turbulence. This issue may also arise from partial blockage of the vents during large excursions. If the vent is too large, it could lead to buckling, potentially compromising the structural integrity of the bobbin.
Collars are one or more turns of material placed between the coil stack and the neck of the joint. They serve to properly dress the lead-out wires from the coil, though these wires may also be glued directly to the bobbin. Additionally, collars help enhance adhesion and maintain the roundness of the bobbin.
Additionally, collars can provide extra wall strength to the bobbin, insulate against temperature differences between the bobbin and the cone, and serve as a correction factor to ensure a proper fit for bobbins that may be too small for the stock cone's inner diameter.
Aluminum bobbins are commonly used for their excellent thermal transfer properties. However, a potential drawback is excessive heating at the neck joint. This heating results not only from the material's thermal conductivity but also from self-heating effects caused by eddy currents. Therefore, it is crucial to select appropriate adhesives if high temperatures are expected at the neck joint.
| 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, eddy currents are not uniform due to the divergence of the current induced at each end of the sheet. This variation creates a tipping or canting force on the voice coil, which is proportional to the cone's velocity.
The wire considerations for voice coils include:
Magnet wire comes in various sizes, insulation coatings, and cross-sectional profiles. The insulation layer, known as the base coat, can be topped with an adhesive bond coat. Typically, the wire is pre-coated with adhesive, which is then reactivated during the winding process. Alternatively, the wire can be wet wound, where the top coat adhesive is applied to the wire that does not have an adhesive coating.
This process occurs during winding. While copper wire is commonly used for speakers, aluminum and copper-clad aluminum wire are sometimes utilized as well. Copper, being more conductive than aluminum, necessitates a larger diameter wire to achieve the same conductivity if aluminum is replaced with copper. Additionally, copper provides superior strength and is easier to solder compared to aluminum.
Special fluxes or mechanical connection techniques are required to join lead-out wires to aluminum voice coils. One advantage of aluminum is its lower mass and greater conductivity per unit weight compared to copper. However, aluminum is prone to work hardening at relatively high operating temperatures, which can lead to brittleness and wire failure.
These variations depend on the type of speaker being produced. Differences can include variations in gauge or the types of fabric strands woven with the wire. For high-powered tweeters and compression drivers, flat conductor strips made from materials like phosphor bronze or beryllium alloy are commonly used.
Options for the appearance and configuration of load-out wires can be chosen based on factors such as wire spacing and the inclusion of a collar between the coil stack and the neck joint.
Most voice coils use round wire, but high-performance speakers often employ flattened wire wound on edge. This design offers several advantages, including a higher wire density in the gap and reduced inductance. A single layer of flat wire has less inductance compared to multiple layers of round wire. The flattened wire can be adjusted to various aspect ratios, with 4:1 and 5:1 being the most common.
Flat wire offers several advantages over round wire coils, including increased wire density in the magnetic gap and reduced inductance. When flattened and coated with insulation, it can significantly enhance performance. Full magnet wire production capabilities are required to achieve this. Square wire is another option that also provides high wire density and avoids issues associated with single-layer flat wire.
This chapter will explore the applications and advantages of voice coils.
In addition to loudspeakers, voice coil actuators are utilized in various applications, including focusing mechanisms, oscillatory systems, mirror tilting, and miniature position control.
Voice coils offer several benefits, including a simple design and straightforward construction process. They exhibit low hysteresis and are compact, allowing for an efficient design footprint. Additionally, voice coils provide high acceleration and are free from cogging or commutation issues.
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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