How to do great cabinet, braces and stuffing

Dimensions

We begin by defining the box volume and the primary vent dimensions using VituixCAD.

We aim to avoid internal dimensions that are multiples of one another, as this can lead to standing waves.
VituixCAD offers a useful tool for this: Tools → Auxiliary → Box Volume

Braces

The I-beam (IPN-style) brace offers the best ratio of stiffness to occupied volume and provides rigidity across the full frequency range.
This is not the case with a simple cross brace centered in the box, which only reinforces at a narrow frequency.

Corners and panel junctions (typically at 90° in rectangular enclosures) are already structurally strong.
It’s in the center of each large panel that additional stiffness is required.

A cross brace without substantial reinforcement is ineffective.
Only strapping structures like the IPN-style brace provide broadband structural support.

Stuffing

Low-density materials like Poly-Fill or pillow stuffing are often used in sealed enclosures to simulate a larger internal volume.
However, they are not particularly effective at absorbing internal reflections.

Poly-Fill is generally avoided in vented boxes, or used in very small amounts.
As we’re already tuned near a Bessel alignment, stuffing in the middle would reduce port efficiency and compromise bass extension — something we want to avoid.

For side-wall damping, the best material is 20 mm (1 inch) waste fiber insulation used in automotive applications:

This is a high-density damping material.
Apply it on each internal side, except the front panel and any brace.
Double the thickness on the top and bottom panels when the speaker is tall.

This treatment naturally adds a bit of virtual volume, helping to compensate for the space occupied by the internal bracing — exactly what we want.

In vented enclosures, absorbing material must not be placed in the direct path between the woofer and the port, as it will reduce the port’s output and overall efficiency.

For the subwoofer case

In sealed subwoofers, we use a lot of Poly-Fill to simulate added volume.
In vented subwoofers, use little to none.

Usually, resonances inside subwoofer boxes are not an issue because the wavelengths involved are much longer than the box’s internal dimensions — so internal standing waves are effectively absent.