Treatment
Bass traps and low-frequency control
Why bass is the hardest part of any room, what a bass trap actually is, and why the corners matter.
7 min read5 sourcesChapter 7 of 24
Schematic
Pressure at a room mode builds up in the corners, where the walls meet — every axial mode of the room has a maximum there. A deep porous trap spanning the corner sits where the air movement associated with that pressure is greatest, and damps it.
Animated illustration of the principle. Not measured data.
Below roughly 150 Hz, sound in a room behaves differently from above it — a gradual change, not a sharp line. The wavelengths are metres long, the room’s dimensions set which notes ring, and the thin panels that fix the mid-range do nothing. Low-frequency control is where most treated rooms fall short, and where the difference between a decorated room and a designed one is most audible.
Why the corners
Every axial mode has a pressure maximum at the walls, and the corners are where two or three of those maxima coincide. Whatever the frequency, the corner is a peak. A trap placed there works on every mode at once, which is why corner trapping is the first move in any small room and why the same panel placed mid-wall does far less.
The floor–wall and ceiling–wall junctions are corners too. In a serious room, the vertical corners are treated first, then the horizontal ones along the front and rear walls.
Porous traps
The simplest bass trap is a lot of absorber: a deep block or triangle of mineral wool or glass wool spanning the corner, often 300–600 mm across. It works on the same friction principle as a wall panel, and the depth is what lets it reach low. Because it is porous, it also absorbs everything above the bass, which is sometimes desirable and sometimes needs a reflective facing to hold the mid and high frequencies in the room.
Porous traps are broadband and forgiving. They do not need to be tuned, they cannot be tuned wrong, and they are the right first step in nearly every room.
Foam “bass traps” a few centimetres thick do not reach the bass. The name is marketing. Depth is the only thing that reaches low frequencies in a porous device.
Tuned traps
When a room has one or two dominant modes that broadband trapping cannot tame, a tuned absorber targets them. A membrane or panel absorber is a sealed box with a flexible front skin that resonates at the problem frequency and damps it. A Helmholtz absorber is a cavity with a neck — a bottle, in effect — tuned the same way. Both act on a narrow band, are ignored by everything else, and can be built to sit in a wall.
They are precise instruments and depend on the room being measured first. A tuned trap at the wrong frequency does nothing.
Traps are one lever of three
Trapping shortens how long modes ring; it does not stop them being excited. Loudspeaker and seat placement decide how strongly each mode is driven and heard, and multiple subwoofers can smooth the response across a whole row of seats. In a well-designed room, placement is settled first, subwoofers are positioned to fight the worst modes, and trapping deals with what remains. Equalisation, if used, is applied last and only to cut.
For integrators and architectsThe numbers, the standards, and what to specify
- Depth
- Porous corner traps: by the quarter-wavelength rule, an effective depth (measured across the corner) of 300 mm approaches high absorption from roughly 290 Hz and 600 mm from roughly 140 Hz, tapering gradually below — a rough guide, not a guarantee. Tested 100 mm board is about 0.56 at 125 Hz. Below 100 Hz, porous trapping needs volumes most rooms cannot spare, and tuned devices or placement take over.
- Membrane absorbers
- Resonant frequency f ≈ 60 / √(m × d) with m the panel surface mass in kg/m² and d the cavity depth in metres (a common approximation). A 6 mm plywood skin (about 4 kg/m²) over a 100 mm cavity tunes near 95 Hz. Damping in the cavity widens the band.
- Helmholtz
- Perforated panel over a sealed cavity; tuning depends on hole size, spacing, panel thickness and cavity depth. Narrow and effective, and the calculation is sensitive to build accuracy.
- Placement before trapping
- Measure at every primary seat. Move the subwoofers before adding a single trap; the largest improvements usually come free.
- Verify
- Waterfall or spectrogram plots show ringing directly. A trap that works shortens the decay at the target frequency; the frequency response alone can hide this.
What to write into the brief
- Corner trapping depth stated in millimetres, and whether faced or open.
- Tuned absorbers only against a measured modal analysis, with the target frequency written down.
Sources
- 01Owens Corning, Fiberglas 700 Series Board — Product Data Sheet (ASTM C423, Type A mounting)Measured absorption coefficients for 25, 50, 75 and 100 mm board.
- 02Zhang et al., DAGA 2021 — porous absorber effectiveness and the quarter-wavelength criterionCites Allard & Atalla, Propagation of Sound in Porous Media.
- 03de Melo Filho et al., Journal of Sound and Vibration, 2018 — mass–air–mass resonance (open access)Gives the double-leaf resonance formula behind f₀ ≈ 60·√((m₁+m₂)/(m₁m₂d)).
- 04Morse & Bolt, “Sound Waves in Rooms”, Reviews of Modern Physics 16(2), 1944Foundational paper on room modes.
- 05Everest & Pohlmann, Master Handbook of Acoustics, 7th ed. (McGraw Hill)Textbook covering small-room acoustics, modes and treatment.
External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.
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