Speaker Distortion Explained
Speaker distortion, explained in one line: anything the system adds to or changes in the signal. In a sound system the large sources are amplifier clipping, the driver's motor and suspension running out of linear travel, coil heating and port turbulence. Each has a known mechanism and a known fix.
Harmonic distortion
A nonlinear system fed a pure tone adds integer multiples of it. A 40 Hz tone comes back with energy at 80 Hz (second harmonic), 120 Hz (third) and so on. Total harmonic distortion (THD) expresses those harmonics as a percentage of the fundamental.
In the sub range this matters more than the percentage suggests. The ear's equal-loudness contours (ISO 226) are far less sensitive at 30 to 40 Hz than at 80 to 120 Hz, so harmonics land where hearing is more sensitive than at the fundamental.
Intermodulation distortion
Two or more tones through a nonlinear system produce new tones at their sums and differences. Intermodulation distortion (IMD) is not harmonically related to the music, so it reads as mud and roughness. In a full-range box, large bass excursion modulates the mids coming off the same cone. That is one reason PA splits bass and mids onto separate drivers with a crossover.
Amplifier clipping
An amplifier's output cannot exceed its supply rails. Ask for more and the peaks are flattened.
- Harmonics: symmetrical clipping adds odd harmonics. Hard clipping a sine towards a square wave adds a series at 3f, 5f, 7f and up.
- Heat: a clipped wave spends longer near full voltage, so average power rises. A full square wave carries twice the power of a sine with the same peak (+3 dB), all of it into the voice coil.
- High-frequency drivers: those added harmonics pass through passive crossovers to tweeters and compression drivers.
The fix is headroom: enough amplifier for the job, sensible gain structure, and a limiter set below clip. See the amp matching guide.
Driver nonlinearity
Bl
Bl is the force factor: flux density in the gap (B) times the length of coil wire in it (l). While the winding stays in the gap, Bl is roughly constant. As the coil travels out, fewer turns sit in the field, Bl falls and force no longer tracks current. The cone stops following the signal, producing both harmonic and intermodulation distortion.
Suspension
Spider and surround stiffen as they stretch, so at large excursion they resist more than at rest and compress the peaks. A double spider, as on the WM18X451, is used to keep the cone centred and the coil aligned in the gap under heavy drive.
Inductance
Coil inductance changes as the coil moves in and out of the motor's iron, so current and force vary with cone position. Shorting rings and copper caps in the motor are used to reduce this.
Xmax and Xmech
Xmax is the one-way excursion within the linear range. Definitions vary: coil overhang, a 10% drop in motor linearity (AES2-1984), or a 10% distortion threshold. Past Xmax distortion climbs fast; at Xmech parts collide or tear. The WM1850/8 is rated 12 mm Xmax over a 1244 cm² cone. Detail in Xmax and excursion.
Other sources
| Source | Mechanism | Reduce it with |
|---|---|---|
| Port noise | High air velocity in an undersized port causes turbulence and chuffing | Larger, flared ports; less drive near tuning |
| Power compression | Copper's resistance rises about 0.39% per °C, so a hot coil draws less power from the same voltage | Larger coils, motor venting, enough drivers that none runs at its limit |
| Cone breakup | The cone stops moving as a rigid piston, adding peaks and colouration | Cross over below breakup |
| Cabinet rattles | Panels, loose fittings or leaks buzz under bass | Bracing, gaskets, sealing |
A coil 100 °C above ambient has about 39% more resistance: roughly 1.4 dB less output at the same drive voltage. Heat is covered in voice coils and heat, cabinet faults in bracing and sealing cabinets.
Running clean
- Enough drivers and cabinets for the level, so none runs at its limit.
- A high-pass filter below the cab's working band.
- Amplifier headroom and limiters.
- Treat a change in character as a warning: flapping, rattling or sudden harshness means turn down now.
If a driver already sounds wrong, see is my speaker blown.
Sources: ISO 226 equal-loudness contours; copper resistance temperature coefficient (about 0.0039 per °C at 20 °C); AES2-1984 Xmax wording as quoted in Klippel Application Note 4, checked October 2026; Wavemark product data. Square wave and power compression figures calculated.
Questions
What causes a speaker to distort?
Mainly amplifier clipping, the cone moving beyond its linear excursion (Bl and suspension nonlinearity), port turbulence, power compression from a hot voice coil, and loose or leaking cabinet parts.
Can a clipping amplifier damage speakers?
Yes. Clipping raises average power, up to double for a fully squared sine at the same peak, and adds high harmonics. That overheats coils and can damage tweeters and compression drivers.
What is Bl in a speaker?
The force factor: magnetic flux density in the gap times the length of coil wire within it. It sets how much force the motor produces per amp, and it falls as the coil travels out of the gap.
What is an acceptable THD for a subwoofer?
There is no single standard figure. Because hearing is less sensitive at 30 to 40 Hz than at the harmonics, sub distortion is more audible than the percentage suggests. Keeping excursion within Xmax matters more than a quoted THD number.