Polyfill in a sealed subwoofer box makes the box behave as if it were larger than it physically is, by about 10 to 25 percent. The fibers absorb heat from the air's compression cycle, which slows the speed of sound inside and makes the driver feel more airspace than the box actually holds. It is a real, useful effect in a sealed box, but it is not magic: it will not rescue a box that is fundamentally too small, and packing it tight kills the benefit.
- Polyfill raises a sealed box's apparent internal volume by about 10 to 25 percent (PUI Audio)
- The mechanism is thermal: fibers absorb compression heat, shifting the air toward isothermal behavior and slowing the speed of sound inside the box
- Use roughly 50 percent loose fill, or about 1 pound per cubic foot, never packed tight (KICKER)
- Sealed boxes only. Polyfill will not rescue an undersized box, and in a box already at target volume it just reduces efficiency. Proline X sealed variants ship with polyfill pre-installed
This is a tuning detail in our how a CNC subwoofer enclosure is built series. For choosing sealed versus ported in the first place, see sealed vs ported, by the numbers.
What Does Polyfill Do in a Sealed Subwoofer Box?
It makes the driver feel a bigger box than it has. Adding polyfill to a sealed enclosure raises its apparent internal volume by about 10 to 25 percent (PUI Audio). The driver behaves as though it is in a larger sealed box, which lowers the system's resonance slightly and can improve low-end extension in a box that is on the small side.
The mechanism is thermal, not just damping. As the cone moves, it compresses and rarefies the air inside the box, and compression normally heats the air. Polyfill fibers absorb and release that heat, shifting the air from adiabatic behavior toward isothermal, which effectively slows the speed of sound inside the enclosure. Slower sound in a fixed box looks, to the driver, like more airspace. The same fibers also damp internal standing waves, absorbing energy that would otherwise reflect around inside the box (Audio Judgement).
Chart: Apparent volume of a 1.0 ft³ sealed box with polyfill
Source: apparent-volume increase of 10 to 25% in a sealed box (PUI Audio).
How Much Polyfill Should You Use?
About 50 percent of the box volume, loose, never packed. The common guideline is to loosely fill roughly half the internal volume, which works out to around one pound of polyfill per cubic foot for the full effect (KICKER). The key word is loose. Polyfill works by letting air move through the fibers, so packing it tight blocks the airflow and kills the thermal effect you are after.
If you are tuning by measurement, the practical method is to build close to the target volume, drop the driver in, and add polyfill in small increments while watching the system Q settle toward target. You are adding apparent volume in small steps, not stuffing the box full and hoping.
| Box volume | Approx. polyfill (loose) |
|---|---|
| 0.5 cubic foot | ~0.5 pound |
| 1.0 cubic foot | ~1 pound |
| 1.5 cubic feet | ~1.5 pounds |
| 2.0 cubic feet | ~2 pounds |
Approximate starting amounts at about 1 pound per cubic foot, loosely distributed (KICKER). Fine-tune by measurement.
Does Polyfill Always Help?
No, and the honest limits are the important part. Polyfill gains you 10 to 25 percent apparent volume, not double. It will not rescue a box that is fundamentally too small for the driver. And in a box already at or above the driver's target volume, it does nothing useful, just reducing efficiency for no acoustic gain. More is not better past the point of effect.
It is also a sealed-box tool. Polyfill belongs in sealed enclosures, where the trapped air is what you are modifying. In a ported box the tuning comes from the port and the vent airflow, and loose fiber in the wrong place can interfere with that, so polyfill is not a default for ported builds. If your sealed box is a little small for the space you have, polyfill is a legitimate way to recover some of the difference. If the box is already right, leave it alone.
How Proline X Uses Polyfill
Our sealed variants ship with polyfill pre-installed, loosely distributed to the right amount for that box and driver. You do not have to source it, weigh it, or guess. It is dialed in as part of building the box to the driver's target, especially on the compact sealed and shallow builds where the apparent-volume gain matters most.
A Proline X Micro Series sealed enclosure built for a ResoniX Gus 10-inch subwoofer. Sealed Micro boxes ship with polyfill pre-installed.
That is the whole point of building to a driver instead of to a catalog number: the volume, the seal, and the fill are all set together so the box performs the way the driver's parameters say it should. Polyfill is one piece of that, handled so you never think about it.
Frequently Asked Questions
Does polyfill actually work in a sealed subwoofer box?
How much polyfill do I put in a subwoofer box?
Can polyfill fix a subwoofer box that is too small?
Should I use polyfill in a ported box?
Do Proline X boxes come with polyfill installed?
Want a Sealed Box Tuned and Filled for Your Driver?
We build Proline X sealed enclosures to your driver's target volume, with polyfill pre-installed to the right amount, CNC-cut from Langboard Elite MDF in our Tullahoma, Tennessee shop.
Send us your driver and install space and we will spec the box. Contact us with the details, or browse the Proline X enclosures and the full subwoofer enclosures collection.
About the Author
Scott Welch is a Multi Time IASCA National and MECA World Sound Quality Champion, an active SQ judge since 2019, and the owner of Audio Intensity in Tullahoma, Tennessee. He cuts every Proline X enclosure on the shop's CNCs and tunes every customer system before it leaves. Audio Intensity is the original US importer for Goldhorn DSP and an authorized dealer for Prodigy, Crescendo, Image Dynamics, Wavtech, Tru Technology, and more.
