Slat (Slotted) Absorber

How it works

The slots and the air behind them form a resonator tuned by the slot ratio, panel thickness, and cavity depth. A porous fill or fabric facing in the cavity broadens and damps the peak. Fresnelcut solves the cavity depth for your target frequency and emits the backing bill of materials so the result is reproducible. See diffuser vs absorber for when each one is the answer.

Worked example: 10% slot ratio, 100 mm cavity

Take 5 mm slots between 45 mm slats (a 10% open ratio, ε = 0.10), 12 mm slat stock, over a 100 mm cavity. The slot's end correction adds effective depth beyond the slat's physical thickness:

Plugging t_eff = 17.9 mm and D = 100 mm into the resonance formula gives f_res = (343.2/2π)·√(0.10/(0.0179·0.100)) ≈ 408 Hz — squarely in the boxy, low-mid range this construction targets. Open this build in the designer to tune it to your own target frequency and get the backing bill of materials.

What you'll need to build it

Beyond the slats themselves you need a frame to hold the 100 mm (or whatever depth you land on) standoff from the wall, and something to damp the cavity — a mineral-wool or open-cell-foam fill, or a resistive fabric facing behind the slats, is what actually broadens and flattens the absorption peak; the slots alone give you a narrow resonance. Fresnelcut's bill of materials lists a porous-fill class and thickness alongside the slat cut list so the build is reproducible.

Slat vs. perforated

Both are tuned resonant absorbers over a cavity with the same underlying physics — only the opening geometry differs. A slat absorber's parallel-slot layout is the more traditional, furniture-like look and is straightforward on a table saw; the perforated-panel absorber swaps slots for a round-hole grid, which is often faster on a laser or CNC and reads as a cleaner, more uniform surface.

Getting the target frequency right

The resonance formula's three levers — slot ratio, effective slot depth, and cavity depth — don't move the target frequency independently: a slot ratio change also shifts the end correction, since narrower slots add more effective depth for the same physical slat thickness. In practice it's easier to fix the slot width and slat thickness to whatever your material and saw comfortably produce, then solve for the cavity depth that lands the resonance where you want it — which is exactly what Fresnelcut does when you enter a target frequency instead of a cavity depth.

Finishing and mounting

Because the slats are the visible surface, this is the absorber type most often left as bare, finished wood rather than covered — stain or clear-coat the front face before assembly, since it's much harder to reach once the cavity is closed up. Mount the finished frame with a standoff equal to the cavity depth you designed for; shorting that gap by mounting flush to the wall shifts the resonance away from the frequency you solved for.