How Speakers Work
A loudspeaker converts an electrical audio signal into mechanical movement and finally into sound waves. Learn how the magnet, voice coil, cone, suspension and enclosure work together to reproduce music.
What Is a Loudspeaker?
A loudspeaker is an electro-mechanical device that converts an electrical signal into acoustic energy.
The amplifier produces an electrical audio signal. That signal is sent to the speaker's voice coil, which interacts with a magnetic field. This interaction causes the voice coil and attached diaphragm to move. The moving diaphragm then produces pressure variations in the air that we perceive as sound.
Electrical signal
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Voice coil
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Magnetic force
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Mechanical movement
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Cone
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Air movement
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Sound
The Basic Parts of a Dynamic Speaker
The most common loudspeaker type is the dynamic loudspeaker. Its main parts include:
- Permanent magnet
- Magnetic pole pieces
- Voice coil
- Voice-coil former
- Diaphragm or cone
- Surround
- Spider
- Dust cap
- Basket or frame
- Terminals
Each component performs a different function, but they must operate together as one mechanical and electrical system.
The Permanent Magnet
The permanent magnet provides the static magnetic field in which the voice coil operates.
Speaker magnets may be manufactured from different magnetic materials. Common examples include ferrite and neodymium.
The magnet itself does not normally move during operation. Instead, it creates a magnetic field concentrated around the voice-coil gap.
Magnet
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โ โcoil โ
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The Magnetic Gap
The voice coil operates inside a narrow magnetic gap.
The geometry of this gap is extremely important because the magnetic field must interact effectively with the voice coil.
A well-designed magnetic circuit concentrates a strong magnetic field through the region where the coil moves.
The voice coil must remain correctly centered in the gap. If the coil rubs against the pole piece, the speaker can produce distortion or mechanical noise and may eventually be damaged.
The Voice Coil
The voice coil is a coil of wire attached to the speaker diaphragm, usually through a former.
When the amplifier sends an alternating electrical current through the voice coil, the coil produces a magnetic field.
The magnetic field of the coil interacts with the permanent magnetic field of the speaker.
This interaction creates a force that moves the coil.
Audio current
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Permanent magnet
Why the Voice Coil Moves
The force acting on a current-carrying conductor in a magnetic field is described by the Lorentz force.
In simplified form:
F = B ร I ร L
where:
- F = force
- B = magnetic flux density
- I = current
- L = effective conductor length in the magnetic field
The direction of the force changes when the direction of the current changes.
Because an audio signal continually changes polarity, the voice coil moves back and forth.
Alternating Current Produces Alternating Motion
Consider a simple sine-wave audio signal.
Voltage
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Time
When the signal changes polarity, the direction of the magnetic force on the voice coil also changes.
The result is back-and-forth movement of the diaphragm.
The Cone or Diaphragm
The diaphragm is the part of the speaker that moves the air.
In a conventional woofer, the diaphragm is usually a cone.
The cone is attached to the voice coil so that movement of the coil causes the cone to move.
Voice coil
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Cone
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Air
The cone must be sufficiently rigid to move air efficiently while minimizing unwanted deformation.
How the Cone Produces Sound
When the cone moves forward, it compresses the air in front of it.
When the cone moves backward, it creates a lower-pressure region.
These pressure variations propagate through the air as sound waves.
Cone movement
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Sound waves
The frequency of the cone's movement determines the frequency of the sound, while the amplitude of movement influences the acoustic output level.
Frequency and Speaker Movement
Frequency describes how many complete cycles occur each second.
1 Hz = 1 cycle per second 100 Hz = 100 cycles per second 1 kHz = 1,000 cycles per second 10 kHz = 10,000 cycles per second
A 100 Hz tone causes the diaphragm to move back and forth 100 times per second.
A 1 kHz tone causes it to complete approximately 1,000 cycles per second.
The mechanical system must therefore respond quickly enough to reproduce the required frequencies.
Amplitude and Sound Level
Increasing the electrical signal generally increases the movement of the voice coil and diaphragm.
Greater diaphragm movement can produce greater sound pressure, provided the speaker remains within its mechanical and thermal limits.
However, doubling electrical power does not simply mean that the speaker produces twice the perceived loudness.
Sound pressure level is normally expressed in decibels (dB), and human hearing responds approximately logarithmically to changes in sound pressure.
The Spider
The spider is a flexible suspension component located near the voice coil.
Its functions include:
- Centering the voice coil
- Controlling mechanical movement
- Providing restoring force
- Helping maintain the correct position of the coil
The spider is an important part of the speaker's mechanical compliance.
The Surround
The surround connects the outer edge of the cone to the speaker basket.
It allows the cone to move while helping maintain its position.
Different speaker designs use different surround materials and geometries.
The surround also contributes to the overall compliance of the speaker.
The Voice-Coil Former
The voice-coil former supports the coil winding and connects the coil mechanically to the diaphragm.
Former materials can include various types of paper, polymer films, aluminium and other engineered materials.
The former must tolerate the temperature and mechanical stresses produced during operation.
The Dust Cap
The dust cap is positioned over the central opening of many cone speakers.
Its primary purpose is to help prevent dust and debris from entering the voice-coil gap.
Depending on its construction and size, it can also influence the acoustic behaviour of the diaphragm.
The Speaker Basket
The basket provides the mechanical structure that holds the speaker components together.
It supports the magnet assembly, spider and surround while maintaining their relative positions.
Baskets may be made from stamped steel, cast aluminium or other materials.
A rigid basket helps maintain alignment during operation.
Speaker Terminals
The speaker terminals provide the electrical connection between the external amplifier wiring and the voice coil.
The signal travels through the terminal, flexible lead wires and voice coil.
Amplifier โ โผ Terminal โ Lead wire โ Voice coil โ Magnetic field โ Mechanical movement
Speaker Polarity
Speaker polarity determines the direction in which the cone initially moves for a particular electrical polarity.
If the positive amplifier terminal is connected to the positive speaker terminal, the speaker moves in its intended polarity direction.
Reversing the connection reverses the acoustic polarity.
Polarity becomes particularly important when multiple speakers operate together.
What Happens When Two Speakers Are Out of Phase?
If two speakers reproduce the same signal but one is connected with reversed polarity, their acoustic outputs can partially cancel each other.
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Speaker B โโโ
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Acoustic cancellation
This can produce reduced output at some frequencies, particularly around frequencies where the acoustic wavelengths and speaker positions cause significant interaction.
Why a Speaker Has Impedance
The voice coil is made from wire. The wire has electrical resistance, but the coil also has inductance.
Consequently, the speaker presents an impedance to the amplifier.
Z = R + jX
The impedance is frequency dependent.
This is why a speaker labelled "8 ฮฉ" does not necessarily measure exactly 8 ฮฉ with a multimeter or remain at 8 ฮฉ throughout the audio frequency range.
DC Resistance vs Speaker Impedance
A multimeter normally measures DC resistance rather than the full AC impedance of the speaker.
For an 8 ฮฉ nominal speaker, the measured DC resistance may be considerably lower than 8 ฮฉ.
This does not necessarily indicate a defective speaker.
The nominal impedance describes the speaker's intended AC load characteristics rather than its simple DC resistance.
Speaker Resonance
A loudspeaker has mechanical resonance.
At its free-air resonance frequency, the mechanical system can produce a significant impedance peak.
Impedance
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Frequency
The exact behaviour depends on the speaker's mechanical and electrical parameters.
Why Enclosures Matter
A loudspeaker does not operate independently from its enclosure.
The enclosure changes the acoustic environment behind the diaphragm and therefore affects the speaker's low-frequency behaviour.
Common enclosure types include:
- Sealed enclosure
- Bass-reflex enclosure
- Transmission line
- Horn enclosure
- Open-baffle design
Sealed Speaker Enclosure
A sealed enclosure traps air behind the driver.
The trapped air behaves approximately like an additional spring acting on the cone.
Cone
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The enclosure volume influences the resulting low-frequency response and system resonance.
Bass-Reflex Enclosure
A bass-reflex enclosure uses a port or vent to interact with the air inside the enclosure.
The port and enclosure form an acoustic resonant system that can increase output around the tuning frequency.
Woofer
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Port dimensions and enclosure volume are important when designing a bass-reflex system.
Why Woofer Cones Are Larger
Low frequencies require the movement of a relatively large volume of air to produce substantial sound pressure.
A larger cone can move more air for a given excursion.
This is one reason woofers generally have larger diaphragms than tweeters.
Why Tweeters Are Smaller
High-frequency sound requires rapid diaphragm movement.
A smaller and lighter diaphragm can respond more easily to high frequencies.
Tweeters therefore generally use much smaller moving assemblies than woofers.
Full-Range Speakers
A full-range driver attempts to reproduce a broad frequency range using a single diaphragm.
This can provide a simple system without a crossover between separate drivers, but a single driver must compromise between low-frequency excursion, high-frequency response, efficiency and dispersion.
Two-Way Speakers
A two-way loudspeaker normally uses:
- A woofer for low and mid frequencies
- A tweeter for high frequencies
A crossover divides the audio signal between the two drivers.
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Amplifier โโ Crossover
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The crossover frequency and slope are selected according to the characteristics of the drivers and the desired acoustic response.
Three-Way Speakers
A three-way loudspeaker normally uses:
- Woofer
- Midrange driver
- Tweeter
The audio spectrum is divided into three frequency regions.
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Amp โโโโโผโโ Midrange
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This allows each driver to operate over a more limited frequency range.
Speaker Crossover
A passive crossover uses capacitors, inductors and resistors to divide the signal between drivers.
A simple first-order low-pass network can use a series inductor:
Amplifier โโโ L โโโ Woofer
A simple first-order high-pass network can use a series capacitor:
Amplifier โโโ C โโโ Tweeter
Higher-order networks use additional components to obtain steeper filter slopes.
Speaker Sensitivity
Sensitivity describes how much acoustic output a speaker can produce for a specified electrical input under defined measurement conditions.
It is normally expressed in dB SPL.
A speaker with higher sensitivity can produce greater acoustic output for the same electrical power than a less sensitive speaker, assuming the specifications are measured under comparable conditions.
Speaker Efficiency
Speaker efficiency describes how effectively electrical input power is converted into acoustic output power.
Most conventional loudspeakers convert only a relatively small fraction of the electrical input power into acoustic energy.
Much of the input energy is dissipated as heat in the voice coil and other parts of the system.
Voice-Coil Heating
When current flows through the voice coil, electrical resistance causes power dissipation.
P = IยฒR
This power becomes heat.
As the voice coil becomes hotter, its electrical resistance generally increases.
This can change the behaviour and efficiency of the speaker during high-power operation.
Mechanical Excursion
Excursion is the distance the diaphragm moves from its rest position.
At low frequencies, a speaker may require significant excursion to produce useful acoustic output.
If the cone is driven beyond its intended mechanical limits, distortion can increase and physical damage may occur.
Therefore, a speaker's power rating alone does not determine how much low-frequency output it can safely produce.
Thermal and Mechanical Limits
A loudspeaker can be limited by both heat and mechanical movement.
| Limit | Cause |
|---|---|
| Thermal | Excessive voice-coil heating |
| Mechanical | Excessive cone excursion |
| Suspension | Spider or surround limitations |
| Magnetic | Magnetic circuit limitations |
Why Speaker Size Matters
Speaker diameter is one factor affecting the amount of air a diaphragm can move.
However, speaker performance is not determined by diameter alone. Cone area, excursion, motor strength, enclosure design and many other parameters also affect performance.
A larger speaker is not automatically better at every frequency.
The Complete Energy Conversion
The operation of a dynamic loudspeaker can be summarized as a chain of energy conversions:
Electrical energy
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Voice-coil magnetic field
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Electromagnetic force
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Mechanical movement
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Cone movement
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Air pressure variation
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Acoustic energy
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Sound reaching your ears
What Happens When You Increase Amplifier Power?
Increasing amplifier output power generally increases the available electrical energy delivered to the speaker.
The resulting cone movement can increase, but the relationship depends on frequency and the speaker's mechanical and electrical characteristics.
Eventually, the speaker reaches one or more limits:
- Voice-coil temperature limit
- Maximum excursion
- Suspension limit
- Magnetic limit
- Mechanical damage limit
Driving beyond these limits can cause distortion or permanent damage.
What Happens When the Amplifier Clips?
If the amplifier is driven beyond its available output voltage, the waveform becomes clipped.
Clean:
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Clipped:
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Clipping generates additional harmonic components. Depending on the speaker system and frequency distribution, these additional components can increase stress on the drivers.
How to Identify a Speaker's Main Components
If you have a conventional cone speaker in front of you, the easiest way to identify its major parts is:
- The large central structure is the cone or diaphragm.
- The flexible outer edge is the surround.
- The central cap is the dust cap.
- The structure behind the cone is the voice-coil and magnetic assembly.
- The corrugated suspension behind the cone is generally the spider.
- The metal or composite structure holding everything together is the basket.
- The external electrical connections are the terminals.
Why Speaker Construction Is a Compromise
A loudspeaker must balance many competing requirements.
- Low-frequency extension
- High-frequency response
- Efficiency
- Power handling
- Low distortion
- Mechanical durability
- Low moving mass
- Controlled excursion
- Suitable impedance
- Manufacturing cost
Improving one characteristic can sometimes make another more difficult to achieve.
From Electrical Signal to Sound
The complete process is surprisingly simple in principle.
- The audio source generates an electrical signal.
- The amplifier increases the signal to a level capable of driving the speaker.
- Current flows through the voice coil.
- The voice coil interacts with the permanent magnetic field.
- An electromagnetic force moves the coil.
- The cone moves with the coil.
- The moving cone creates pressure variations in the air.
- The pressure variations travel through the air.
- Your ears detect those variations as sound.
Key Takeaways
- A dynamic speaker converts electrical energy into acoustic energy.
- The voice coil interacts with a permanent magnetic field to create mechanical force.
- The cone or diaphragm moves air to produce sound.
- The spider and surround control and support cone movement.
- The speaker basket maintains the mechanical alignment of the components.
- Speaker impedance varies with frequency.
- Speaker enclosures strongly influence low-frequency performance.
- Woofers generally handle lower frequencies while tweeters handle higher frequencies.
- Crossovers divide the audio spectrum between multiple drivers.
- Both thermal and mechanical limits determine how much power a speaker can safely handle.