06 · Bass traps

Slot Column

Airtight modules with a few slots in the front, tuned to one room mode you pick.

size: 300–700 wide, 150–500 deep, up to 2800 mm tall
tuning: the slot width is calculated for the mode
range: about 30–100 Hz (calculated)
package: cut list, hardware, print files for your printer

Overview

How it works

A tuned trap for one room mode: airtight plywood modules stacked floor to ceiling, a few vertical slots in the front. You pick the mode of your room; the slot width is calculated.

A Helmholtz resonator. The air in each slot is a mass, the air in the box a spring: at one frequency the air in the slots swings hard, and the wool behind them turns it into heat.

f₀ = c / 2π · √( r / (t · D) ) r = open part of the front, t = neck length with its end corrections, D = depth of air, c = 343 m/s

Where

Where the mode presses hardest: the rear or front wall for the length modes (between the diffusers), the side walls for the width modes, low in a corner for all of them.

Built simply

Square-cut plywood; each slot has two cheek strips behind the front boards, so the neck is as long as you set. Printed spacers set every slot while you screw.

One module in plan. The slot and its neck are the mass, the air in the box the spring.

Figures

Sizes and numbers

Mode	300 deep, neck 60	300 deep, neck 120	400 deep, neck 120
40 Hz	4.1 mm	7.8 mm	11.3 mm
50 Hz	6.9 mm	12.9 mm	18.8 mm
60 Hz	10.7 mm	19.6 mm	28.8 mm
80 Hz	22.3 mm	38.4 mm	more slots
100 Hz	39.8 mm	more slots	more slots

Calculated, not measured

Slot width for one slot across a 450 mm column, 50 mm of wool behind the necks. Wider slots are easier to build accurately; the configurator warns when a slot gets narrower than 5 mm. Calculated, not yet measured: measure one module in your room before building the rest.

Safety

Screw it to the wall before you let go of it. Safety information →

Build

Details

01300–700 mm wide, 150–500 deep, up to 2800 tall, in 300–900 mm modules; 1 to 7 slots across; necks 18–120 mm; wool 0–100 mm; optional fabric in any colour.
02Every module airtight. Glue and acoustic sealant in every joint, screwed to the wall and to the module below from inside. Two printed brackets on top.