Room Modes in a 15x20 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 15 ft width both resonate at 112.5 Hz, so bass at that note will be much louder than its neighbours.
• Your 15 ft width and 9 ft ceiling height both resonate at 187.6 Hz, so bass at that note will be much louder than its neighbours.
• The proportions (1 : 1.67 : 2.22) fall inside the Bolt area, the range of room ratios that spreads modes most evenly.
• Below about 145 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

Your 20 ft length and 15 ft width share a mode near 112.5 Hz. When two dimensions resonate at the same note, their boosts add up, so that note stacks on top of itself and comes out noticeably louder than the bass around it.

If you use this room for movies, that stacked note tends to show up as boom on specific low-frequency effects rather than an even rumble. If it is a music or mixing room, that same spot makes some bass notes read louder on playback than they actually are on the recording, which makes mixing bass by ear risky here.

Height, width and length work out to 1 : 1.67 : 2.22, which lands inside the Bolt area. Rooms with that proportion usually need less correction than a room shaped like a cube or a hallway.

How to fix it, in order

1. Put your first traps in the four vertical corners where floor meets ceiling; that is where every mode in this room reaches its loudest point, ceiling height included.
2. Full absorption at 112.5 Hz would take about 2.5 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. Avoid the exact middle of the 20 ft length for your seat or mix position; try around 7.6 ft from the front wall (38% of the length) as a starting point, then nudge from there by ear.
4. Test more than one subwoofer position. Corner placement drives the most output but also the least even bass; moving it along a wall or using two subs at different spots tends to average out this room’s peaks.
5. Confirm the fix with an REW measurement at the listening position, paying closest attention below 145 Hz; above that, general room reverb matters more than any single mode.

Every mode below 200 Hz

The table below lists every mode under 200 Hz for this room: 98 in all, 15 axial, 46 tangential and 37 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 15x20 rooms

Will a 15x20 room work for a home theater?

This size fits a dedicated home theater or media room well. At 80/100, the modal score is good, so treatment here is mostly fine-tuning rather than fixing real problems.

Where should I put bass traps in a 15x20 room?

The four vertical corners first. Full absorption at 112.5 Hz would take roughly 2.5 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 subwoofer size is right for a 15 by 20 ft room?

There is no fixed sub size tied to 300 sq ft; that number mostly sets this room's mode frequencies (98 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 15x20 room have one loud bass note?

The short answer for this room: two of its dimensions reinforce the same note near 112.5 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 15x20 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 112.5 Hz, since that note's own quarter wavelength (about 2.5 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 145 Hz to see what still needs work.

Similar room sizes

• 14x20 ft, 9 ft ceiling
• 15x20 ft, 8 ft ceiling