Room Modes in an 11x21 ft Room with a 10 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 26.8 Hz, set by the 21 ft length.
• Your 21 ft length is almost exactly twice your 11 ft width, so their modes fall close together without quite stacking.
• Between 26.8 Hz and 51.2 Hz there are no modes at all, so notes in that 24.4 Hz gap will sound thinner than the bass around them.
• Mode density drops in the 31.5 Hz third-octave band (0 modes against 1 in the band below), so bass will sound uneven from note to note.
• The proportions (1 : 1.10 : 2.10) fall outside the Bolt area because the room is long and narrow for its height, so modes bunch up along the length.
• Below about 156 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

Between 26.8 Hz and 51.2 Hz there is no mode to reinforce anything, a gap of 24.4 Hz. Notes that fall in that gap sound thinner and quieter than notes just above or below it.

If you use this room for movies, that gap tends to show up as boom on specific low-frequency effects rather than an even rumble. If it is a music or mixing room, that same spot makes some bass notes read louder on playback than they actually are on the recording, which makes mixing bass by ear risky here.

This room's ratio (1 : 1.10 : 2.10) is outside the Bolt area: the room is long and narrow for its height, which bunches modes up along the length. Treatment can still get it sounding good, but the shape is not helping as much as it could.

How to fix it, in order

1. Put your first traps in the four vertical corners where floor meets ceiling; that is where every mode in this room reaches its loudest point.
2. At 31.5 Hz the quarter wavelength is about 8.9 ft, deeper than any practical panel, so a porous trap alone will not fully absorb that note. Build corner traps as thick as you can fit (6 to 12 in, straddling the corner with an air gap behind) to take the edge off, then handle the note itself with a membrane or pressure trap tuned near it, careful seat position, and more than one subwoofer with EQ.
3. Avoid the exact middle of the 21 ft length for your seat or mix position; try around 8 ft from the front wall (38% of the length) as a starting point, then nudge from there by ear.
4. Test more than one subwoofer position. Corner placement drives the most output but also the least even bass; moving it along a wall or using two subs at different spots tends to average out this room’s peaks.
5. Confirm the fix with an REW measurement at the listening position, paying closest attention below 156 Hz; above that, general room reverb matters more than any single mode.

Every mode below 200 Hz

Below are all 86 modes this room produces under 200 Hz: 13 axial, 42 tangential, 31 oblique. Axial modes, which only involve one pair of opposite surfaces, ring the loudest. Tangential modes (four surfaces) come next, and oblique modes (all six surfaces) are the weakest of the three.

Questions about 11x21 rooms

Will an 11x21 room work for a home theater?

This size suits a bedroom theater or dedicated music room, though the 60/100 modal score signals some work ahead, mainly because there is a gap of 24.4 Hz between 26.8 Hz and 51.2 Hz with no mode in between.

Where should I put bass traps in an 11x21 room?

Start in the four floor-to-ceiling corners; every mode in this room peaks there. The 31.5 Hz mode itself would need about 8.9 ft of depth to fully absorb, more than any panel can give, so build corner traps as thick as you can fit (6 to 12 in) and pair them with a membrane trap tuned near 31.5 Hz.

What subwoofer size is right for an 11 by 21 ft room?

There is no fixed sub size tied to 231 sq ft; that number mostly sets this room's mode frequencies (86 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 11x21 room have weak bass notes?

The short answer for this room: there is a gap of 24.4 Hz between 26.8 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 11x21 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 31.5 Hz, since that note's own quarter wavelength (about 8.9 ft) is too deep for any panel. From there, move your seat to about 8 ft from the front wall, then measure with REW below 156 Hz to see what still needs work.

Similar room sizes

• 10x21 ft, 10 ft ceiling
• 12x21 ft, 10 ft ceiling
• 11x20 ft, 10 ft ceiling
• 11x22 ft, 10 ft ceiling
• 11x21 ft, 8 ft ceiling
• 11x21 ft, 9 ft ceiling