Why Sub Drivers Fail
Why sub drivers fail comes down to two mechanisms: more heat in the voice coil than it can shed, or more cone travel than the suspension and motor allow. Clipping, missing high-pass filters and water are the usual routes to one or the other. Identify which one killed a driver before fitting the next, or it goes the same way.
Thermal or mechanical
| Type | Cause | Typical evidence |
|---|---|---|
| Thermal | Sustained power above what the coil can shed as heat | Burnt smell, darkened coil, open circuit, deformed former, rubbing |
| Mechanical | Cone driven beyond its safe excursion | Torn surround or spider, coil jammed or knocked out of the gap, bottoming noises, failed glue joints |
Thermal failure
Almost all the electrical power into a sub driver ends up as heat. A driver radiating all its input as sound into half space would measure about 112 dB at 1 W / 1 m. The WM1850/8 is rated 93.5 dB, which is 10(93.5−112)/10 ≈ 1.4% efficient: at 1800 W, roughly 1775 W is heat in the coil. That heat has to leave through the former, the gap and the motor. If it comes in faster than it leaves, the coil temperature climbs, adhesives soften, insulation breaks down, turns short and the wire eventually burns open.
Coil size is the main lever: a larger coil has more surface to shed heat. The WM1850/8 uses a 125 mm coil in a vented motor; the WM18X451 a 115 mm (4.5") copper coil. Power compression feeds the problem: copper's resistance rises about 0.39% per °C, so a hot coil gives less output for the same voltage and operators turn up further. See voice coils and heat.
Clipping
A clipped amplifier flattens the waveform peaks. At the same peak voltage, a sine clipped all the way to a square wave carries twice the average power, so the coil heats faster, and the added odd harmonics are content a sub was never meant to reproduce.
A small amplifier is not "safer". Driven into hard clipping it can cook a driver; a properly sized amplifier with a limiter below clip is the safer setup. See amp matching and power ratings explained.
Over-excursion
Xmax is the travel over which the coil stays in the gap's linear region. Past it, distortion rises fast; past the mechanical limit the coil former strikes the back plate, the spider stretches or tears, and the surround rips. For constant output, excursion quadruples with each octave down, so mechanical failures happen at the bottom of the range.
No high-pass filter
The most common route to mechanical damage. Below a reflex box's tuning the port stops loading the cone and the driver behaves almost as if unbaffled: excursion rises sharply while useful output falls. Horns and scoops unload below their cut-off in the same way. A high-pass filter a little below the cab's useful low limit keeps that energy out. See cabinet tuning and Xmax and excursion.
Other triggers
- Dropped microphones, cables pulled while live, and turntable feedback: large low-frequency transients.
- Heavy EQ boost at the very bottom on a mixer or DSP.
- A driver in a cab it does not suit: one designed for horn loading can be poorly controlled in a large reflex box, and vice versa.
Water and damp
Paper cones and many adhesives are weakened by water, and terminals and motor parts corrode. Rain on an outdoor rig, condensation in a cold van and storage in a damp garage all count. Cover cabs outdoors, let them dry and warm before running hard, and store them dry.
Age and handling
Suspensions fatigue over years of hard use. A dropped cab can shift the motor or bend the basket. Cones stacked against hard objects get punctured. See loading and stacking safely.
Prevention
- High-pass filter on every sub channel.
- Amplifier with headroom, limiters set to protect the driver.
- Watch clip lights and back off gain, not just EQ.
- Match the driver to the cab type.
- Keep cabs dry.
Already failed? Start with is my speaker blown, then weigh reconing against replacement. A replacement WM1850/8 is £275 for an 1800 W AES rating: 15.3p per rated watt.
Sources: 112 dB at 1 W / 1 m corresponds to 1 W acoustic radiated into half space (I = 1/2π W/m² at 1 m re 10⁻¹² W/m²); copper resistance temperature coefficient about 0.0039 per °C; Wavemark WM1850/8 spec sheet and WM18X451 product data. Efficiency and clipping figures calculated and rounded; the efficiency estimate assumes the sensitivity was measured at 1 W / 1 m in half space.
Questions
What is the most common cause of subwoofer failure?
Overheating the voice coil through sustained high power or amplifier clipping, and over-excursion from running without a high-pass filter. Inspection usually shows which: burnt or open coils point to heat, torn suspensions or a knocked coil point to excursion.
Can an underpowered amp blow a speaker?
Yes. Driven into hard clipping it delivers more average power, up to double for a sine clipped to a square wave at the same peak, which heats the coil. A properly sized amp with a limiter is safer.
Do I need a high-pass filter on my subs?
Yes. Below a reflex box's tuning, or a horn's cut-off, the cone is no longer loaded and excursion rises sharply for little output. A high-pass filter a little below the cab's working range prevents this.
How much of an amplifier's power becomes heat in a sub driver?
Nearly all of it. A driver rated 93.5 dB at 1 W / 1 m is about 1.4% efficient, so around 98.6% of the input ends up as heat in the voice coil and motor.
Can rain damage speaker drivers?
Yes. Water softens paper cones, weakens adhesives and corrodes terminals and motor parts. Cover cabs outdoors and let them dry before use.