Room Modes in a 9x20 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.
• Your width and ceiling height are both 9 ft, so their modes land on the same notes (62.5, 125.0 and 187.6 Hz), doubling the boost at each.
• Between 28.1 Hz and 56.3 Hz there are no modes at all, so notes in that 28.2 Hz gap will sound thinner than the bass around them.
• Mode density drops in the 40 and 80 Hz third-octave bands, where fewer modes fall than in the band below, so bass will sound uneven from note to note.
• The proportions (1 : 1.00 : 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 187 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

Because your 9 ft width and 9 ft ceiling are close in size, their modes stack near 62.5 Hz. Expect that one note to sound louder, and ring longer, than the rest of the bass in this room.

In a home theater, this shows up as one bass note in an action scene sounding far louder than the rest of the mix, usually a kick drum hit or an LFE cue landing right on that stacked note. In a music room, the same thing means certain bass notes on a track jump out while others next to them feel buried.

At 1 : 1.00 : 2.22, this room sits outside the Bolt area because the room is long and narrow for its height, which bunches modes up along the length. Expect to lean on bass traps and seat position a bit more than in a room with friendlier proportions.

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, including the ones set by your ceiling height.
2. At 62.5 Hz the quarter wavelength is about 4.5 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. 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 187 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: 66 in all, 13 axial, 32 tangential and 21 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 9x20 rooms

Will a 9x20 room work for a home theater?

You can use this room as a bedroom theater or dedicated music room, but its bass will fight you: a 25/100 modal score, mainly because two of its dimensions reinforce the same note near 62.5 Hz, means committed corner trapping and careful seat placement are not optional here.

Where should I put bass traps in a 9x20 room?

Start in the four floor-to-ceiling corners; every mode in this room peaks there. The 62.5 Hz mode itself would need about 4.5 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 62.5 Hz.

What subwoofer size is right for a 9 by 20 ft room?

There is no fixed sub size tied to 180 sq ft; that number mostly sets this room's mode frequencies (66 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 9x20 room have one loud bass note?

The short answer for this room: two of its dimensions reinforce the same note near 62.5 Hz. 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 a 9x20 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 62.5 Hz, since that note's own quarter wavelength (about 4.5 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 187 Hz to see what still needs work.

Similar room sizes

• 8x20 ft, 9 ft ceiling
• 10x20 ft, 9 ft ceiling
• 9x19 ft, 9 ft ceiling
• 9x20 ft, 8 ft ceiling