Voice Coils and Heat
Voice coils and heat decide how a sub sounds in hour four, not minute one. A driver of 93.5 dB sensitivity converts roughly 1.4% of its input to sound; the rest heats the coil. How fast the motor gets that heat out sets power compression and, in the end, coil life.
The efficiency figure
One acoustic watt radiated into half space gives about 112 dB at 1 m. So a 1 W / 1 m sensitivity converts to efficiency as 10(sensitivity − 112) / 10. For the WM1850/8 at 93.5 dB that is about 1.4%. At 1800 W in, around 1775 W is heat, concentrated in the coil and gap.
Power compression
Copper's resistance rises by about 0.39% per °C. A coil running 150 °C above ambient has about 59% more resistance. At a fixed amplifier voltage, power falls in proportion (P = V² / R), so output drops by 10·log₁₀(1.59) = 2 dB, before counting any shift in the driver's response. The system sounds right at soundcheck and flat by the end of the night, and turning it up adds more heat. See power ratings explained.
Coil size
- Area to shed heat. Coil surface scales with diameter for a given winding height: a 125 mm coil has 25% more circumference than a 100 mm one.
- Motor strength. A larger coil allows more wire in the gap and a stronger motor (higher Bl).
- The trade. More coil and former add moving mass, which the motor has to carry; the design balances thermal capacity against Bl and Mms. See Thiele/Small parameters.
Formers and winding
The former is the tube the coil is wound on. Glass fibre and polyimide (Kapton) formers stay dimensionally stable at high temperature. Winding on both the inside and outside of the former puts more wire in the gap and spreads heat across two faces.
Cooling the motor
Vented pole pieces and back plates let the moving cone pump air through the gap and past the coil, so cooling rises with excursion. Finned or exposed motor parts help move heat from the magnet assembly to the air. Cab design matters too: a sealed, unventilated box traps that heat.
Coil specs compared
| Driver | Coil | Motor | Continuous power | Price |
|---|---|---|---|---|
| WM1850/8 | 125 mm | Ferrite, vented | 1800 W AES | £275 |
| WM18X451 | 115 mm (4.5"), copper | Ferrite, double spider | 1800 W RMS | £345 |
| B&C 18SW115 | 116 mm, copper on glass fibre former | Neodymium | 1700 W nominal | £444 (Thomann UK) |
The WM1850/8 carries the largest coil of the three at the lowest price. Why big coils matter for sustained sub-bass: why sub drivers fail and dub and the sound system.
Sources: copper temperature coefficient 0.393% per °C (annealed copper, IEC standard value); efficiency from sensitivity and P = V²/R are standard relationships; B&C Speakers published data for the 18SW115, checked October 2026; Thomann UK listed price for the 18SW115 8 Ω, checked 11 October 2026; Wavemark WM1850/8 spec sheet and WM18X451 product data. Calculated values rounded.
Questions
What is power compression in a speaker?
As the voice coil heats, its resistance rises (about 0.39% per degree C for copper), so the amplifier delivers less power and output falls. A coil 150 degrees C above ambient loses about 2 dB from resistance alone.
How much of an amplifier's power becomes sound?
Very little. A sub driver of 93.5 dB sensitivity converts about 1.4% of its input to sound; the rest becomes heat in the coil and motor.
Does a bigger voice coil handle more power?
Generally yes. A larger diameter gives more surface to shed heat and room for a stronger motor, at the cost of more moving mass. Venting and former material also affect how much heat a driver can handle.
What does a vented motor do?
Vents in the pole piece and back plate let the cone's motion pump air past the coil, carrying heat out of the gap. The harder the driver works, the more air it moves.