Speakers
A loudspeaker is an electroacoustic device that converts an electrical audio signal into mechanical movement and then into sound. Speakers are used in amplifiers, televisions, radios, computers, vehicles, PA systems, headphones and home audio equipment.
What Is a Speaker?
A loudspeaker converts electrical energy from an audio amplifier into mechanical movement of a diaphragm. The movement of the diaphragm creates pressure variations in the surrounding air that are perceived as sound.
Audio Signal
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Voice Coil
│
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Magnetic Force
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Diaphragm Movement
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Air Pressure Changes
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Sound
How Does a Speaker Work?
A conventional dynamic speaker uses a permanent magnetic field and a voice coil attached to a diaphragm.
When the audio amplifier sends current through the voice coil, the interaction between the magnetic field and the coil produces a force. This force moves the diaphragm.
Amplifier
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Voice Coil
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Magnetic Interaction
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Cone / Diaphragm
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Air Movement
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Sound
Main Types of Speakers
- Woofer
- Midrange driver
- Tweeter
- Subwoofer
- Full-range driver
- Coaxial speaker
- Horn driver
- Compression driver
- Electrostatic speaker
- Planar magnetic speaker
Dynamic Speaker
The dynamic loudspeaker is one of the most common speaker designs. It uses a voice coil positioned in a magnetic field.
Diaphragm
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▼
┌───────────┐
│ Voice Coil│
└───────────┘
│
Magnetic
Gap
│
Magnet
The amplifier's alternating audio current causes the voice coil to move back and forth, producing diaphragm movement.
Speaker Diaphragm
The diaphragm is the part of the speaker that moves air to produce sound.
Depending on the speaker design, the diaphragm may be made from materials such as:
- Paper
- Polypropylene
- Aluminium
- Fabric
- Polymer materials
- Composite materials
- Other specialized materials
The diaphragm material affects mass, stiffness, damping and therefore the acoustic behavior of the driver.
Speaker Cone
Many woofers and midrange drivers use a cone-shaped diaphragm.
The cone provides a relatively large radiating surface while allowing the voice coil to drive the diaphragm.
Speaker Dust Cap
The dust cap is located near the center of many cone speakers.
Its primary purpose is to help protect the voice-coil area from dust and contamination.
Its shape and material can also influence the acoustic behavior of the speaker.
Speaker Surround
The surround connects the outer edge of the cone to the speaker frame.
It allows the cone to move while helping to control the mechanical behavior of the diaphragm.
Common surround materials include foam, rubber and fabric.
Speaker Spider
The spider is a flexible suspension component located around the voice coil.
It helps center the voice coil in the magnetic gap and provides mechanical restoring force.
Cone │ ▼ Surround ───────── Frame │ ▼ Voice Coil │ ▼ Spider │ ▼ Magnet Structure
Speaker Frame
The frame, sometimes called the basket, supports the speaker's mechanical components.
The frame must provide sufficient mechanical rigidity to maintain the relationship between the cone, voice coil and magnetic structure.
Speaker Magnet
The magnet creates the magnetic field used by the voice coil.
Common speaker magnet materials include:
- Ferrite
- Neodymium
- Other permanent-magnet materials
The magnet material is only one part of the magnetic system. The shape of the pole pieces, magnetic gap and overall motor structure also affect speaker performance.
Voice Coil
The voice coil is a winding of wire attached to the speaker diaphragm. It is positioned within the magnetic gap.
When current flows through the coil, electromagnetic forces move the coil and attached diaphragm.
Audio Current
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▼
┌────────────┐
│ Voice Coil │
└────────────┘
│
▼
Diaphragm Movement
Voice Coil Former
The voice coil is normally wound around a cylindrical former.
The former provides mechanical support for the winding and transfers the coil movement to the diaphragm.
Former materials can include different types of paper, aluminium, polyimide and other suitable materials depending on the speaker design.
Voice Coil Wire
The voice coil winding uses fine conductive wire.
The wire material, diameter, winding arrangement and number of turns affect the electrical resistance, inductance, mass and power-handling characteristics of the speaker.
Speaker Suspension
The suspension consists primarily of the surround and spider.
It allows controlled movement of the diaphragm while maintaining the voice coil within the magnetic gap.
Speaker Impedance
Speaker impedance describes the opposition presented by the speaker to an alternating audio signal.
Common nominal speaker impedances include:
- 4 Ω
- 6 Ω
- 8 Ω
- 16 Ω
The actual impedance of a speaker varies with frequency.
Speaker Resistance vs Impedance
Resistance is the DC resistance measured using a conventional multimeter.
Impedance is frequency-dependent and includes both resistance and reactive effects.
Therefore, an 8 Ω speaker will normally not measure exactly 8 Ω with a multimeter.
Speaker DC Resistance
A multimeter measures the DC resistance of the voice coil.
The measured resistance is normally lower than the speaker's nominal impedance.
For example, a speaker marked 8 Ω may measure several ohms of DC resistance, depending on its design.
Speaker Power Rating
Speaker power ratings describe how much electrical power the speaker can handle under specified conditions.
Power ratings can be expressed in watts.
Common specifications include:
- Continuous power
- RMS power
- Program power
- Peak power
These terms are not necessarily defined identically by every manufacturer, so the manufacturer's specification should be checked.
Speaker RMS Power
RMS power is commonly used as a practical way of describing the continuous electrical power associated with a speaker under specified test conditions.
The exact test conditions and terminology should always be checked in the manufacturer's documentation.
Speaker Peak Power
Peak power describes a higher short-duration power level that a speaker may tolerate under specified conditions.
Peak power should not be interpreted as the normal continuous operating power of the speaker.
Speaker Sensitivity
Sensitivity describes the acoustic output produced by a speaker for a specified electrical input and measurement condition.
It is commonly expressed in dB SPL.
A higher sensitivity generally means that less amplifier power is required to achieve a particular sound level, all else being equal.
Speaker Frequency Response
Frequency response describes how the acoustic output changes across different frequencies.
Output
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│ ┌───────────────┐
│ / \
│_____/ \____
└──────────────────────────────►
Frequency
A speaker's frequency response is affected by its design, enclosure, installation and measurement conditions.
Woofer
A woofer is designed primarily to reproduce low and lower-midrange frequencies.
Woofers generally have relatively large diaphragms and substantial excursion capability compared with small high-frequency drivers.
Midrange Driver
A midrange driver is designed to reproduce the middle portion of the audio spectrum.
It is commonly used in multi-way speaker systems between the woofer and tweeter.
Tweeter
A tweeter is designed primarily to reproduce high-frequency sound.
Tweeters generally use smaller, lighter diaphragms than woofers so that they can respond rapidly to high-frequency signals.
Subwoofer
A subwoofer is designed to reproduce very low-frequency sound.
Subwoofers commonly use relatively large diaphragms and are designed for substantial low-frequency excursion.
Full-Range Speaker
A full-range driver attempts to reproduce a wide range of frequencies using a single driver.
A single driver can simplify the system, although a single diaphragm cannot necessarily provide the same performance across the entire audio range as a properly designed multi-way system.
Coaxial Speaker
A coaxial speaker combines two or more drivers in approximately the same physical location.
A common arrangement places a tweeter in front of or near the center of a larger woofer.
Two-Way Speaker
A two-way speaker system normally contains:
- Woofer
- Tweeter
A crossover divides the audio signal so that each driver receives the appropriate frequency range.
Three-Way Speaker
A three-way system commonly contains:
- Woofer
- Midrange driver
- Tweeter
The crossover divides the audio spectrum among the three drivers.
Speaker Crossover
A crossover separates an audio signal into frequency ranges suitable for different drivers.
┌── Woofer
Audio ──► Crossover
├── Midrange
│
└── Tweeter
Crossovers can be passive or active.
Passive Crossover
A passive crossover operates after the amplifier and uses components such as capacitors, inductors and resistors.
Amplifier
│
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Passive Crossover
├────► Woofer
├────► Midrange
└────► Tweeter
Active Crossover
An active crossover operates at a lower signal level before the power amplifiers.
Source │ ▼ Active Crossover ├──► Amplifier ──► Woofer ├──► Amplifier ──► Midrange └──► Amplifier ──► Tweeter
Active systems can provide independent amplification for each driver.
Speaker Crossover Frequency
The crossover frequency is the region where the signal transitions between drivers.
For example, a two-way speaker may use a crossover that sends lower frequencies to the woofer and higher frequencies to the tweeter.
The correct crossover frequency depends on the drivers and system design.
Crossover Slope
Crossover slope describes how rapidly the signal level changes outside the intended frequency range.
Common slopes are described in dB per octave, such as 6 dB/octave, 12 dB/octave, 18 dB/octave and 24 dB/octave.
Speaker Enclosure
The enclosure is an important part of a loudspeaker system.
The enclosure controls the acoustic behavior of the rear radiation from the driver and can significantly affect low-frequency response.
Sealed Speaker Enclosure
A sealed enclosure traps the rear radiation of the driver inside an airtight cabinet.
The enclosed air acts as an additional spring on the driver's suspension.
Bass Reflex Enclosure
A bass-reflex enclosure uses a port to couple the air inside the cabinet to the outside environment.
Speaker │ ▼ ┌──────────────┐ │ │ │ Cabinet │ │ │ │ ────┼──► Port └──────────────┘
The port and enclosure are designed to reinforce output around a specified tuning frequency.
Speaker Port
The port is an opening or duct in a bass-reflex enclosure.
Its dimensions influence the tuning frequency and airflow behavior.
Incorrect port design can cause unwanted noise or poor bass response.
Speaker Enclosure Materials
Speaker cabinets can be constructed from materials such as:
- MDF
- Plywood
- Particle board
- Wood
- Plastic
- Composite materials
The enclosure should have sufficient rigidity for the intended application.
Speaker Baffle
The baffle is the front panel on which the speaker driver is mounted.
Baffle dimensions and shape can affect acoustic radiation, especially around lower frequencies.
Speaker Diffraction
When sound waves encounter cabinet edges, they can bend and interfere with the direct sound.
This effect is called diffraction and can influence the frequency response of the system.
Speaker Phase
The phase relationship between drivers becomes important in multi-way speaker systems.
Incorrect phase relationships around a crossover region can cause cancellation or uneven frequency response.
Speaker Polarity
Speaker terminals are commonly marked positive and negative.
Maintaining consistent polarity is particularly important when multiple speakers operate together.
Amplifier + ───── + Speaker Amplifier - ───── - Speaker
Speaker Out of Phase
If one speaker is wired with reversed polarity relative to another, their acoustic outputs can interfere with each other.
This can produce reduced output at some frequencies, particularly when the speakers reproduce similar frequency ranges.
Speaker Wiring
A basic passive speaker connects to the amplifier output using two conductors.
Amplifier + ───────── + Speaker - ───────── - Speaker
The cable and connectors should be appropriate for the amplifier power and installation.
Series Speaker Connection
Multiple speakers can be connected in series.
Amplifier + ── Speaker ── Speaker ── Amplifier -
For simple resistive approximations, series impedances add. Actual speaker impedance is frequency-dependent, so real systems require more careful analysis.
Parallel Speaker Connection
Speakers can also be connected in parallel.
┌── Speaker ──┐
Amplifier + ─┤ ├─ Amplifier -
└── Speaker ──┘
Parallel connection can significantly reduce the total impedance seen by the amplifier.
The amplifier must be capable of safely driving the resulting load.
Speaker Impedance and Amplifier
The amplifier must be designed to operate with the connected speaker load.
Connecting a load with an impedance lower than the amplifier's specified minimum can increase output current and cause overheating, protection activation or damage.
Speaker Sensitivity and Amplifier Power
Speaker sensitivity affects how much amplifier power is required to reach a particular sound level.
A more sensitive speaker can produce a higher acoustic output for the same electrical input under comparable measurement conditions.
Speaker Efficiency
Speaker efficiency describes how effectively electrical input power is converted into acoustic output.
Loudspeaker efficiency is generally low compared with many electrical energy conversion systems, with much of the input power ultimately becoming heat.
Speaker Excursion
Excursion is the distance that the diaphragm moves from its rest position.
Low-frequency reproduction generally requires greater diaphragm movement than high-frequency reproduction at comparable acoustic levels.
Maximum Excursion
A speaker has mechanical limits on how far its diaphragm can move.
Exceeding these limits can cause distortion or physical damage.
Speaker Distortion
Distortion occurs when the acoustic output is not a faithful representation of the input signal.
Possible sources include:
- Nonlinear suspension
- Voice-coil movement outside the magnetic gap
- Magnetic nonlinearities
- Diaphragm breakup
- Mechanical limitations
- Thermal compression
Speaker Voice-Coil Heating
Electrical power dissipated in the voice coil produces heat.
As the coil temperature increases, its resistance increases. This can alter the speaker's behavior and reduce the amount of acoustic output obtained for a given amplifier signal.
Thermal Compression
Thermal compression occurs when heating of the voice coil changes the speaker's electrical and acoustic behavior.
At high power levels, increasing voice-coil resistance can reduce the effective output compared with the expected increase from amplifier power alone.
Speaker Resonance
A speaker driver has a natural mechanical resonance.
The resonance frequency is influenced by the moving mass and the mechanical compliance of the suspension.
For a driver in a particular enclosure, the overall system resonance can be different from the driver's free-air resonance.
Free-Air Resonance
The free-air resonance frequency, commonly represented as Fs, is a fundamental parameter used when analyzing a speaker driver.
It is especially important when designing low-frequency enclosures.
Thiele-Small Parameters
Thiele-Small parameters describe important electro-mechanical characteristics of speaker drivers.
Common parameters include:
- Fs
- Qts
- Qes
- Qms
- Vas
- Re
- Le
- Sd
- Xmax
These parameters are particularly useful when designing speaker enclosures and analyzing low-frequency performance.
Speaker Fs
Fs is the driver's free-air resonance frequency.
It is an important parameter when determining how a driver behaves at low frequencies.
Speaker Vas
Vas represents the equivalent compliance volume associated with the driver's suspension.
It is commonly used with other Thiele-Small parameters when designing enclosures.
Speaker Qts
Qts is the total Q factor of the driver and combines the electrical and mechanical Q contributions.
It is one of the important parameters used when evaluating suitable enclosure types.
Speaker Xmax
Xmax describes the maximum linear excursion of a driver under the manufacturer's specified definition.
Higher excursion capability can be important for high-output low-frequency reproduction.
Speaker Testing
A speaker can be tested using several methods depending on the fault being investigated.
- Visual inspection
- DC resistance measurement
- Continuity testing
- Low-level audio testing
- Frequency-response measurement
- Impedance measurement
Testing Speaker Resistance
Set a multimeter to an appropriate resistance range and measure across the speaker terminals.
A finite resistance normally indicates that the voice coil has continuity.
An open-circuit reading can indicate a broken voice coil or connection.
Testing a Speaker for Short Circuit
A very low resistance compared with the expected voice-coil resistance can indicate a possible shorted winding or another fault.
The measured value should be compared with the manufacturer's specifications where possible.
Testing Speaker Polarity
Speaker polarity can sometimes be checked using a small DC test voltage, provided the test is appropriate for the driver.
The cone should move in a predictable direction when the polarity is applied.
Only a brief, low-level test should be used; do not apply excessive DC to a speaker.
Testing a Tweeter
Tweeters can have different electrical and mechanical constructions.
A simple resistance measurement can help determine whether the voice coil is open, but it does not fully verify tweeter performance.
Testing should be performed with a suitable low-level signal and appropriate protection.
Testing a Woofer
A woofer can be inspected for:
- Voice-coil continuity
- Damaged cone
- Damaged surround
- Loose dust cap
- Rubbing voice coil
- Mechanical damage
Voice Coil Rubbing
A damaged or misaligned voice coil can rub against the magnetic gap.
This may produce a scratching sound when the cone is moved carefully by hand.
Do not apply excessive force to the cone during this test.
Speaker Troubleshooting
When a speaker produces no sound, check:
- Amplifier output.
- Speaker cable.
- Speaker terminals.
- Voice-coil continuity.
- Crossover components.
- Driver connections.
- Physical damage.
Speaker Produces Distorted Sound
Possible causes include:
- Damaged voice coil
- Rubbing voice coil
- Damaged surround
- Damaged cone
- Excessive amplifier clipping
- Overdriving the speaker
- Damaged crossover components
Speaker Has No Bass
Possible causes include:
- Incorrect enclosure
- Incorrect crossover
- Reversed speaker polarity
- Insufficient amplifier capability
- Damaged woofer
- Air leakage in a sealed enclosure
- Incorrect bass-reflex port design
Speaker Has No Treble
Possible causes include:
- Failed tweeter
- Open crossover capacitor
- Incorrect wiring
- Damaged crossover
- Amplifier or source problem
Speaker Makes a Rattling Noise
Possible causes include:
- Loose dust cap
- Damaged cone
- Loose surround
- Loose grille
- Loose enclosure component
- Damaged voice coil
Speaker Replacement
When replacing a speaker driver, the replacement should match the requirements of the system.
Important parameters include:
- Impedance
- Power rating
- Sensitivity
- Frequency range
- Physical diameter
- Mounting-hole dimensions
- Cutout diameter
- Mounting depth
- Voice-coil configuration
- Enclosure requirements
How to Identify a Speaker
- Read the markings on the magnet or frame.
- Identify the nominal impedance.
- Identify the power rating.
- Measure the frame diameter.
- Measure the cutout diameter.
- Measure the mounting depth.
- Determine the driver type.
- Check the terminal arrangement.
- Look for manufacturer and model information.
How to Select a Speaker
- Determine the required frequency range.
- Determine the required acoustic output.
- Determine the amplifier's minimum impedance.
- Determine the required power handling.
- Check sensitivity.
- Choose the appropriate driver type.
- Determine the enclosure requirements.
- Check physical dimensions.
- Check crossover requirements.
- Verify the manufacturer's specifications.
Speaker and Amplifier Matching
The speaker and amplifier should be matched for impedance, power and intended application.
The amplifier should not be connected to a speaker load below its specified minimum impedance.
Likewise, an amplifier capable of producing substantially more power than a speaker can safely handle may damage the speaker if excessive signal levels are used.
Speaker Clipping
Amplifier clipping occurs when the amplifier is driven beyond the voltage it can reproduce cleanly.
The resulting waveform contains substantial distortion and can produce excessive heating in some speaker drivers, particularly tweeters.
Speaker Protection
Speaker systems can use protection circuits to reduce the risk of damage.
Protection methods can include:
- Fuses
- PTC devices
- Protection relays
- Electronic limiters
- High-pass filtering for tweeters
- Thermal protection
Tweeter Protection
Tweeters are particularly sensitive to excessive low-frequency energy.
A crossover or high-pass filter prevents unsuitable low-frequency energy from being sent to the tweeter.
Amplifier
│
▼
Crossover
│
└──── High Frequencies ───► Tweeter
Speaker Fuses
Some systems use fuses or other protection components to limit the current delivered to speaker drivers.
The protection device must be selected for the specific application.
Speaker Phase and Multiple Drivers
When multiple drivers operate together, their relative phase and acoustic alignment affect the resulting sound.
Crossover design and physical driver placement are important for achieving smooth integration between drivers.
Speaker Cabinet Damping
Internal damping materials can be used in speaker enclosures to control internal reflections and resonances.
The appropriate amount and type of damping depend on the enclosure design.
Speaker Grille
A grille protects the speaker from physical damage while allowing sound to pass through.
The grille material and construction can influence acoustic output, particularly at higher frequencies.
Speaker Mounting
A speaker should be securely mounted to prevent vibration and air leakage where the enclosure design requires a sealed connection.
Loose mounting can produce unwanted noise and vibration.
Speaker Faults
| Symptom | Possible Cause |
|---|---|
| No sound | Open voice coil, wiring fault, crossover fault or amplifier problem |
| Distorted sound | Damaged cone, rubbing voice coil, clipping or overdriving |
| No bass | Polarity problem, enclosure problem, crossover fault or woofer damage |
| No treble | Failed tweeter or crossover problem |
| Rattling | Loose component, damaged cone or mechanical fault |
| Very low output | Incorrect impedance, damaged driver or amplifier problem |
| Intermittent sound | Loose terminal, broken wire or intermittent voice-coil connection |
| Voice coil rubbing | Mechanical misalignment or damaged suspension |
Advantages of Loudspeakers
- Can reproduce a wide range of audio frequencies.
- Available in many sizes and power levels.
- Different driver types can be optimized for different frequencies.
- Can be combined into multi-way systems.
- Can be designed for home audio, professional audio or specialized applications.
Limitations of Loudspeakers
- Speaker performance depends strongly on enclosure design.
- Drivers have mechanical and thermal limits.
- Impedance varies with frequency.
- High power can produce significant heat.
- Mechanical components can wear or become damaged.
- Incorrect crossover design can cause poor frequency response.
Common Speaker Design Mistakes
- Connecting a speaker below the amplifier's minimum impedance.
- Ignoring speaker sensitivity.
- Ignoring enclosure requirements.
- Using an incorrect crossover frequency.
- Connecting a tweeter without suitable high-pass protection.
- Reversing polarity between multiple speakers.
- Exceeding the driver's excursion limits.
- Exceeding the voice coil's thermal capability.
- Choosing a replacement based only on physical diameter.
- Ignoring mounting depth and cutout dimensions.
Key Points
- A loudspeaker converts electrical audio signals into sound.
- Dynamic speakers use a voice coil and magnetic field to move a diaphragm.
- Woofers reproduce lower frequencies.
- Midrange drivers reproduce middle frequencies.
- Tweeters reproduce higher frequencies.
- Subwoofers are designed for very low frequencies.
- Speaker impedance varies with frequency.
- Speaker power ratings must be interpreted according to the manufacturer's test conditions.
- Sensitivity describes acoustic output for a specified input condition.
- Crossovers divide the audio spectrum between drivers.
- Speaker enclosures strongly influence low-frequency performance.
- Thiele-Small parameters are important when designing low-frequency speaker systems.
- Voice-coil heating can limit speaker output at high power.
- Speaker polarity should be consistent when multiple drivers operate together.
- Replacement speakers must match electrical, acoustic and mechanical requirements.