Bass is the hardest part of any room to control. Low notes build up in some places, cancel in others and keep ringing after they should have stopped. Bass traps are absorbers designed for low frequencies.
Bass trap basics
Beginner
Why bass misbehaves
Every room has resonant frequencies, called room modes, set by its dimensions. The strongest are between parallel walls. The first one is at the speed of sound divided by twice the room length. In a 5 m long room that’s 343 ÷ 10 = 34.3 Hz, then 68.6 Hz, 102.9 Hz and so on.
Why corners
Every room mode has a pressure maximum at the walls, and corners are where walls meet. A thick trap straddling a corner also sits a long way from both walls, where it absorbs best. That’s why corners are the usual place to start.
Types of bass trap
| Type | How it works | Good for |
|---|---|---|
| Porous corner trap | Deep mineral wool, often triangular or straddling the corner | Broad absorption down to around 100 Hz, depending on depth |
| Membrane or panel trap | A sealed box with a flexible front panel that vibrates | Targeting lower frequencies in less depth |
| Helmholtz resonator | A box with slots or holes tuned to one frequency | A specific problem mode |
Be realistic
A 100 mm RWA45 slab absorbs about 44% at 100 Hz. GIK Acoustics notes that a 100 Hz wave reaches peak air movement nearly 0.9 m from the wall. Porous traps help a lot above about 100 Hz, but the deepest bass needs far more depth, or electronic help such as multiple subwoofers, Bass Control or ART.
Performance of any specific trap depends on its design and your room.
Bass traps in more depth
Advanced
Porous versus pressure traps
Porous traps work on air movement, so they need depth and distance from the wall. Membrane traps and Helmholtz resonators work on pressure, so they belong right against a boundary, where pressure is highest. Adding mineral wool inside a membrane trap widens the range of frequencies it works over.
Helmholtz tuning
A Helmholtz resonator’s frequency depends on the area of its opening, the volume of the box and the effective length of the neck: f = (c ÷ 2π) × √(A ÷ (V × L)). Small changes in any of these move the tuning, so build and measure.
Model before you build
REW‘s Room Simulator predicts modes and the bass response at your seat for a rectangular room. Its limits: rectangular rooms only, up to 20 mode orders per axis, and absorption that doesn’t vary with frequency. Use it to compare seat and subwoofer positions, then measure the real room.
Active and passive together
Dirac says ART measurably shortens bass decay below 150 Hz, and that combining active and passive treatment might achieve the best results in some rooms. Passive treatment is still the main tool from about 100 Hz up.