03 · Diffusion and absorption

Tube Trap

A column of mineral wool round a wooden cage. Round and free-standing, or half-round against the wall.

diameter: 250–600 mm, up to 2800 mm tall
wool: 30–100 mm round an air core
range: from about 140 Hz up at Ø 400 with 100 mm (calculated)
package: cut list, hardware, print files for your printer

Overview

How it works

A broadband absorber: a column of mineral wool round a wooden cage, round and free-standing or half-round against the wall. No tuning: it soaks up a wide band from the upper bass up.

The wool wall absorbs like any porous absorber; the air in the hollow core works as air behind it, so a fat tube reaches lower than its wool alone. Half of the tube carries a thin reflective sheet under the fabric: the bass goes through it, the treble comes back, and you choose how lively the room stays by turning it.

Where

Round: in the corners, beside the speakers, behind the seat. Half-round: against the rear wall between the diffusers, or on a side wall.

Built simply

Plywood caps and inner discs, hardwood staves between them, a threaded rod through the middle of the round one; the wool is a roll wrapped round the cage, the fabric a sleeve over it.

A round tube in plan. The reflective half faces the room for a livelier sound, the wall for a drier one.

Figures

Sizes and numbers

Round, 2400 mm	thinner wool	thicker wool	weight
Ø 250 mm	50 mm: 329 Hz · 0.2 m²	60 mm: 286 Hz · 0.3 m²	≈ 8.6–8.8 kg
Ø 300 mm	50 mm: 278 Hz · 0.3 m²	90 mm: 187 Hz · 0.6 m²	≈ 10.2–11 kg
Ø 400 mm	50 mm: 212 Hz · 0.7 m²	100 mm: 141 Hz · 1.3 m²	≈ 14.6–15.6 kg
Ø 500 mm	50 mm: 172 Hz · 1.2 m²	100 mm: 119 Hz · 1.9 m²	≈ 20.5–21.5 kg
Ø 600 mm	50 mm: 144 Hz · 1.9 m²	100 mm: 103 Hz · 2.7 m²	≈ 27.4–28.9 kg

Calculated, not measured

Each cell: wool thickness, from which frequency it absorbs half or more, and its absorption at 125 Hz in m² (sabins). Unfaced wool roll 20–40 kg/m³, the core counted as air behind it. Calculated, not yet measured.

Safety

A tall round tube can tip. Keep it out of walkways or tie it to the wall. Gloves and a dust mask when you cut mineral wool. Safety information →

Build

Details

01Diameter 250–600 mm, height 600–2800 mm; round on 3 printed feet, or half-round on a back board with 2 printed brackets to the wall.
02Inner discs every 600 mm at most; staves about every 150 mm round them. Fabric in the colour you choose; reflective sheet of kraft paper or thin plastic, or none.