THE 12 VOLT ENCYCLOPEDIA · VOLUME IX
Polyfill in Subwoofer Boxes: The Science, Quantity, Pillow Question and When Not to Use It
Fiberfill is not imaginary air and not bass fertilizer. It changes how acoustic energy and heat move inside a cabinet. In a suitable sealed enclosure it can lower Q and make the driver behave as though compliance were somewhat greater. In a ported box, careless fill can obstruct airflow and move alignment unpredictably.
What the fibers actually do
As air oscillates through a fibrous network, friction and repeated heat exchange dissipate acoustic energy. The process damps internal modes and can make compression/expansion closer to isothermal, increasing effective acoustic compliance. The box has not grown physically; the driver simply sees a changed acoustic load.
How much apparent volume is gained
Rules claiming a guaranteed twenty, thirty or forty percent are too confident. Effect depends on fiber type, density, distribution, enclosure size and frequency. Light or moderate fill in a sealed box may create a useful measured change; packing it solid eventually adds resistance and can reduce benefits. Design near the real volume and verify with impedance.
Loose fill versus wall lining
Loose fill forces more moving air through fibers and is common in sealed boxes. Wall lining controls reflections while leaving the central air path and a vent clear. Foam, felt, fiberglass, wool, denim and polyester differ in airflow resistance and safety. Open-cell acoustic foam is not the same as closed-cell packaging foam.
Can a cheap pillow be used?
Many inexpensive pillows contain polyester fiber similar in base chemistry to audio-branded fill. That does not make every pillow identical. Fire treatment, fiber length, density, contamination, fabric and consistency can differ. Remove no material blindly in a hot vehicle; use clean, stable fiber whose behavior and safety are understood. The price of branded material often buys known weight, packaging and consistency—not supernatural acoustics.
How to quantity it
Weigh the material rather than describing handfuls. Distribute it evenly without hard compressed clumps. A common sealed-box starting range is roughly 0.5 to 1.0 pound per cubic foot, but it is only a starting range; some designs need less or more. Measure resonance/Q change and stop when additional density no longer helps the target.
Securing material
In sealed boxes, teasing fiber uniformly and keeping it clear of the cone, terminals and pole vent may be enough; mesh or light attachment can prevent migration. In ported or multi-chamber systems, use proper mesh, adhesive intended for the material or wall lining so fibers cannot be blown into the duct. Never allow adhesive overspray onto suspension or voice-coil ventilation.
When not to use it
Do not toss loose fill into a high-velocity port, horn throat, narrow transmission path or bandpass chamber without modeling its resistance. Do not use it to conceal a box that is grossly too small, a leak, a panel resonance or a bad tuning. Do not put flammable or degrading household material near hot components.
How to prove the effect
Measure the driver’s free-air parameters if needed, then sweep cabinet impedance before and after a weighed change. In sealed systems, observe resonance and peak shape. In vented systems, watch both peaks and Fb. Repeat at the same temperature and low voltage. Listening can confirm preference, but measurement tells whether the fiber changed the alignment or only the story.
Frequently Asked Questions
Does polyfill physically increase volume?
No. It changes acoustic compliance and damping so the driver may behave as if volume were somewhat larger.
Is pillow stuffing identical?
Sometimes the fiber is similar, but density, treatment and consistency are not guaranteed.
Should polyfill be glued down?
Secure it when migration could reach a driver vent or port; avoid blocking airflow.
Can I use fill in a ported box?
Often as controlled wall lining, but loose fill near the port can change losses and cause obstruction.
How much should I add?
Weigh a conservative starting amount and verify the actual response or impedance change.
Final Word
Enclosure design is controlled energy management. Choose a topology because its size, passband, efficiency, excursion and construction demands fit the vehicle and listener—not because a nickname wins an internet argument. When the drawing, math, build and measurement agree, the “sorcery” becomes repeatable engineering.