Room Modes in a 13x20 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 20 ft length and 13 ft width both resonate near 84.4 and 168.8 Hz, so bass at those notes will be much louder than its neighbours.
• Between 28.1 Hz and 43.3 Hz there are no modes at all, so notes in that 15.2 Hz gap will sound thinner than the bass around them.
• The proportions (1 : 1.44 : 2.22) fall inside the Bolt area, the range of room ratios that spreads modes most evenly.
• Below about 155 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

The modes from your 20 ft length and 13 ft width land on top of each other near 84.4 Hz. That stack means one specific low note gets reinforced twice, so it jumps out over everything nearby.

For a home theater, expect that stacked note 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 room's proportions (1 : 1.44 : 2.22, height to width to length) fall inside the Bolt area, the range acousticians consider best for spreading modes evenly. That is a genuine advantage of this room's shape, not something you can add later with treatment.

How to fix it, in order

1. Treat the four floor-to-ceiling corners first; they are common to every mode this room produces.
2. Full absorption at 84.4 Hz would take about 3.3 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. Move your listening position off-center along the 20 ft length; roughly 7.6 ft from the front wall (38% back) is a common starting spot before fine-tuning.
4. Walk the subwoofer around the front of the room while playing a bass-heavy track and listen from your seat: corner placement is loudest but least even, and a second sub or an off-corner spot often fills in this room’s weak points.
5. Once traps are in, measure with REW from the listening position. Focus on frequencies below 155 Hz (this room's Schroeder frequency); that is where individual modes, not general reverb, are running the show.

Every mode below 200 Hz

The table below lists every mode under 200 Hz for this room: 86 in all, 14 axial, 41 tangential and 31 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 13x20 rooms

Is a 13 by 20 ft room good for a music room or home theater?

It can work as a dedicated home theater or media room, but do not expect an easy ride: this room scores 64/100 because two of its dimensions reinforce the same note near 84.4 Hz. Treatment will make a real difference here.

What is the best corner for bass traps in a 13x20 room?

The four vertical corners first. Full absorption at 84.4 Hz would take roughly 3.3 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.

What size subwoofer does a 13x20 room need?

Subwoofer size is less about this room's 260 sq ft and more about the output headroom you want; room size mainly changes where the 86 modes below 200 Hz fall. A room this size needs more sub output to reach the same level as a smaller one, and running two subs at different spots evens out the response between seats.

Why is bass boomy in one spot in a 13 by 20 ft room?

Here it comes down to this: two of its dimensions reinforce the same note near 84.4 Hz. That is a property of the room's shape, not your equipment, so treatment, not a better sub, is the fix.

What is the fastest fix for bass in a 13x20 room?

Corner bass traps come first, as thick as you can fit (6 to 12 in) plus a membrane trap tuned near 84.4 Hz, since that note's own quarter wavelength (about 3.3 ft) is too deep for any panel. After that, reposition your seat near 7.6 ft from the front wall and verify with REW below 155 Hz.

Similar room sizes

• 12x20 ft, 9 ft ceiling
• 14x20 ft, 9 ft ceiling
• 13x20 ft, 8 ft ceiling