Room Modes in an 11x20 ft Room with a 9 ft Ceiling

Mode spectrum

Each line is one standing wave. Axial modes, the strongest kind, stand tallest; tight bunches and wide empty stretches are where the bass will sound uneven.

Axial modes by dimension

What this means for your room

• The lowest room mode is 28.1 Hz, set by the 20 ft length.
• Between 28.1 Hz and 51.2 Hz there are no modes at all, so notes in that 23.1 Hz gap will sound thinner than the bass around them.
• Mode density drops in the 40 Hz third-octave band (0 modes against 1 in the band below), so bass will sound uneven from note to note.
• The proportions (1 : 1.22 : 2.22) fall outside the Bolt area because the room is long and narrow for its height, so modes bunch up along the length.
• Below about 169 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

This room has a hole of 23.1 Hz between 28.1 Hz and 51.2 Hz where no mode helps the bass along. Anything tuned to that range reads weaker than its neighbors.

For a home theater, expect that gap to color explosions and sub-bass hits unevenly rather than smoothly. For a music room, it means you cannot fully trust what you hear in the low end at the listening position, since a note that sounds loud in the room may be perfectly balanced on the recording.

The proportions (1 : 1.22 : 2.22) fall outside the Bolt area, the range room ratios usually spread modes best over, because the room is long and narrow for its height, which bunches modes up along the length. That does not rule the room out, but treatment is doing more of the work here than shape is.

How to fix it, in order

1. Start with bass traps in the four floor-to-ceiling corners; every mode in this room peaks there.
2. Full absorption at 40.0 Hz would take about 7 ft of trap depth, well past what any room can fit. Fill the corners as deep as you reasonably can (6 to 12 in, with an air gap behind), then lean on a membrane trap tuned near that frequency, your seat position and a second subwoofer with EQ to tame the note itself.
3. Do not sit dead-center on the 20 ft length. Start near 7.6 ft from the front wall, about 38% of the way back, and adjust from there.
4. For the subwoofer, try a few spots before settling: a front corner usually gives the most output but also the most uneven bass, while pulling it off the wall or adding a second sub often smooths out peaks like the ones this room has.
5. After treating, run an REW sweep from the seat. Everything below 169 Hz is where these modes live, so that is the range worth checking before you call the room done.

Every mode below 200 Hz

The table below lists every mode under 200 Hz for this room: 74 in all, 13 axial, 36 tangential and 25 oblique. Axial modes bounce between just two parallel surfaces (say, the two side walls) and are the loudest and most audible; tangential modes involve four surfaces and are quieter; oblique modes bounce off all six surfaces and are the faintest. Start with the axial rows; they cause most of the boomy or thin spots you will actually hear.

Questions about 11x20 rooms

Will an 11x20 room work for a home theater?

This size suits a bedroom theater or dedicated music room, though the 65/100 modal score signals some work ahead, mainly because there is a gap of 23.1 Hz between 28.1 Hz and 51.2 Hz with no mode in between.

Where should I put bass traps in an 11x20 room?

The four vertical corners first. Full absorption at 40.0 Hz would take roughly 7 ft of trap depth, so treat that note with a tuned membrane or pressure trap instead, and use thick porous corner traps (6 to 12 in) for everything above it.

What subwoofer size is right for an 11 by 20 ft room?

There is no fixed sub size tied to 220 sq ft; that number mostly sets this room's mode frequencies (74 of them under 200 Hz). At this size you will want more headroom than a small room needs, and two subwoofers usually beat one at keeping bass even from seat to seat.

Why does my 11x20 room have weak bass notes?

The short answer for this room: there is a gap of 23.1 Hz between 28.1 Hz and 51.2 Hz with no mode in between. Bass unevenness like that is built into the shape of the room and shows up regardless of what speakers or sub you use.

How do I fix bass problems in an 11x20 room?

Put bass traps in the four floor-to-ceiling corners first, as thick as you can fit (6 to 12 in) plus a membrane trap tuned near 40.0 Hz, since that note's own quarter wavelength (about 7 ft) is too deep for any panel. From there, move your seat to about 7.6 ft from the front wall, then measure with REW below 169 Hz to see what still needs work.

Similar room sizes

• 10x20 ft, 9 ft ceiling
• 12x20 ft, 9 ft ceiling
• 11x20 ft, 8 ft ceiling