Voice Coil Heating and Power Compression
Your subwoofer's rated power handling is measured with a cold voice coil. In the real world, voice coils heat up - and when they do, everything changes.
What power compression is
Power compression is the reduction in acoustic output that occurs when a voice coil heats up during sustained operation. As the coil temperature rises, its DC resistance (Re) increases, which reduces the current flowing through it for a given voltage. Less current means less force, which means less cone displacement and less sound output.
This is not a failure mode. It is normal physics. The resistance of copper and aluminium wire increases predictably with temperature.
The scale of the problem
A voice coil that measures a few ohms when cold can rise by 50% or more at sustained operating temperatures. The hotter the coil, the more current it rejects, and the less output you get.
At moderate operating temperatures, you might lose a decibel or so — noticeable if you are measuring, but not dramatic. At the extreme temperatures that competition subwoofers regularly reach, losses can exceed 3 dB. That is half the acoustic power, gone to heat. You are putting in serious wattage but getting substantially less output than the cold-coil spec sheet suggests.
Why this matters for your box design
Power compression affects vented enclosures differently than sealed:
Impedance shift. The increased Re raises the entire impedance curve, which changes the interaction between the driver and amplifier. For amplifiers that deliver constant voltage (most car audio amps), the impedance rise reduces power delivery, compounding the output loss.
Q factor changes. Qes (electrical Q) is proportional to Re. As Re rises, Qes rises, and therefore Qts rises. This shifts the system response - a driver that was well-damped at room temperature becomes under-damped at operating temperature, with a peaked, boomy response.
Tuning shift. In a vented enclosure, the changed Q shifts the system's response shape. The practical effect is that the box sounds tighter and more controlled when cold, then becomes looser and boomier as it warms up during a listening session.
Thermal time constants
Voice coils do not heat up instantly. A subwoofer's thermal behaviour has two time constants:
Short-term (voice coil to former/magnet): Seconds. The voice coil heats quickly because it is a thin wire with low thermal mass. Heat conducts to the former and magnet gap.
Long-term (magnet/motor to ambient): Minutes. The magnet assembly absorbs heat from the coil and dissipates it to the air and enclosure. This is slow because the magnet has high thermal mass.
On bass transients (brief, loud hits), the coil heats briefly but the thermal mass of the motor absorbs it. On sustained bass content (electronic music, test tones, movie explosions), the coil reaches thermal equilibrium at a much higher temperature.
This is why subwoofers can handle brief peaks well above their rated power but lose output on sustained heavy content.
Mitigation strategies
Ventilated motor structures. Drivers with pole vents, spider vents, or aerodynamic cooling channels dissipate heat faster, reducing the equilibrium temperature. These drivers show less power compression at sustained power levels.
Larger voice coils. A larger coil has more thermal mass and surface area for heat dissipation. A 3-inch voice coil handles sustained power better than a 2-inch coil of the same resistance.
Aluminium vs copper. Aluminium wire is lighter (lower Mms, higher efficiency) but has higher resistivity and heats faster per unit of current. Copper is heavier but handles heat better. Some drivers use copper-clad aluminium wire as a compromise.
Derating. The most reliable approach is to design for 60–75% of rated power as the continuous operating point. If the driver is rated at 800W, design the system as if it will receive 500–600W sustained. This keeps the coil temperature in the range where compression is minimal (under 1.5 dB).
How RokketBox models thermal effects
RokketBox's simulation includes power compression modelling based on the voice coil resistance, thermal constants, and power input. The impedance and SPL curves show the effect of coil heating at the specified power level, giving you a realistic picture of sustained output rather than the optimistic cold-coil spec sheet numbers.