Room Modes in a 17x20 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 17 ft width both resonate near 165.5 and 196.9 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 43.4 Hz and 56.3 Hz there are no modes at all, so notes in that 12.9 Hz gap will sound thinner than the bass around them.
• Mode density drops in the 40 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 : 2.13 : 2.50) fall inside the Bolt area, the range of room ratios that spreads modes most evenly.
• Below about 144 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

The modes from your 20 ft length and 8 ft ceiling land on top of each other near 140.7 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 : 2.13 : 2.50, 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. 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 140.7 Hz the quarter wavelength is about 2 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 144 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: 98 in all, 15 axial, 47 tangential and 36 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 17x20 rooms

Is a 17 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 54/100 because two of its dimensions reinforce the same note near 140.7 Hz. Treatment will make a real difference here.

What is the best corner for bass traps in a 17x20 room?

Start in the four floor-to-ceiling corners; every mode in this room peaks there. The 140.7 Hz mode itself would need about 2 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 140.7 Hz.

What size subwoofer does a 17x20 room need?

Subwoofer size is less about this room's 340 sq ft and more about the output headroom you want; room size mainly changes where the 98 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 17 by 20 ft room?

Here it comes down to this: two of its dimensions reinforce the same note near 140.7 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 17x20 room?

Corner bass traps come first, as thick as you can fit (6 to 12 in) plus a membrane trap tuned near 140.7 Hz, since that note's own quarter wavelength (about 2 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 144 Hz.

Similar room sizes

• 16x20 ft, 8 ft ceiling
• 18x20 ft, 8 ft ceiling