Room Modes in a 13x20 ft Room with an 8 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.
• Your 20 ft length and 8 ft ceiling height both resonate at 140.7 Hz, so bass at that note 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.
• Mode density drops in the 100 Hz third-octave band (6 modes against 7 in the band below), so bass will sound uneven from note to note.
• The proportions (1 : 1.63 : 2.50) fall inside the Bolt area, the range of room ratios that spreads modes most evenly.
• Below about 165 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

Because your 20 ft length and 13 ft width are close in size, their modes stack near 84.4 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 a ratio of 1 : 1.63 : 2.50, this room sits inside the Bolt area, the zone where modes tend to spread out rather than pile up. Shape is doing you a favor here.

How to fix it, in order

1. Treat the four floor-to-ceiling corners first; they are common to every mode this room produces, and your ceiling height is part of the problem.
2. At 84.4 Hz the quarter wavelength is about 3.3 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. 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 165 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: 78 in all, 13 axial, 38 tangential and 27 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?

This size is workable for a dedicated home theater or media room, but its modal score of 49/100 is low, driven by the fact that two of its dimensions reinforce the same note near 84.4 Hz. Go in expecting a serious treatment plan.

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

Start in the four floor-to-ceiling corners; every mode in this room peaks there. The 84.4 Hz mode itself would need about 3.3 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 84.4 Hz.

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 78 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 165 Hz.

Similar room sizes

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