What Is Speaker Distortion?

Distortion occurs when the output of a loudspeaker differs from the ideal output that would be produced by a perfectly linear system.

If a speaker receives a pure sine wave, an ideal linear speaker would produce only that same frequency.

A real loudspeaker can generate additional frequencies.

Input:

1 kHz sine wave

        ↓

    Loudspeaker

        ↓

Output:

1 kHz
2 kHz
3 kHz
4 kHz
...

The additional components are produced by nonlinear behaviour in the loudspeaker.

Why Loudspeakers Produce Distortion

A loudspeaker contains many physical elements that must operate linearly for accurate reproduction.

Important sources of nonlinearity include:

• Voice-coil motor behaviour
• Magnetic-field variations
• Suspension nonlinearity
• Large cone excursion
• Voice-coil position in the magnetic gap
• Cone breakup
• Mechanical rubbing
• Voice-coil heating
• Cabinet vibration
• Port turbulence

Linear vs Nonlinear Behaviour

A linear system produces an output proportional to its input.

Input doubles
     ↓
Output doubles

A nonlinear system does not maintain this proportional relationship.

Input doubles
     ↓
Output does not exactly double
     ↓
Additional frequency components appear

Loudspeaker distortion becomes increasingly important as the driver is operated closer to its mechanical, thermal or acoustic limits.

Harmonic Distortion

One of the most common forms of loudspeaker distortion is harmonic distortion.

If the input signal is a pure tone at 1 kHz, nonlinear behaviour can produce harmonics at integer multiples of the fundamental frequency.

Fundamental:
1 kHz

2nd harmonic:
2 kHz

3rd harmonic:
3 kHz

4th harmonic:
4 kHz

5th harmonic:
5 kHz

These additional components are harmonically related to the original signal.

Second Harmonic Distortion

The second harmonic is twice the fundamental frequency.

For a 100 Hz fundamental:

Fundamental = 100 Hz
2nd harmonic = 200 Hz

For a 1 kHz fundamental:

Fundamental = 1 kHz
2nd harmonic = 2 kHz

Third Harmonic Distortion

The third harmonic occurs at three times the fundamental frequency.

Fundamental = 100 Hz
3rd harmonic = 300 Hz

Higher-order harmonics can also be generated by nonlinear loudspeaker behaviour.

Total Harmonic Distortion — THD

THD stands for Total Harmonic Distortion. It expresses the combined level of harmonic components relative to the fundamental signal.

A simplified expression is:

THD = √(V₂² + V₃² + V₄² + ...) / V₁

where:

• V₁ = fundamental component
• V₂ = second harmonic
• V₃ = third harmonic
• V₄ = fourth harmonic

The result is commonly expressed as a percentage.

Example of THD

Suppose a speaker is driven with a 100 Hz sine wave and the measured output contains:

100 Hz  = fundamental
200 Hz  = 1 V
300 Hz  = 0.5 V

If the fundamental is 10 V, the harmonic components can be combined using the THD relationship.

The important point is that THD expresses the total harmonic content relative to the fundamental.

THD Percentage

A distortion specification might be written as:

THD = 1%

This means the combined harmonic content is approximately 1% of the fundamental according to the measurement conditions and definition used.

The measurement frequency and output level should always be considered when comparing THD specifications.

THD vs Frequency

Loudspeaker distortion is normally frequency-dependent.

A driver can have low distortion in one frequency range and much higher distortion in another.

THD
│
│       /\
│      /  \
│     /    \____
│____/          \____
│
└────────────────────► Frequency

For this reason, a single THD number does not completely describe a loudspeaker.

THD vs SPL

Distortion also changes with output level.

A speaker may have relatively low distortion at moderate SPL but produce significantly more distortion at high SPL.

This is because cone excursion, motor nonlinearity and thermal effects become increasingly important as the input level rises.

Harmonic Distortion Measurement

A common method is to drive the speaker with a sine wave and measure the acoustic output using a microphone.

Signal generator
       ↓
   Amplifier
       ↓
    Speaker
       ↓
   Microphone
       ↓
Spectrum analysis
       ↓
THD calculation

The measurement can be repeated at different frequencies and output levels.

Second- and Third-Harmonic Graphs

Speaker testing software can display harmonic distortion as separate curves.

Distortion
   │
   │  H3 ──────────
   │
   │  H2 ───────
   │
   └──────────────────► Frequency

Examining individual harmonics can reveal more information than a single THD value.

Low-Frequency Distortion

Low-frequency operation can produce substantial distortion because the cone must move a large distance to produce significant acoustic output.

As frequency decreases, the required excursion for a given acoustic output generally increases.

This makes excursion-related distortion particularly important in woofers and subwoofers.

Excursion Distortion

A loudspeaker suspension and motor system are not perfectly linear over unlimited excursion.

As the cone moves farther from its rest position:

• The magnetic force can change.
• The suspension stiffness can change.
• The voice coil can move out of the optimal magnetic gap position.
• The effective BL product can change.

These effects can produce distortion.

BL Nonlinearity

The motor force is commonly represented by:

F = BL × I

where BL is the motor force factor and I is voice-coil current.

In an ideal motor, BL would remain constant throughout the operating range.

In a real loudspeaker, BL can vary with voice-coil position.

This variation contributes to nonlinear distortion.

BL Curve

A driver can be characterized by measuring motor force as the voice coil moves through its operating range.

BL
│
│        ─────────
│      /           \
│    /               \
│───                   ───
│
└────────────────────────► Position

A flatter BL curve generally indicates more consistent motor force over the relevant excursion range.

Suspension Nonlinearity

The spider and surround provide the mechanical restoring force that returns the cone toward its rest position.

Their stiffness can change with excursion.

This means the suspension may not behave like a perfect linear spring.

Suspension nonlinearity can therefore contribute to harmonic distortion.

Spider Nonlinearity

The spider controls the voice-coil position and provides mechanical restoring force.

At large excursion, its stiffness can change.

Different spider designs produce different nonlinear behaviour.

Surround Nonlinearity

The surround allows the cone to move while maintaining the mechanical connection between the cone and basket.

At large excursions, the surround can become significantly stiffer or behave differently from its small-signal operation.

This can contribute to distortion.

Voice-Coil Position

The position of the voice coil within the magnetic gap is important.

Many drivers are designed so that the coil operates in a region where the magnetic force is relatively uniform.

Large excursion can move the coil away from this optimal region.

Voice-Coil Overhang and Underhang

Two common motor geometries are:

• Overhung voice coil
• Underhung voice coil

The geometry affects how BL changes with voice-coil position and therefore influences linearity and excursion behaviour.

Overhung Voice Coil

In an overhung motor, the voice coil is longer than the magnetic gap.

A portion of the coil therefore remains outside the gap during normal operation.

The design can provide useful excursion capability, but BL can change as the coil moves.

Underhung Voice Coil

In an underhung motor, the voice coil is shorter than the magnetic gap.

The coil can remain within a relatively uniform magnetic region over a defined excursion range.

This can provide excellent motor linearity when correctly designed.

Magnetic Saturation

The magnetic circuit can also become nonlinear when operated under strong conditions.

Magnetic saturation can change the motor force and contribute to distortion.

Motor design, pole geometry, magnet size and gap configuration all affect this behaviour.

Voice-Coil Inductance Nonlinearity

The voice coil has inductance, represented approximately by:

Le

The inductance can change with voice-coil position and current because of the magnetic circuit.

This can produce additional nonlinear behaviour, particularly at higher frequencies.

Inductance Modulation

If voice-coil inductance changes as the cone moves, the electrical behaviour of the driver also changes.

The varying inductance can generate distortion and can also affect the driver's frequency response.

Mechanical Rubbing

A voice coil that rubs against the magnetic gap can produce severe distortion.

Possible causes include:

• Damaged voice coil
• Misalignment
• Damaged spider
• Damaged cone
• Improper reconing
• Foreign material in the magnetic gap

Rubbing may sometimes be audible even at low signal levels.

Distortion in Repaired Speakers

A repaired driver should be checked for distortion as well as basic electrical continuity.

A voice coil can have normal DC resistance while still producing mechanical distortion.

A low-level sine-wave test and impedance measurement can help identify problems after reconing.

Voice-Coil Heating

High electrical power heats the voice coil.

As the voice coil temperature increases, its resistance increases.

This changes the electrical behaviour of the driver and can reduce the increase in acoustic output expected from additional power.

Power Compression

Power compression occurs when increasing electrical input does not produce the expected increase in SPL.

Voice-coil heating is one of the most important causes.

For example, doubling electrical power would ideally produce about 3 dB more output, but a real speaker may produce less because of thermal and mechanical limitations.

Thermal Distortion

Thermal effects can alter:

• Voice-coil resistance
• Power compression
• Frequency response
• Electrical damping
• Maximum SPL

These effects become increasingly important during high-power operation.

Cone Breakup

At sufficiently high frequencies, a cone may stop behaving like a perfectly rigid piston.

Different regions of the cone can begin moving with different amplitudes and phases.

This behaviour is known as cone breakup.

It can create frequency-response irregularities and harmonic or other nonlinear behaviour.

Rigid Piston Behaviour

At lower frequencies, a well-designed cone can approximate piston-like motion.

Low frequency:

   ┌─────────────┐
   │             │
   │    CONE     │  ← moves approximately
   │             │     as one unit
   └─────────────┘

As frequency increases, the cone can develop flexural modes.

Cone Breakup Modes

Piston mode:

──────────────
──────────────

Breakup:

──────╱╲──────
─────╱  ╲─────
────╱    ╲────

These modes can create peaks and irregularities in the frequency response.

Intermodulation Distortion

Intermodulation distortion (IMD) occurs when two or more different frequencies interact through a nonlinear system and produce additional frequencies.

For example, if a speaker receives:

f₁ = 500 Hz
f₂ = 1,000 Hz

nonlinearity can produce components related to combinations such as:

f₂ - f₁
f₂ + f₁
2f₁ - f₂
2f₂ - f₁
...

IMD can be particularly useful for detecting nonlinear behaviour that may not be obvious from a single-tone harmonic distortion test.

Why IMD Matters in Loudspeakers

Music contains many frequencies simultaneously.

A speaker therefore operates with multiple tones at the same time.

A nonlinear driver can cause these frequencies to interact and create new components that were not present in the original signal.

Harmonic Distortion vs IMD

Distortion and Frequency Response

Frequency response and distortion are different measurements.

A speaker can have a very smooth frequency response while still producing substantial distortion.

Conversely, a speaker can have some frequency-response irregularity but relatively low nonlinear distortion.

Both measurements are therefore important.

Distortion Measurement at Different Levels

A useful loudspeaker test measures distortion at multiple output levels.

Low SPL
   ↓
Moderate SPL
   ↓
High SPL
   ↓
Maximum useful output

This reveals how distortion changes as the driver approaches its operating limits.

Distortion vs Excursion

For many woofers, distortion rises significantly as cone excursion increases.

This is one reason subwoofer systems must be designed with adequate excursion capability.

Reducing the low-frequency load can sometimes dramatically reduce distortion.

High-Pass Filtering and Distortion

A high-pass filter can protect a woofer from excessive low-frequency excursion.

For example, a PA speaker may use a high-pass filter below its intended operating range.

This can reduce unnecessary cone excursion and improve usable maximum SPL.

Distortion and Enclosure Design

The enclosure can influence distortion indirectly by changing the driver's acoustic loading and excursion.

A properly designed enclosure can help control cone movement in its intended operating range.

An incorrectly designed enclosure can allow excessive excursion or create other acoustic problems.

Distortion in Bass Reflex Speakers

Near the tuning frequency of a bass reflex enclosure, the port contributes significant acoustic output.

This can reduce the amount of cone excursion required from the driver around the tuning frequency.

Below the tuning frequency, however, cone excursion can increase rapidly.

A suitable high-pass filter may therefore be required in some applications.

Port Noise and Distortion

A bass reflex port can itself become a source of unwanted acoustic noise at high output levels.

Causes can include:

• High air velocity
• Port turbulence
• Sharp port edges
• Insufficient port area
• Port compression

Good port design can reduce these problems.

Distortion in Horn Speakers

Horn-loaded systems can have very high output capability, but they can also develop distortion from:

• Driver excursion
• Diaphragm nonlinearity
• Motor nonlinearity
• Horn resonances
• Airflow effects
• Diaphragm breakup

The distortion characteristics depend strongly on the compression driver and horn combination.

Distortion in Tweeters

Tweeter distortion can increase when the driver receives excessive low-frequency energy.

This is one reason the crossover frequency must be selected appropriately.

A tweeter operated below its safe range can experience excessive excursion and potentially mechanical damage.

Distortion and Crossover Frequency

Choosing an appropriate crossover frequency can reduce distortion by keeping each driver within its suitable operating range.

The crossover should consider:

• Driver frequency response
• Driver distortion
• Power handling
• Directivity
• Acoustic slope
• Impedance

Distortion and Crossover Slope

A steeper crossover slope can provide greater attenuation outside a driver's intended operating range.

However, crossover slope must be considered together with the driver's natural acoustic response.

A higher electrical filter order does not automatically produce the same acoustic filter order.

Distortion and Directivity

Distortion measurements can change with measurement angle.

The acoustic output of harmonics may have different radiation patterns from the fundamental.

This is another reason speaker distortion should ideally be evaluated under controlled and documented conditions.

Measuring Distortion with a Microphone

A microphone can capture the acoustic output of the loudspeaker.

Measurement software can then analyze the fundamental and harmonic components.

Speaker
   ↓
Acoustic output
   ↓
Measurement microphone
   ↓
Audio interface
   ↓
Computer
   ↓
FFT / spectrum analysis
   ↓
THD / harmonic measurements

FFT Spectrum Analysis

An FFT, or Fast Fourier Transform, converts a time-domain signal into a frequency-domain representation.

This allows the fundamental and harmonic components to be displayed separately.

Amplitude
│
│       │
│       │ Fundamental
│       │
│       │
│       │      │
│       │      │  H2
│       │      │
│       │          │ H3
└────────────────────────► Frequency
        f     2f    3f

Measurement Distance

The microphone distance should be controlled when comparing distortion measurements.

Changing the distance changes the acoustic level reaching the microphone and can also change the influence of room reflections.

Repeatable measurements require a consistent setup.

Room Reflections and Distortion Testing

Room reflections can complicate acoustic measurements.

Reflected sound can interfere with the direct signal and make the frequency-domain measurement more difficult to interpret.

Controlled environments, gated measurements or appropriate measurement techniques can help reduce these effects.

Distortion Measurement with a Near-Field Setup

A microphone placed close to a driver can reduce some room effects.

However, near-field distortion measurements must be interpreted carefully because the acoustic field near the driver is different from the far field.

The measurement method should be appropriate for the intended analysis.

Distortion Measurement Level

The test level should be stated when reporting distortion.

For example:

THD = 1% @ 90 dB SPL

is much more useful than:

THD = 1%

because distortion changes with operating level.

Distortion Measurement Frequency

The frequency should also be specified.

For example:

THD = 2% @ 100 Hz, 95 dB SPL

provides substantially more information than an unspecified THD value.

Distortion Graphs

A useful loudspeaker distortion graph may show:

• Fundamental SPL
• Second harmonic
• Third harmonic
• Total harmonic distortion
• Measurement level
• Frequency

Distortion
│
│ H3 ────────────
│
│ H2 ────────
│
│ THD ─────────────
│
└────────────────────► Frequency

Comparing Two Speakers

When comparing distortion between two speakers, use equivalent:

• Measurement distance
• Frequency
• SPL
• Bandwidth
• Microphone position
• Measurement method

Otherwise the comparison may be misleading.

Low Distortion Does Not Automatically Mean Better Sound

Distortion is an important performance parameter, but it is not the only factor that determines perceived sound quality.

A complete loudspeaker evaluation should also consider:

• Frequency response
• Directivity
• Maximum SPL
• Impedance
• Transient behaviour
• Enclosure performance
• Application requirements

Distortion and Human Hearing

The audibility of distortion depends on the frequency, level, type and spectral distribution of the distortion products.

Different harmonic orders can also be perceived differently.

Therefore a lower numerical THD value does not always directly predict a proportional improvement in perceived sound quality.

Harmonic Order

Distortion products can occur at different harmonic orders.

Fundamental = f

2nd harmonic = 2f
3rd harmonic = 3f
4th harmonic = 4f
5th harmonic = 5f

A distortion spectrum can therefore provide information about the type of nonlinearity occurring in the driver.

Even and Odd Harmonics

Harmonics are often discussed as even-order and odd-order components.

For a fundamental frequency f:

Even:
2f
4f
6f
...

Odd:
3f
5f
7f
...

The relative levels of these components can provide clues about the symmetry of the nonlinear behaviour.

Asymmetrical Nonlinearity

If the positive and negative halves of cone movement do not behave symmetrically, even-order harmonic components can be generated.

This can result from asymmetry in the motor or suspension system.

Symmetrical Nonlinearity

Some nonlinearities are more symmetrical around the driver's rest position.

Such behaviour can produce stronger odd-order harmonic components.

Real loudspeakers can exhibit combinations of several nonlinear mechanisms.

Distortion from a Damaged Speaker

A damaged loudspeaker can produce substantially higher distortion than a healthy driver.

Possible causes include:

• Voice-coil rubbing
• Damaged spider
• Torn surround
• Loose cone
• Damaged tinsel lead
• Shifted magnet structure
• Foreign material in the magnetic gap

Simple Diagnostic Test

A practical first test for a suspected damaged speaker is to gently move the cone by hand while the speaker is disconnected.

The movement should normally be smooth and free of scraping noises.

This test does not replace electrical or acoustic measurement, but it can reveal obvious mechanical problems.

Impedance Testing and Distortion

Impedance testing can complement distortion measurements.

An abnormal impedance curve may reveal:

• Voice-coil problems
• Mechanical resonance problems
• Enclosure problems
• Crossover problems

See: Impedance Testing .

Distortion After Reconing

After reconing a speaker, distortion should be checked at an appropriate low-to-moderate level before the driver is subjected to high power.

An incorrectly centered voice coil can produce rubbing and severe distortion.

The driver should therefore be mechanically inspected and electrically tested before high-power operation.

Reducing Loudspeaker Distortion

Several design approaches can reduce distortion.

• Improve motor linearity
• Improve suspension linearity
• Increase useful excursion capability
• Use an appropriate voice-coil geometry
• Improve magnetic-gap design
• Reduce cone breakup
• Use appropriate crossover points
• Use adequate enclosure loading
• Reduce port turbulence
• Provide sufficient thermal capacity

Reducing Excursion Distortion

Excursion-related distortion can be reduced by keeping the driver within its linear operating range.

This can be achieved through:

• Appropriate enclosure design
• High-pass filtering
• Using a larger driver
• Using multiple drivers
• Increasing Xmax
• Reducing the required low-frequency output

Reducing Thermal Distortion

Thermal compression can be reduced by improving the thermal design of the driver.

Possible approaches include:

• Improved voice-coil cooling
• Better magnetic-gap ventilation
• Appropriate voice-coil size
• Improved heat transfer
• Operating the driver within its thermal limits

Reducing Cone Breakup

Cone breakup can be controlled through:

• Appropriate cone material
• Controlled cone geometry
• Increased cone stiffness
• Improved damping
• Appropriate crossover frequency

The goal is to keep the driver operating in a controlled region.

Reducing Distortion Through Crossover Design

A crossover can reduce distortion by preventing a driver from operating outside its suitable frequency range.

For example, a tweeter should not be required to reproduce substantial low-frequency energy.

Similarly, a woofer can be protected from frequencies where its cone breakup becomes excessive.

Distortion and Speaker Efficiency

High sensitivity does not automatically mean low distortion.

A high-sensitivity driver may still have significant nonlinear distortion at high output levels.

Sensitivity and distortion should therefore be evaluated separately.

Distortion and Maximum SPL

A useful loudspeaker should maintain acceptable distortion while producing the required SPL.

Maximum SPL should therefore ideally be specified together with a distortion limit.

For example:

Maximum SPL:
120 dB @ 1 m

THD limit:
specified measurement criterion

The measurement conditions are essential to interpreting such a specification.

Distortion Measurement Checklist

• Speaker type identified
• Driver operating range known
• Measurement microphone calibrated or characterized
• Measurement distance fixed
• Test frequency specified
• Test SPL specified
• Fundamental measured
• Harmonics measured
• THD calculated
• Second and third harmonics examined
• Distortion measured at multiple SPL levels
• Room effects considered
• Mechanical condition checked
• Impedance checked where appropriate
• Thermal effects considered for high-level tests

Practical Distortion Testing Workflow

1. Inspect the speaker mechanically.
2. Check the voice-coil continuity.
3. Measure DC resistance.
4. Perform a low-level impedance test if required.
5. Connect the measurement microphone.
6. Choose a safe test frequency.
7. Apply a low-level sine wave.
8. Measure the fundamental and harmonic components.
9. Calculate or display THD.
10. Repeat at several frequencies.
11. Repeat at increasing SPL levels.
12. Observe where distortion begins to rise significantly.
13. Investigate the likely mechanical, electrical or acoustic cause.

Distortion Test Example

A woofer can be tested at:

50 Hz
60 Hz
80 Hz
100 Hz
150 Hz
200 Hz

At each frequency, measure distortion at several SPL levels.

For example:

85 dB
90 dB
95 dB
100 dB
105 dB

The resulting data can show where the driver begins to become nonlinear.

Understanding a Distortion Map

A distortion map can show distortion as a function of both frequency and SPL.

SPL
│
│       Low distortion
│      ───────────────
│    ─────────────────
│  ──────────────
│
└──────────────────────► Frequency

As SPL increases, the distortion regions generally become more pronounced.

Distortion and Speaker Design

A complete loudspeaker development process should evaluate:

Frequency response
        +
Impedance
        +
Distortion
        +
Directivity
        +
Maximum SPL
        +
Power compression
        ↓
Complete loudspeaker evaluation

No single measurement can completely describe loudspeaker performance.

Key Takeaways

• Loudspeaker distortion occurs when the acoustic output is not a linear reproduction of the input signal.
• Harmonic distortion produces additional frequencies related to the fundamental.
• THD expresses the combined harmonic content relative to the fundamental.
• Distortion changes with frequency and SPL.
• Large cone excursion is an important source of woofer distortion.
• BL nonlinearity can produce distortion as the voice coil moves through the magnetic gap.
• Suspension nonlinearity can originate from the spider and surround.
• Voice-coil inductance can also behave nonlinearly.
• Mechanical voice-coil rubbing can cause severe distortion.
• Cone breakup can produce unwanted frequency components and response irregularities.
• Voice-coil heating can cause power compression and alter the speaker's behaviour.
• Intermodulation distortion occurs when multiple frequencies interact through a nonlinear system.
• Frequency response and distortion are different measurements and should both be evaluated.
• High-pass filtering can reduce excessive low-frequency excursion.
• Proper crossover design can keep drivers within their suitable operating ranges.
• Impedance testing is useful when diagnosing abnormal speaker behaviour.
• A repaired or reconed speaker should be tested before being subjected to high power.
• Low distortion alone does not guarantee that a speaker is suitable for every application.
• Distortion measurements should always specify frequency, SPL, distance and measurement conditions.