Short answer

Start at about ½ lb of fiber fill per cubic foot of net box volume, fluffed and loosely filled, then measure and adjust. That’s the figure Parts Express prints on its current Acousta-Fill listing for “most sealed and vented enclosures,” and the one Mahogany Sound’s Acousta-Stuf instruction sheet suggests for sealed boxes. For a ported box, the same sheet gives ¼ to ½ lb per cubic foot, says to keep an open path between the driver and the vent, and recommends against dense packing. Both are starting points, not rules: the sheet itself says some experimentation will usually be needed. The math is simple. Work out your net volume first (how to calculate net box volume), then multiply. A 1.0 ft³ sealed box (about 28 liters) starts at ½ lb.

What stuffing actually does

Rod Elliott (ESP) points out that even when the panels are dead, the air inside the box is not. Damping material absorbs sound inside the enclosure. How you put it in matters: added loosely, ESP says, it makes the enclosure acoustically larger; packed in tightly, the enclosed volume gets smaller. Either way it changes how the driver reacts with the box, mainly at or near the driver’s resonant frequency. ESP adds that in many, though by no means all, commercial designs, the driver’s interaction with the filled volume is expected to be modeled and measured, and the fill adjusted to suit. Two things stuffing is not:

Sealed boxes

Mahogany Sound’s sheet says a sealed box can be completely filled, loosely, at about ½ lb per cubic foot. The fiber ships compressed, so fluff or tease it to an even consistency before it goes in, and keep it clear of the driver so it can’t interfere with the cone or block any vents the driver needs for cooling. On packing a sealed box more densely, the sources pull in different directions. Mahogany Sound says denser packing can act like lowering the system’s Q, similar to making the enclosure larger, at the cost of efficiency. ESP says tightly packed fill makes the enclosed volume smaller. Both agree on loose fill as the place to start, so begin there and let a measurement decide whether to change anything.

Ported (vented) boxes

Builders argue about whether a vented box should be damped at all. ESP’s view is that it’s essential: without it, upper bass and midrange energy bounces around inside the cabinet, and you can often hear it through the vent. Elliott’s test: if you can hear anything coming out of the vent that isn’t at the tuned frequency, the box needs damping. How much, and where:

  • Line the walls, or fill lightly with a clear path to the port. Mahogany Sound says vented boxes are usually lined, can be filled at ¼ to ½ lb per cubic foot, and should keep an open path between the driver and the vent. Dense packing, it says, only takes up space and effectively makes the box smaller.
  • Expect some loss of port output. In an ESP article, Robert C. White explains that volume filling a reflex box adds loss and reduces port output. In his 8-liter test box, a light fill (about half the density of standard polyester stuffing, roughly 12 kg/m³) cut port output by 3 to 4 dB. He found the effect on box Q minimal, and says his tests are not definitive.
  • A thin lining alone mostly works on the midrange. White’s measurements show a typical 25 mm (1 in) polyester lining doesn’t give really useful attenuation until around 1 kHz.
  • No fiberglass. ESP rates fiberglass as very good damping, but not for vented boxes, because glass fibers may be blown out of the port.

Because fill changes the port’s output, re-check tuning after you stuff a ported box (how to calculate port length).

Fiber vs foam

ESP says foam is generally not suitable as the damping material: it usually doesn’t work very well, and it tends to disintegrate after a few years. White’s test of a 38 mm acoustic foam tile gave a rough result, and he preferred the polyester fiber. Used as a wall lining together with fiber fill, though, foam made the output noticeably smoother in his test box, and he concluded that lining the inner walls with acoustic foam is beneficial. Elliott’s editor’s note adds a counterexample: one design by a respected designer used open-cell foam to line the walls, and he found the foam completely useless for damping the box’s resonances. Treat foam as a lining, not the damping. ESP’s general advice on fiber: to be effective, it should feel coarse to the touch, so there’s plenty of friction between the fibers.

Measure, then adjust

Stuff to the starting figure, then measure, change one thing, and measure again (REW basics for DIY speaker builders). Log the weight of fill you used in your build notes, along with net volume and tuning. Choosing between box types in the first place? See sealed vs ported.

Parts

Parts Express lists its Acousta-Fill nylon polyfill in 1 lb and 5 lb bags (links below). Its listing gives the same starting point used above: ½ lb per cubic foot, loosely filled throughout. At that rate a 1 lb bag covers about 2 ft³ of net volume. We haven’t tested it against other fills. The Acousta-Stuf instruction sheet quoted above is for an earlier Parts Express fiber product that’s no longer listed. Links to Parts Express may earn us a commission; see our disclosure. Building the box itself? See MDF vs plywood for speaker boxes.