What Is a Speaker Voice Coil?

A voice coil is a coil of wire positioned inside the magnetic gap of a loudspeaker.

When an audio current flows through the coil, it interacts with the magnetic field produced by the permanent magnet. This interaction creates a force that moves the coil.

The coil is mechanically connected to the speaker diaphragm, so the movement of the voice coil causes the cone or diaphragm to move.

Amplifier
    │
    ▼
Electrical current
    │
    ▼
Voice coil
    │
    ▼
Magnetic force
    │
    ▼
Mechanical movement
    │
    ▼
Speaker cone
    │
    ▼
Sound

The Voice Coil Is the Speaker Motor

A dynamic loudspeaker can be thought of as an electromagnetic motor. The permanent magnet provides the magnetic field and the voice coil provides the controlled magnetic field that interacts with it.

The force generated by the motor is related to:

F = B × I × L

where:

• F = force
• B = magnetic flux density
• I = current through the coil
• L = effective conductor length in the magnetic field

The BL product is commonly used to describe the motor strength of a loudspeaker.

Basic Voice-Coil Construction

A conventional voice coil consists of several main parts:

• Voice-coil wire
• Former
• Adhesive
• Connection leads
• Mechanical attachment to the diaphragm

The wire is wound into a cylindrical coil around the former.

        Voice-coil winding

       ┌───────────────┐
       │ ))))))))))))) │
       │ ))))))))))))) │
       │ ))))))))))))) │
       └───────────────┘
              │
            Former

The completed coil is positioned in the magnetic gap with very small clearance between the coil and the surrounding magnetic structure.

The Voice-Coil Former

The former supports the winding and maintains the shape of the coil.

It must withstand mechanical movement, heat and the forces generated during operation.

Common former materials include:

• Paper
• Polymer films
• Aluminium
• Other heat-resistant engineered materials

The choice of former material affects thermal behaviour, mechanical strength, mass and manufacturing cost.

Voice-Coil Wire

The wire used for a voice coil must have suitable electrical, mechanical and thermal properties.

Copper is widely used because of its good electrical conductivity and availability.

Aluminium wire can also be used in some designs because its lower density can help reduce moving mass.

The wire is normally coated with electrical insulation so that adjacent turns do not electrically short together.

Why the Wire Is Insulated

A voice coil contains many turns of wire positioned very close together.

If the wire were bare metal, adjacent turns would touch electrically and bypass the intended winding.

The insulation allows the turns to remain electrically separate while still forming one continuous coil.

Turn 1  ─────────────
          insulation
Turn 2  ─────────────
          insulation
Turn 3  ─────────────

The insulation must tolerate the operating temperature of the coil.

Single-Layer Voice Coil

A single-layer voice coil uses one layer of wire around the former.

It can provide low moving mass and a relatively simple construction.

The achievable electrical resistance depends on the wire diameter, number of turns and mean circumference of the winding.

Multi-Layer Voice Coil

A multi-layer voice coil uses several layers of wire.

Adding layers allows more wire to be placed in a given axial length, which can increase the total conductor length and therefore the electrical resistance.

Multi-layer designs can also provide a larger amount of conductor inside the magnetic field, increasing motor force for a given current.

However, additional winding mass and thermal considerations must be taken into account.

Voice-Coil Diameter

Voice coils are available in many diameters.

A larger voice coil can provide advantages such as:

• Greater thermal capacity
• Larger winding area
• Potentially greater power handling
• Improved mechanical robustness

However, a larger coil can also increase moving mass.

The ideal diameter therefore depends on the application.

Common Voice-Coil Sizes

There is no universal set of voice-coil sizes, but common designs include approximately:

Actual sizes vary considerably between manufacturers and applications.

Voice-Coil Winding Height

The axial height of the winding is an important design parameter.

It determines how much of the conductor remains within the magnetic gap as the coil moves.

The relationship between winding height and magnetic-gap height affects the driver's excursion capability and motor behaviour.

Short-Coil and Long-Coil Designs

Voice coils can be broadly described according to how their winding height relates to the magnetic gap.

In a short-coil design, the winding is shorter than the magnetic gap.

In a long-coil design, the winding extends beyond the magnetic gap.

Long-coil designs can provide greater linear excursion because more winding remains available to interact with the magnetic field as the coil moves.

However, the exact performance depends on the magnetic circuit and geometry.

Voice-Coil Impedance

The resistance of the voice-coil wire contributes to the speaker's electrical impedance.

The coil also has inductance.

Therefore, the voice coil is not simply a resistor.

Voice coil:

Resistance + Inductance

      R + jX

The inductive reactance is:

XL = 2πfL

As frequency increases, the inductive component becomes more significant.

DC Resistance of a Voice Coil

The DC resistance is commonly represented by Re in loudspeaker specifications.

It depends primarily on:

• Wire material
• Wire diameter
• Total wire length
• Temperature

A thicker wire generally has lower resistance per unit length.

A longer wire generally has greater resistance.

Calculating Voice-Coil Resistance

The resistance of a wire can be estimated from:

R = ρL / A

where:

• R = resistance
• ρ = resistivity of the conductor
• L = wire length
• A = conductor cross-sectional area

This relationship explains why wire diameter, wire length and material all affect voice-coil resistance.

Voice-Coil Resistance and Speaker Impedance

An important distinction is:

Voice-coil DC resistance
        ≠
Speaker nominal impedance

For example, a speaker marketed as 8 Ω may have a voice-coil DC resistance of several ohms.

The nominal impedance includes the frequency-dependent behaviour of the complete driver.

Why Voice Coils Heat Up

Electrical power is dissipated in the voice-coil resistance.

P = I²R

As current increases, heating increases rapidly because power is proportional to the square of the current.

The heat must be transferred from the winding to the former, magnetic structure and surrounding air.

Voice-Coil Temperature

Copper and aluminium conductors have temperature-dependent resistance.

As the voice coil becomes hotter, its resistance generally increases.

This affects the current drawn from the amplifier and changes the electrical behaviour of the speaker.

At high power levels, this can contribute to power compression.

Power Compression

Power compression occurs when increasing electrical input power produces a smaller-than-expected increase in acoustic output.

Voice-coil heating is one important cause.

As the coil becomes hotter:

Temperature increases
        │
        ▼
Voice-coil resistance increases
        │
        ▼
Electrical behaviour changes
        │
        ▼
Acoustic output becomes less proportional
to additional input power

Voice-Coil Cooling

Effective heat removal is important in high-power speakers.

Cooling can occur through:

• Conduction through the former
• Heat transfer to the magnetic structure
• Air movement around the coil
• Ventilation through the pole piece
• Ventilation through the magnet structure
• Movement of air caused by the cone

Professional high-power drivers often use carefully designed magnetic and ventilation systems to improve thermal performance.

Voice-Coil Gap

The voice coil operates inside a narrow magnetic gap.

The clearance between the coil and magnetic structure must be carefully controlled.

Too much clearance can reduce magnetic coupling.

Too little clearance can cause the coil to rub against the pole piece or top plate when the speaker moves or becomes mechanically misaligned.

Voice-Coil Rubbing

Voice-coil rubbing occurs when the coil contacts the magnetic structure.

Typical symptoms include:

• Scratchy sound
• Buzzing
• Mechanical scraping
• Distortion
• Noise when the cone is moved by hand

Possible causes include:

• Damaged suspension
• Over-excursion
• Misaligned voice coil
• Damaged former
• Deformed cone
• Magnet assembly movement

Voice-Coil Alignment

The voice coil must remain centered in the magnetic gap.

The spider and surround help maintain this alignment during cone movement.

Correct:

     │   │
     │coil│
     │   │
   ┌─┴───┴─┐
   │ gap   │


Incorrect:

      ╲coil
       ╲
        ╲
   ┌────────┐
   │  gap   │

Poor alignment can cause rubbing, distortion and reduced efficiency.

Voice-Coil Lead Wires

Flexible lead wires connect the moving voice coil to the stationary speaker terminals.

These wires must flex repeatedly as the cone moves.

They therefore need suitable flexibility and mechanical attachment.

A damaged lead wire can cause intermittent operation or an open speaker circuit.

Single Voice Coil Speakers

A single-voice-coil speaker has one electrical winding.

It normally has two external terminals.

Terminal (+)
     │
     ▼
Voice coil
     │
     ▼
Terminal (-)

This is the most common arrangement for conventional loudspeakers.

Dual Voice Coil Speakers

A dual voice coil (DVC) speaker has two electrically separate voice coils on the same moving assembly.

Each coil normally has its own pair of terminals.

        ┌── Coil 1 ──┐
        │             │
Cone ───┤             │
        │             │
        └── Coil 2 ──┘

DVC speakers are particularly common in subwoofers.

The two coils can be wired in series or parallel to obtain different overall impedances.

Dual Voice Coil Wiring

Suppose a dual-voice-coil subwoofer has two 4 Ω coils.

Series connection:

4 Ω + 4 Ω = 8 Ω

Parallel connection:

4 Ω || 4 Ω = 2 Ω

This allows the same driver to be configured for different amplifier loads.

The amplifier must still be capable of operating safely at the chosen impedance.

Voice-Coil Wire Gauge

The diameter of the voice-coil wire affects:

• Electrical resistance
• Current capability
• Thermal behaviour
• Winding size
• Moving mass

A thicker wire generally has lower resistance and can carry greater current, but it also occupies more space and may increase moving mass.

Voice-coil design is therefore a balance between electrical, mechanical and thermal requirements.

Number of Turns

The number of turns affects both electrical resistance and motor characteristics.

More turns generally mean:

• More conductor length
• Higher resistance
• Greater inductance
• Potentially greater conductor length in the magnetic field

The exact relationship depends on the coil geometry and magnetic circuit.

Voice-Coil Winding Height and Xmax

The relationship between the winding height and magnetic-gap height strongly affects the linear excursion of the driver.

As the coil moves, the amount of conductor inside the strongest part of the magnetic field changes.

A carefully designed motor maintains a relatively constant BL product over the intended excursion range.

BL Product

The BL product is one of the important motor parameters of a loudspeaker.

BL = B × L

A stronger magnetic field and greater effective conductor length in the field can increase the motor force.

For a given current:

F = BL × I

The BL product is normally specified in N/A.

Voice Coil and Moving Mass

The voice coil contributes to the moving mass of the speaker.

A heavier coil can influence:

• Resonant frequency
• High-frequency response
• Transient behaviour
• Required driving force
• Efficiency

The designer must balance coil size and thermal capability against moving mass.

Voice-Coil Inductance

The voice coil behaves partly like an inductor.

Its inductance causes the electrical impedance to rise as frequency increases.

XL = 2πfL

For example, if the voice coil has 1 mH inductance:

At 100 Hz:

XL ≈ 0.63 Ω


At 1 kHz:

XL ≈ 6.28 Ω


At 10 kHz:

XL ≈ 62.8 Ω

The actual impedance of a real speaker is more complicated because the coil's inductance can also vary with frequency and position, and the mechanical system contributes to the overall impedance.

Voice Coil and Frequency Response

Voice-coil inductance can influence the high-frequency response of a driver.

As frequency increases, the electrical impedance can increase, reducing current for a given amplifier voltage.

This is one of the reasons a conventional woofer generally becomes less efficient at very high frequencies.

Why Woofers Have Large Voice Coils

Woofers are often expected to handle significant amounts of power. Their voice coils therefore need to tolerate substantial current and heat.

Large voice coils can provide:

• Greater winding area
• Greater thermal mass
• Improved heat dissipation
• Greater mechanical strength

However, increasing the size of the moving assembly can also increase moving mass.

Why Tweeter Voice Coils Are Small

Tweeters must respond to high-frequency signals.

Low moving mass is therefore particularly important.

Tweeter voice coils are typically much smaller than large woofer voice coils.

Their power handling is usually lower, but their small moving mass allows rapid movement.

Voice Coil and Speaker Resonance

The voice coil contributes to the moving mass, while the spider and surround provide mechanical compliance.

Together they form a mechanical resonant system.

A simplified relationship is:

Fs = 1 / (2π√(Mms × Cms))

Increasing moving mass tends to lower resonance, while increasing suspension stiffness tends to raise resonance.

Voice-Coil Adhesives

The winding must remain firmly attached to the former during repeated high-speed movement.

Adhesives used in speaker construction must withstand:

• Heat
• Mechanical vibration
• Repeated flexing
• Electrical insulation requirements

Poor bonding can allow the winding to loosen or deform during operation.

Voice-Coil Overheating

A severely overheated voice coil can suffer permanent damage.

Possible symptoms include:

• Burning smell
• Increased distortion
• Changed impedance
• Open circuit
• Voice-coil rubbing
• Reduced output

If the winding insulation or adhesive is damaged, the speaker may fail even after it has cooled down.

How a Burned Voice Coil Can Fail

A voice coil can fail in several ways.

Testing a Voice Coil With a Multimeter

A basic resistance test can reveal obvious electrical faults.

Set the multimeter to resistance mode and measure across the speaker terminals.

A normal speaker should show a finite resistance.

A completely open circuit may indicate a broken voice coil, lead wire or connection.

An unusually low resistance may indicate a shorted winding or another problem, although the expected value depends on the speaker design.

Multimeter Testing Is Not Enough

A resistance measurement cannot determine whether:

• The voice coil is correctly centered.
• The coil rubs against the magnetic gap.
• The coil has shorted turns.
• The speaker has the correct frequency response.
• The suspension is healthy.
• The driver has the correct Thiele-Small parameters.

Additional mechanical and impedance tests may therefore be required.

Voice-Coil Polarity

The direction of movement of the voice coil depends on the polarity of the electrical signal.

When multiple speakers operate together, maintaining correct polarity helps ensure that their acoustic outputs combine correctly.

A simple polarity test can be performed on some conventional speakers using a small DC source for a very brief moment, observing the direction of cone movement.

This should only be done briefly and at a very low voltage appropriate for the driver. Never use a high-power supply for this test.

Voice-Coil Repair

A damaged voice coil can sometimes be replaced by reconing the speaker.

A recone kit may contain:

• Replacement cone
• Voice coil
• Former
• Spider
• Surround
• Dust cap
• Adhesives

The new voice coil must be accurately centered in the magnetic gap before the adhesives fully cure.

Why Voice-Coil Centering Is Critical During Repair

The magnetic gap is narrow.

Even a small alignment error can cause the coil to rub.

During reconing, the coil is normally centered mechanically while the adhesive connections are made.

After curing, the cone should move smoothly without scraping.

Voice-Coil Diameter vs Power Handling

A larger voice coil often allows greater thermal capacity, but diameter alone does not determine power handling.

Other factors include:

• Wire size
• Winding height
• Former material
• Magnetic-gap design
• Ventilation
• Cooling
• Adhesive system
• Operating frequency

Voice-Coil Design Is a Trade-Off

Designing a voice coil involves balancing several competing requirements.

Larger wire
   │
   ├── Lower resistance
   ├── Higher current capability
   └── More space / mass


More turns
   │
   ├── Higher resistance
   ├── More conductor in magnetic field
   ├── Greater inductance
   └── More winding mass


Larger coil
   │
   ├── Greater thermal capability
   ├── Greater mechanical strength
   └── Potentially greater moving mass

Voice Coil and Speaker Efficiency

The motor strength, moving mass, suspension and magnetic circuit all affect the efficiency of the driver.

A strong motor does not automatically make a speaker efficient. The complete electro-mechanical system must be considered.

This is why the BL product should be evaluated together with Mms, suspension characteristics, electrical resistance and other parameters.

Voice Coil and Xmax

The voice-coil geometry is closely related to the driver's linear excursion.

The objective is to keep the electromagnetic force reasonably consistent as the coil moves through the magnetic gap.

When the coil moves too far, BL can change significantly, increasing distortion.

Therefore, maximum excursion is not simply a mechanical distance. The linear behaviour of the motor is also important.

Voice-Coil Inductance and Shorting Rings

Some advanced loudspeaker motors use conductive shorting rings or similar magnetic-circuit techniques to reduce variations in voice-coil inductance and improve linearity.

These techniques can help reduce certain forms of distortion and improve high-frequency impedance behaviour.

They are particularly useful in high-performance driver designs.

Voice-Coil Former Materials

The exact performance depends on the particular material and construction.

Voice Coil in a Woofer

A woofer voice coil is designed to handle significant excursion and power while operating over a relatively low frequency range.

The design typically emphasizes:

• Thermal capacity
• Mechanical durability
• Excursion capability
• Motor strength

Voice Coil in a Tweeter

A tweeter voice coil is generally optimized for very low moving mass and rapid response.

The design emphasizes:

• Low moving mass
• High-frequency response
• Accurate magnetic coupling
• Appropriate thermal capacity

The tweeter must also be protected from excessive low-frequency energy by an appropriate crossover.

Voice Coil in a Subwoofer

Subwoofer voice coils are often designed for substantial power handling and excursion.

Large voice coils, robust formers, strong adhesives and improved cooling are commonly used in high-power designs.

Dual-voice-coil configurations are also common in subwoofers because they allow different amplifier load configurations.

Voice-Coil Parameters at a Glance

Key Takeaways

• The voice coil is the electromagnetic motor of a dynamic loudspeaker.
• It consists primarily of insulated wire wound around a former.
• The voice coil interacts with the permanent magnetic field to produce mechanical force.
• Voice-coil resistance contributes to the speaker's electrical behaviour.
• Voice-coil inductance causes impedance to increase with frequency.
• Larger voice coils can provide greater thermal capability but may also increase moving mass.
• Wire diameter and winding length determine much of the coil's DC resistance.
• Voice-coil temperature increases as electrical power is dissipated.
• Cooling is essential for high-power speaker operation.
• The voice coil must remain accurately centered in the magnetic gap.
• Dual voice coils allow different electrical wiring configurations.
• Voice-coil geometry strongly influences BL, excursion and distortion.
• A voice coil can fail electrically, mechanically or thermally.