Room Modes in a 13x28 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 20.1 Hz, set by the 28 ft length.
• Your 28 ft length is almost exactly three times your 9 ft ceiling height, so their modes fall close together without quite stacking.
• Between 20.1 Hz and 40.2 Hz there are no modes at all, so notes in that 20.1 Hz gap will sound thinner than the bass around them.
• Mode density drops in the 25 and 50 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 : 1.44 : 3.11) fall outside the Bolt area because the room is long and narrow for its height, so modes bunch up along the length.
• Below about 131 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

This room has a hole of 20.1 Hz between 20.1 Hz and 40.2 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 : 1.44 : 3.11) 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, including the ones set by your ceiling height.
2. Full absorption at 25.0 Hz would take about 11.3 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 28 ft length. Start near 10.6 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 131 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: 116 in all, 16 axial, 55 tangential and 45 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 13x28 rooms

Is a 13x28 room good for a home theater?

At 364 sq ft, this size works as a dedicated home theater or media room, but a modal score of 55/100 means it is workable, not effortless: there is a gap of 20.1 Hz between 20.1 Hz and 40.2 Hz with no mode in between, so plan on real bass trapping.

Where do bass traps go in a 13 by 28 ft room?

The four vertical corners first. Full absorption at 25.0 Hz would take roughly 11.3 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.

Do I need a big subwoofer for a 13x28 room?

Room size here mainly shapes where the modes land, not the sub size on its own; this room has 116 modes below 200 Hz to work around either way. Larger rooms like this one ask more of a subwoofer's output, and a second sub in a different spot helps smooth out the peaks and dips across seats.

Why do some bass notes sound weak in a 13x28 room?

In this room, the main cause is that there is a gap of 20.1 Hz between 20.1 Hz and 40.2 Hz with no mode in between. Room modes reinforce specific notes more than others no matter how good your speakers are, and that unevenness is what you are hearing.

How many bass traps does a 13 by 28 ft room need?

Start by treating the four corners with traps as thick as you can fit (6 to 12 in) plus a membrane trap tuned near 25.0 Hz, since that note's own quarter wavelength (about 11.3 ft) is too deep for any panel. Next, shift your listening position toward 10.6 ft from the front wall, then confirm progress with an REW sweep under 131 Hz.

Similar room sizes

• 12x28 ft, 9 ft ceiling
• 14x28 ft, 9 ft ceiling
• 13x27 ft, 9 ft ceiling
• 13x28 ft, 8 ft ceiling