Short answer: build it to the right internal volume, make it stiff, and make it airtight. 18 mm MDF or birch ply both work. Brace the big panels, glue every joint, gasket every driver and terminal, and use damping to match the box type: stuffing in sealed boxes, wall lining in ported ones. A plain box built accurately beats a fancy one built sloppily.
The three jobs of a cabinet
- Be the right size inside. The internal volume (and port size, if it has one) sets the bass. Build to the design's internal dimensions, and remember braces and the driver itself take up room.
- Stay still. Walls that flex or ring add their own sound. Stiffness and bracing deal with it.
- Not leak. A leak in a sealed box acts like a badly tuned port, and in a ported box it messes with the tuning. Leaks also whistle.
MDF or plywood?
| MDF | Birch / hoop pine ply | |
|---|---|---|
| Cost | Cheaper | Dearer |
| Flatness and consistency | Very flat, no voids | Good ply is flat; cheap ply can have voids |
| Stiffness for its weight | Lower; heavy | Higher; lighter for the same stiffness |
| Holding screws | Poor in the edges | Good |
| Edges | Smooth, but soak up paint | Show the layers (some people love it) |
| Machining | Cuts and routs cleanly | Can splinter; needs sharp tools |
| Dust | Very fine and nasty: wear a proper mask | Normal wood dust |
Our take: MDF is the easy, cheap, forgiving choice for painted cabinets and subwoofers. Good birch ply is stiffer and lighter, holds screws, and looks great with a clear finish, at a higher price. 18 mm is the usual thickness for both. Go thicker (or double up) for baffles on big drivers.
Whatever you use, wear a dust mask rated for fine dust when cutting or sanding MDF, and use dust extraction if you have it.
Joints
- Butt joints with glue are plenty strong for a speaker. Our Sealed 18SW500 design uses them throughout because they're simple to cut and assemble.
- Rebates (rabbets) make alignment easier and give more glue area.
- Mitres look the cleanest under paint or veneer, but need accurate cuts.
One honest warning from experience: painted butt joints often show faint lines over time as the material moves with the seasons, however good the paint. Veneer hides it better.
Use PVA wood glue on every joint, clamp it square, and wipe off the squeeze-out before it sets.
Bracing
You can't really over-brace a cabinet, within reason. Braces stop panels flexing and push their resonances higher, where they're easier to damp.
- Window braces: a panel with big holes cut out, running across the inside. Strong, and lets air move freely.
- Dowel or rod braces: connect opposite walls at the middle of each panel.
- Shelf braces: a full panel with a big opening, often between woofer and tweeter sections.
- Block braces: simple blocks glued at panel centres.
Brace the biggest unsupported panels first, usually the sides and back. Account for the volume the braces take, or the box ends up smaller inside than designed.
Damping and stuffing
Match it to the box type.
Sealed boxes: a loose fill of polyester stuffing (polyfill) or wool. It soaks up internal reflections and makes the box act a little bigger than it is. Don't pack it tight. For scale, our Sealed 18SW500 guide suggests about 800 g to 1 kg in a 90 L sub box, loosely.
Ported boxes: line the walls rather than filling the box. Put acoustic felt or foam on one wall of each parallel pair (for example the back, the top and one side). Keep the area around the port's inner end clear, or you'll choke it.
Panel damping: bitumen or butyl sheets on large panels lower the "knock". Useful on thin or unbraced panels, less necessary on a well-braced 18 mm box.
From our range: acoustic felt sheets and butyl sound deadener.
Ports
- Build the port exactly to the design's size and length. Port length sets the tuning.
- Flared ends cut down port noise. A plain tube works, but rounding over the ends of a slot port or using a flared tube is better.
- Give the inner end room. Keep at least one port diameter between the inner end and the nearest wall.
- Rear ports and walls: a rear port too close to the wall behind it can sound boomy. Front ports are less fussy about placement.
There's more on port sizing in Sealed vs ported vs passive radiator, and a range of ports if you're not building a slot.
Sealing it up
- Glue every joint, and run a bead of glue or neutral-cure silicone along the inside seams if you can reach them.
- Gasket the drivers. Foam gasket tape under the flange seals it and stops buzzes.
- Seal the terminals. A binding post plate or terminal cup needs a gasket too.
- Seal any wire holes with silicone or hot glue.
- The cone test (sealed boxes): with the drivers in, gently push the woofer cone in and hold it (the woofer only, never a tweeter). It should feel much stiffer than in free air and spring back when you let go. If it goes in easily, or slowly creeps inward while you hold it, look for a leak.
Mounting the drivers
- Flush-mount where you can: route a rebate so the driver flange sits level with the baffle. A router with a circle jig makes clean holes and rebates; a jigsaw works for through-holes if you're careful. (Some subwoofers, like the 18SW500, have a thick enough flange that a second baffle layer with a larger hole does the same job.)
- Chamfer the back of the woofer hole so the cone's rear wave isn't choked by a thick baffle.
- Use threaded inserts or T-nuts with bolts for heavy drivers, rather than wood screws in MDF. Screws strip out of MDF edges, especially if the driver comes out a few times. A mounting kit with bolts and T-nuts is cheap insurance.
- Measure the actual driver before you cut. Datasheet cut-out sizes are usually right, but not always.
Finishing
- Seal MDF edges first. They soak up paint. A sanding sealer, shellac or thinned PVA, sanded back, stops them looking fuzzy.
- Paint: primer, then a few light coats, sanding between them. A roller gives a nice even finish on flat panels.
- Veneer: hides joints well and looks premium. Iron-on veneer is a reasonable first attempt.
- Vinyl wrap or speaker carpet: quick and tough, good for subs and PA boxes.
Tools that make it easier
- A straight-cut method: circular saw with a guide rail, track saw, table saw, or have the timber yard cut your panels.
- Router with a circle jig.
- Plenty of clamps (more than you think).
- A square, a drill, and a sanding block.
- A dust mask and hearing protection.
Quick questions
Is 16 mm MDF OK? For small speakers, yes, with sensible bracing. For subs and bigger boxes, 18 mm or more is better.
Should I use screws as well as glue? Glue does the holding. Screws or brads just clamp it while the glue dries, and they're optional if you have enough clamps.
How much stuffing goes in a sealed box? Enough to loosely fill it, not compressed. Start with less; you can always add more.
Can I use silicone instead of gasket tape? You can, but it makes drivers hard to remove. Gasket tape is cleaner and reusable.