This article provides an in-depth discussion of voice coils. You will learn:
This chapter delves into the design attributes and operational roles of voice coils.
A voice coil is a coil of wire, usually composed of copper, aluminum, or copper-clad aluminum, wrapped around a bobbin or a former. It attaches to the speaker cone's apex. The interaction between the voice coil and the motor structure's magnetic field results in the movement of the speaker cone.
Current flowing through a speaker or subwoofer's voice coil produces a magnetic field within the gap between the voice coil and magnet structure, prompting the coil's movement. When a music waveform is introduced, the cone replicates the audio signal.
In a voice coil actuator, the current interacts with inductors within a permanent magnetic field, generating force. This force is proportional to the product of the magnetic flux and the current passing through the coil, as explained by the Lorentz force equation.
F = B × I
Where \( F \) is the Force (N), \( B \) is the Magnetic Flux Density (Tesla), and \( I \) is the Current (Amps).
The force created throughout the actuator's stroke is typically constant, with slight reduction at the start and end.
The actuator's moving element can either be the coil assembly or the permanent magnetic field.
Most voice coil actuators utilize a moving coil design, featuring a coil wound on a bobbin, which moves in and out of a permanent magnetic field that is enclosed in a steel housing.
The moving magnet design is another common voice coil actuator type, where the magnet assembly moves while the coil remains stationary. This actuator is typically equipped with a shaft-mounted permanent magnetic field and features end caps with bearings, usually providing an integrated bearing system.
The design specifications of voice coils include:
To move freely and frequently, a speaker's components should be lightweight, minimizing inertia and enabling accurate high-frequency sound reproduction. Voice coils must be delicate yet strong enough to endure the stress of distorted signals or high volumes.
A balance between delicacy and strength is necessary due to the potential for damage by excessive current or distorted signals, thus construction quality and durability must be optimized.
Power handling describes the heat endurance of various voice coil components, like wire, insulation, adhesives, and former material. To manage heat dissipation, many subwoofers and speakers incorporate cooling features, such as heat sinks or vented pole pieces.
These cooling mechanisms aid in expelling heat from the voice coil, thus improving its power handling capabilities.
The voice coil's position in the magnetic gap also enhances cooling. Due to Ohmic heating—where excessive current increases heat—a ribbon wire, a flattened wire type, can be wound to improve packing density and heat dissipation compared to round wire coils.
Some coils feature a surface-sealed bobbin and collar that can be submerged in ferrofluid, aiding the cooling process by transferring heat into the magnetic structure.
Applying excessive power at low frequencies can push the voice coil beyond normal limits, potentially causing distortion or mechanical damage.
Copper wire is favored for voice coil winding in speakers because it efficiently conducts electricity and withstands high temperatures, making it cost-effective and versatile.
To enhance sensitivity or frequency response, all-aluminum wire may be employed to reduce moving mass. Aluminum wire weighs substantially less than copper, though its conductivity is only 61% of copper's.
To achieve the same functionality and power handling as copper, an aluminum wire coil requires a 56% larger cross-sectional area. The decision between copper and aluminum depends on the speaker's specific design and application needs.
Historically, paper bobbins were used for winding voice coils in traditional speakers, but with the advent of powerful amplifiers, alloy 1145 aluminum foil replaced paper, enhancing higher power level durability. Today, high-fidelity loudspeaker voice coils can withstand temperatures up to 302°F (150°C) or even 356°F (180°C).
Professional speakers utilize advanced thermoset composite materials to endure extreme thermal and mechanical stresses, tolerating temperatures above 572°F (300°C). Aluminum offers low cost, strong structural properties, and ease of bonding.
Increased amplifier power has highlighted aluminum's limitations. While it transfers heat efficiently to the loudspeaker's adhesive bonds, this could lead to thermal degradation or burning of voice coils.
The movement of aluminum bobbins in the magnetic gap can produce eddy currents, elevating temperature and risking long-term voice coil durability. Despite polyimide plastic film being a developed alternative without aluminum's drawbacks, it is expensive and softens when heated.
Hisco P450 was introduced to counteract this softening in professional speakers. This thermoset composite combines thin glass fiber cloth and polyimide resin, merging polyimide benefits with glass fiber's temperature resistance and stiffness. Hisco P450 withstands high temperatures up to 572°F (300°C) and physical stress, maintaining frequency response at lower temperatures.
Voice coil winding wires are typically copper with an insulated coating or an adhesive layer. Anodized aluminum flat wire may also be utilized, providing an insulating oxide layer more resistant to dielectric breakdown than enamel coatings on other wires.
This results in lightweight, low-inductance voice coils suitable for small, extended-range speakers, although their primary power limitation comes from adhesives' thermal softening point.
Impedance describes the resistance a voice coil offers to passing current. There’s an inverse relationship between coil impedance and current: as impedance lowers, current increases through the coil.
A 2-ohm voice coil exerts less resistance than a 4-ohm coil, enabling the amplifier to deliver more power to the speaker or subwoofer.
Voice coil-equipped components like speakers can have various configurations impacting their impedance or resistance. For example, speakers may have a single voice coil configuration but different impedance options from the same manufacturer.
Speakers with dual voice coil configurations may also vary in impedance. Single voice coil equipment is simpler to wire, while dual or multiple coil setups provide versatile wiring options to match power handling capabilities with the amplifier.
To faithfully reproduce sound, a component with a voice coil, like a speaker, must ensure the voice coil stays suspended within the magnetic gap between the center pole piece and the top plate.
The voice coil should move freely but not enough to disengage from the speaker's permanent magnet and the voice coil current-induced varying magnetic field.
A speaker's motor setup requires the voice coil to stay centered within the magnetic gap while allowing limited vertical movement. Wounded around a rigid cylinder, this maintains the coil’s circular shape. It is also centered by the diaphragm's inner ring, supported by the driver surround or spider suspension mechanism.
The former's material varies with power handling necessities; paper is common for lower-rating drivers, while high-power loudspeakers often use materials like Kapton or Nomex. Material choice influences large drivers' tonal quality, significant for devices such as subwoofers.
It's critical for the gap to stay clear of obstructions to allow the voice coil's free movement over time, and the dust cap fulfills this role. Like the former, various dust cap materials affect sound production's tonal characteristics.
Materials used for dust caps include paper, rubber, or sturdy plastics. The dust cap's shape also impacts the driver's tonal quality, with options such as simple domes, domes with secondary cones, and corrugated discs each contributing uniquely to the driver's sound.
The dust cap's shape alters output variability across the driver’s pass band's different frequencies.
Typically comprising copper or aluminum wire connected to the driver's terminal, this coil channels and transfers the current from the amplifier's source signal within a sound system, generating a magnetic field through the wire.
Two magnets wield forces upon each other through their fields. This principle applies to voice coils in speaker drivers, where a permanent magnet’s field engages with the current-induced magnetic field in the coil, acting much like two magnets.
The "voice coil" term now generally describes any solenoid-based movement mechanism within a magnetic field, such as the coil controlling read-write heads in disk drives.
In this context, a lightweight wire coil sits within a robust field generated by rare earth magnets, forming part of the motor in the servo system that precisely positions read-write heads. The coil, driven by an electric control signal, produces force to accurately position the heads.
Voice coil specifications include force constant, linear stroke, peak force, torque constant, and peak torque. The force constant measures force production per ampere-turn of coil excitation, applicable to voice coil motors, specified in units like pounds or Newtons per ampere-turn.
The peak force denotes the maximum and continuous force from a linear voice coil actuator or motor, while the torque constant indicates the produced torque per ampere of coil excitation, typically in pounds per ampere or similar units.
Angular stroke measures rotary actuators' and motors’ maximum displacement angle, with peak torque a pivotal factor in selecting rotary devices. The electrical time constant tracks how quickly voice coil current reaches 63% of its final value under step input voltage conditions.
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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