What Does the Magnet Do in a Speaker?

The permanent magnet provides a strong, relatively constant magnetic field around the voice coil.

When electrical current flows through the voice coil, the magnetic field produced by the coil interacts with the permanent magnetic field. This interaction produces a force that moves the voice coil.

Electrical signal
       │
       ▼
   Voice coil
       │
       │ interacts with
       ▼
Permanent magnetic field
       │
       ▼
    Mechanical force
       │
       ▼
       Cone
       │
       ▼
      Sound

The Speaker Magnetic Circuit

The magnet itself is only one part of the magnetic system.

A conventional loudspeaker motor normally includes:

• Permanent magnet
• Back plate
• Centre pole
• Top plate
• Magnetic gap
• Voice coil

These components form a magnetic circuit that concentrates magnetic flux into the narrow gap where the voice coil operates.

Basic Speaker Motor Structure

                 Top plate
          ┌───────────────────┐
          │       Gap         │
          │   ┌─────────┐     │
          │   │  Coil   │     │
          │   └─────────┘     │
          └────────┬──────────┘
                   │
                Pole piece
                   │
             ┌─────┴─────┐
             │  Magnet   │
             └─────┬─────┘
                   │
               Back plate

The exact geometry varies considerably between loudspeaker designs.

Why Is the Magnetic Gap So Important?

The voice coil sits inside a narrow magnetic gap.

The objective is to produce a strong and reasonably uniform magnetic field through the portion of the voice coil that interacts with the magnetic circuit.

A stronger and more uniform magnetic field can improve the motor's ability to convert current into mechanical force.

Magnetic Flux Density

Magnetic flux density is represented by B and is measured in teslas (T).

The force acting on a conductor in a magnetic field can be simplified as:

F = B × I × L

where:

• F = force
• B = magnetic flux density
• I = current
• L = effective conductor length

This relationship explains why the magnetic field inside the speaker gap is so important.

The BL Product

Loudspeaker designers commonly use the BL product to describe an important part of the motor strength.

BL = B × L

The force can then be expressed as:

F = BL × I

A higher BL value generally means that a given current can produce greater electromagnetic force, assuming the other conditions remain comparable.

Permanent Magnet

The permanent magnet provides the static magnetic field required for the speaker motor.

Unlike an electromagnet, it does not require continuous electrical power to maintain its magnetic field.

The magnet is combined with steel components that guide and concentrate the magnetic flux through the voice-coil gap.

Common Magnet Materials

Several permanent-magnet materials have been used in loudspeakers.

• Ferrite
• Neodymium
• Alnico
• Samarium-cobalt in specialized applications

Each material has different magnetic, mechanical, thermal and economic characteristics.

Ferrite Magnets

Ferrite magnets are widely used in loudspeakers.

They are relatively inexpensive and can provide good magnetic performance for many applications.

Ferrite magnet assemblies are often physically larger and heavier than equivalent high-energy rare-earth magnet systems.

This makes ferrite particularly attractive when weight and compactness are not the primary concerns.

Neodymium Magnets

Neodymium magnets have very high magnetic energy density compared with traditional ferrite magnets.

This allows a strong magnetic circuit to be made relatively small and light.

Neodymium is therefore common in professional loudspeakers where reduced weight is valuable.

The magnetic assembly can be substantially lighter than a comparable ferrite design.

Alnico Magnets

Alnico magnets were widely used in older and classic loudspeaker designs.

They have useful magnetic properties and can provide a distinctive construction approach.

Alnico is more expensive and mechanically different from ferrite and neodymium, so it is less common in many modern mass-produced speakers.

Ferrite vs Neodymium

The magnet material alone does not determine the performance of the complete speaker motor.

Magnet Size Does Not Tell the Whole Story

A common misconception is that a larger magnet automatically means a better or more powerful speaker.

The performance of the motor depends on the complete magnetic circuit.

Important factors include:

• Magnet material
• Magnet geometry
• Magnetic flux density
• Gap geometry
• Pole-piece dimensions
• Top-plate thickness
• Voice-coil geometry
• Magnetic saturation

A well-designed compact motor can outperform a poorly designed motor with a physically larger magnet.

The Pole Piece

The pole piece is the central part of the magnetic circuit.

It guides magnetic flux toward the voice-coil gap.

The pole is positioned inside the voice coil.

Its geometry strongly influences the magnetic field distribution in the gap.

The Top Plate

The top plate is the magnetic steel component located above the magnet.

It forms one side of the magnetic gap and helps direct magnetic flux around the voice coil.

             Top plate
        ┌────────────────┐
        │     │    │     │
        │     │Gap │     │
        │     │    │     │
        └─────┼────┼─────┘
              │ Coil
              │
           Pole piece

The Back Plate

The back plate is located behind the magnet and forms part of the magnetic return path.

Together with the pole and top plate, it completes much of the magnetic circuit.

The back plate can also contribute to the mechanical strength of the motor assembly.

The Magnetic Gap

The magnetic gap is the narrow region between the pole piece and top plate where the voice coil operates.

The gap must be accurately manufactured because the voice coil moves within it with very small clearance.

The magnetic field in this region is one of the most important characteristics of the loudspeaker motor.

Why the Gap Must Be Narrow

A carefully designed narrow gap allows a strong magnetic field to be concentrated around the voice coil.

However, the clearance cannot simply be reduced without limit.

The coil must still be able to move without contacting the magnetic structure.

Manufacturing tolerances, thermal expansion, mechanical alignment and large excursion must all be considered.

Voice-Coil Clearance

The voice coil is positioned very close to the magnetic structure.

If the coil becomes misaligned, it can rub against the pole or top plate.

Possible causes include:

• Damaged spider
• Damaged surround
• Deformed voice-coil former
• Mechanical shock
• Magnet displacement
• Incorrect reconing

Magnetic Saturation

The steel components of the magnetic circuit can become magnetically saturated.

When saturation occurs, increasing magnetizing force does not produce a proportional increase in magnetic flux.

This can limit the effectiveness of further increasing magnet size or magnetic field strength.

Good motor design therefore requires the steel components to be properly dimensioned.

Why the Pole Piece Can Become Saturated

The magnetic flux is concentrated through relatively small regions of the magnetic circuit.

If the cross-sectional area of a steel component is insufficient, the magnetic flux density can become too high.

The steel then approaches saturation.

Large flux
    │
    ▼
Small steel area
    │
    ▼
High flux density
    │
    ▼
Possible saturation

Magnetic Circuit Efficiency

The purpose of the magnetic circuit is to direct as much useful magnetic flux as practical through the voice-coil gap.

Flux that leaks away from the intended path does not contribute as effectively to the motor force.

Good magnetic-circuit design therefore attempts to minimize unnecessary leakage while maintaining the required field geometry.

Magnetic Flux Leakage

Not all magnetic flux remains concentrated in the intended gap.

Some flux can escape into surrounding air or structural components. This is known as magnetic leakage or stray flux.

Motor geometry can be designed to control this behaviour.

Magnet Polarity

A permanent magnet has two magnetic poles, commonly identified as north and south.

The speaker motor uses the magnetic field between these poles to produce the required field in the gap.

The polarity of the magnet must be considered when assembling the magnetic circuit.

Magnet Orientation

The magnet must be installed with the intended orientation relative to the pole and return path.

The magnetic circuit is designed so that the flux follows the desired path through the gap.

An incorrect magnetic assembly can substantially reduce or alter the motor's performance.

Magnet Assembly

A conventional loudspeaker motor can be thought of as a stack of magnetic and steel components.

          ┌───────────────┐
          │   Top plate   │
          └──────┬────────┘
                 │
              Gap │
                 │
          ┌──────┴────────┐
          │    Magnet     │
          └──────┬────────┘
                 │
          ┌──────┴────────┐
          │  Back plate   │
          └───────────────┘
                 │
            Pole piece

Actual motor geometries can be much more sophisticated.

Magnet and Voice-Coil Force

When current flows through the voice coil, the coil experiences a force because it is inside the permanent magnetic field.

The simplified relationship is:

F = BL × I

This is the fundamental operating principle of the conventional moving-coil loudspeaker.

Alternating Current and Cone Movement

Audio is an alternating electrical signal.

As the direction of current through the voice coil changes, the direction of electromagnetic force also changes.

Current →
    │
    ▼
Coil moves outward


Current ←
    │
    ▼
Coil moves inward

The cone therefore moves back and forth according to the audio waveform.

Magnet Strength and Speaker Performance

A stronger magnetic field can increase the motor force for a given current.

However, speaker performance does not depend on magnet strength alone.

The complete system includes:

• Magnetic circuit
• Voice coil
• Cone
• Suspension
• Enclosure
• Electrical impedance

All of these factors must work together.

Magnet and Speaker Sensitivity

Motor strength can influence sensitivity, but it is not the only factor that determines sensitivity.

Moving mass, electrical resistance, suspension and acoustic loading also have important effects.

A large magnet does not automatically guarantee high sensitivity.

Magnet and Moving Mass

The magnet itself is normally part of the stationary structure.

Its mass therefore does not directly become part of the moving mass Mms.

This is one reason a powerful but heavy ferrite motor can still be used successfully in a speaker: the magnet remains stationary while the cone and voice coil move.

Ferrite Magnet Weight

Ferrite motor assemblies can be relatively heavy.

The weight can be significant in large woofers and professional speakers.

However, the weight of the stationary magnet assembly does not have the same effect on acoustic performance as adding weight to the cone or voice coil.

Neodymium Motor Advantages

The high magnetic energy density of neodymium allows designers to create strong magnetic circuits with smaller magnets.

This can significantly reduce the weight of professional loudspeakers.

For portable PA systems, this can be a major advantage.

Magnet Temperature

Permanent magnets have temperature-dependent magnetic properties.

High temperatures can reduce magnetic performance, and excessive temperature can cause permanent changes in some magnet materials.

The exact temperature limits depend on the magnet material and grade.

Thermal management is therefore important in high-power loudspeaker designs.

Neodymium and Heat

Neodymium magnets require particular attention to temperature because their magnetic properties are temperature dependent.

The allowable temperature depends on the specific grade and construction.

A speaker using a neodymium motor should therefore be designed around the manufacturer's specified temperature limits.

Ferrite and Heat

Ferrite magnets are widely used in high-power loudspeakers and have useful temperature characteristics.

However, ferrite motors are not immune to thermal effects.

Excessive heating can still affect the magnetic circuit and the speaker's overall performance.

Magnet Demagnetization

Permanent magnets can lose magnetic strength under certain conditions.

Potential causes include:

• Excessive temperature
• Very strong opposing magnetic fields
• Mechanical damage
• Improper handling

The likelihood and severity depend strongly on the magnet material and grade.

Speaker Magnet and Magnetic Gap Temperature

During high-power operation, the voice coil becomes hot.

Heat can transfer into nearby components of the magnetic assembly.

The motor must therefore be designed to tolerate the thermal environment produced by the voice coil.

Ventilated Pole Pieces

Many high-power speakers use a vent through the pole piece.

This can allow air movement behind the voice coil.

The ventilation can help remove heat and reduce air compression behind the moving coil.

        Cone
          │
       Voice coil
          │
          ▼
      ┌───────┐
      │  Gap  │
      └───┬───┘
          │
       Pole vent
          │
          ▼
       Air flow

Magnetic Shorting Rings

Some advanced loudspeaker motors use conductive rings or sleeves to modify the behaviour of the magnetic circuit.

These components can help reduce variations in voice-coil inductance and certain forms of nonlinear behaviour.

They are commonly found in higher-performance driver designs.

Faraday Rings

A conductive shorting ring is often referred to as a Faraday ring in loudspeaker motor design.

It can reduce changes in voice-coil inductance with position and frequency and can help reduce certain types of distortion.

The exact effect depends on the motor geometry and implementation.

Underhung and Overhung Motors

Voice-coil geometry can be classified broadly into underhung and overhung designs.

The difference relates to the relationship between the voice-coil winding height and magnetic-gap height.

Overhung Voice Coil

In an overhung design, the voice-coil winding is longer than the magnetic gap.

A significant portion of the winding remains outside the strongest region of the magnetic field.

The design can provide substantial excursion capability, depending on the geometry.

Underhung Voice Coil

In an underhung design, the voice-coil winding is shorter than the magnetic gap.

The coil can remain within a relatively uniform magnetic field over a useful range of movement.

Underhung designs can provide very good motor linearity but require a carefully designed magnetic circuit.

Magnetic Gap Uniformity

Ideally, the magnetic field in the working portion of the gap should remain as uniform as possible over the intended operating range.

Changes in magnetic field strength as the voice coil moves can produce nonlinear force.

Good motor design attempts to control these variations.

Flux Density and Excursion

As the voice coil moves, the amount of conductor inside the strongest part of the magnetic field can change.

If the motor force changes significantly with displacement, the speaker can produce nonlinear distortion.

Therefore, motor design is closely related to the speaker's linear excursion capability.

Magnetic Circuit and Distortion

Several motor characteristics can contribute to nonlinear distortion:

• Variation of BL with displacement
• Variation of voice-coil inductance
• Magnetic saturation
• Flux modulation
• Non-uniform magnetic fields

Advanced loudspeaker motors use various techniques to reduce these effects.

Magnet and Voice-Coil Inductance

The voice coil is an inductor as well as a resistive load.

The magnetic circuit can influence the coil's inductance and how that inductance changes with frequency and coil position.

This is one reason sophisticated motor designs use conductive shorting elements and carefully shaped magnetic structures.

Speaker Magnet Shapes

The permanent magnet can have different shapes depending on the motor design.

Common forms include:

• Ring magnets
• Disc magnets
• Segment magnets
• Specialized magnet assemblies

The shape is selected to work with the required magnetic circuit.

Ring Magnets

Ring-shaped magnets are common in conventional loudspeaker motors.

The central opening accommodates the pole structure.

        ┌─────────────┐
        │   MAGNET    │
        │    ┌───┐    │
        │    │   │    │
        │    └───┘    │
        └─────────────┘

The ring geometry works naturally with the cylindrical voice-coil structure.

Segmented Magnets

Some motor designs use multiple magnet segments rather than one continuous ring.

Segmented construction can be useful for manufacturing or specialized magnetic-circuit arrangements.

Magnet and Speaker Basket

The basket supports the magnetic assembly and moving suspension.

The basket must be mechanically rigid so that the magnetic gap remains properly aligned with the moving voice coil.

A strong magnetic motor is not useful if the mechanical structure cannot maintain accurate alignment.

Magnet Assembly Alignment

The pole piece, top plate and magnet must remain accurately aligned.

Even a small displacement can alter the magnetic gap.

A damaged or shifted magnet assembly can therefore cause voice-coil rubbing.

What Happens When a Speaker Magnet Moves?

The permanent magnet is normally firmly attached to the magnetic structure.

If the magnet or pole assembly becomes loose after mechanical impact, the magnetic gap can become misaligned.

This can cause the voice coil to rub and can make the speaker unusable.

A shifted magnet assembly is generally a serious repair problem.

Magnet and Speaker Power Handling

The magnet itself does not determine the speaker's power handling.

Power handling depends on the entire driver, including:

• Voice-coil size
• Voice-coil cooling
• Magnetic circuit
• Cone
• Suspension
• Enclosure
• Frequency range

A larger magnet can provide stronger motor force, but it does not automatically mean that the speaker can handle more thermal power.

Magnet and Speaker Sensitivity

A strong motor can contribute to high sensitivity, but sensitivity is determined by the complete electro-mechanical system.

Important factors include:

• BL
• Mms
• Re
• Suspension
• Radiating area
• Enclosure

Magnet and Subwoofers

Subwoofer motors are often designed to provide strong force over large cone excursions.

Important considerations include:

• BL linearity
• Magnetic gap geometry
• Voice-coil length
• Cooling
• Flux density
• Mechanical excursion

A powerful subwoofer motor is therefore much more than simply a large magnet.

Magnet and Tweeters

Tweeter motors are much smaller than typical woofer motors, but the magnetic gap must still provide a strong and well-controlled field.

Because the moving assembly is small, motor geometry can be optimized for very low moving mass and high-frequency response.

Why Some Speakers Have Large Magnets

Large magnets can be used when a strong magnetic field and substantial motor structure are required.

Large ferrite magnets are particularly common in powerful woofers and professional speakers.

However, the size of the magnet should always be evaluated together with the rest of the motor design.

Why Some Speakers Have Small Magnets

A small magnet does not necessarily mean a weak motor.

Modern high-energy magnet materials can produce strong magnetic fields from relatively compact magnets.

The magnetic circuit can also be optimized to concentrate flux efficiently in the gap.

Magnet Material Comparison

Magnetic Motor Parameters

Common Speaker Magnet Myths

Myth: Bigger Magnet Always Means Better Speaker

Not necessarily. Motor geometry, flux density, BL, moving mass, suspension and enclosure all matter.

Myth: Neodymium Always Sounds Better

Magnet material alone does not determine sound quality. A well-designed ferrite motor can perform extremely well.

Myth: A Heavy Speaker Is Always More Powerful

Weight may come from the magnet, basket or other components and does not directly determine acoustic output.

Myth: Magnet Strength Determines Wattage

Power handling is strongly influenced by the voice coil, cooling, mechanical limits and other parts of the speaker.

Inspecting a Speaker Magnet

When examining a speaker motor, look for:

• Loose magnet assembly
• Cracks or damage
• Signs of impact
• Corrosion
• Shifted top plate
• Damage around the pole piece
• Evidence of voice-coil rubbing

Do not attempt to disassemble a magnet assembly unless you understand the risks and have the appropriate equipment.

Speaker Magnet Repair

A shifted or detached magnet assembly can be difficult to repair because the magnetic gap must be restored to extremely accurate alignment.

Simply gluing the magnet back into place without accurately aligning the pole and top plate can result in permanent voice-coil rubbing.

Professional reconing or motor replacement may be preferable for valuable drivers.

Magnetic Circuit and Speaker Design

The magnetic circuit must be designed together with the voice coil.

        Magnetic circuit
              │
              ▼
       Magnetic field
              │
              ▼
         Voice coil
              │
              ▼
           BL force
              │
              ▼
        Cone movement
              │
              ▼
             Sound

This is the electromagnetic foundation of a conventional dynamic loudspeaker.

Key Takeaways

• The permanent magnet provides the static magnetic field for a dynamic loudspeaker.
• The magnet works together with the pole piece, top plate and back plate to form a magnetic circuit.
• The voice coil operates inside a narrow magnetic gap.
• Magnetic flux density in the gap strongly affects motor force.
• The simplified motor relationship is F = BL × I.
• Ferrite, neodymium and alnico are among the permanent-magnet materials used in loudspeakers.
• Neodymium allows compact and lightweight motor assemblies.
• A large magnet does not automatically mean a better speaker.
• Magnetic-circuit geometry is just as important as magnet material.
• Magnetic saturation can limit motor performance.
• Magnetic leakage reduces the efficiency of the intended magnetic circuit.
• Voice-coil alignment depends on an accurately constructed magnetic gap.
• Ventilated pole pieces can help with voice-coil cooling.
• Shorting rings can reduce certain forms of inductance variation and distortion.
• The magnetic motor, voice coil, cone and suspension must be designed as one complete system.