Room Modes in a 15x21 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 26.8 Hz, set by the 21 ft length.
• Your 21 ft length and 15 ft width both resonate at 187.6 Hz, so bass at that note will be much louder than its neighbours.
• Your 21 ft length and 9 ft ceiling height both resonate at 187.6 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.
• Between 26.8 Hz and 37.5 Hz there are no modes at all, so notes in that 10.7 Hz gap will sound thinner than the bass around them.
• Mode density drops in the 31.5 Hz third-octave band (0 modes against 1 in the band below), so bass will sound uneven from note to note.
• The proportions (1 : 1.67 : 2.33) fall inside the Bolt area, the range of room ratios that spreads modes most evenly.
• Below about 141 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

Your 21 ft length and 15 ft width share a mode near 187.6 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 : 2.33, 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. 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 187.6 Hz the quarter wavelength is about 1.5 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 21 ft length. Start near 8 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 141 Hz is where these modes live, so that is the range worth checking before you call the room done.

Every mode below 200 Hz

This room has 103 modes below 200 Hz, 15 axial, 49 tangential and 39 oblique. Read the axial rows as the ones you will hear most clearly. Tangential and oblique modes add texture but usually only become audible when they land close to an axial mode.

Questions about 15x21 rooms

Will a 15x21 room work for a home theater?

This size suits a dedicated home theater or media room, though the 55/100 modal score signals some work ahead, mainly because two of its dimensions reinforce the same note near 187.6 Hz.

Where should I put bass traps in a 15x21 room?

Start in the four floor-to-ceiling corners; every mode in this room peaks there. The 187.6 Hz mode itself would need about 1.5 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 187.6 Hz.

What subwoofer size is right for a 15 by 21 ft room?

There is no fixed sub size tied to 315 sq ft; that number mostly sets this room's mode frequencies (103 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 15x21 room have one loud bass note?

The short answer for this room: two of its dimensions reinforce the same note near 187.6 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 15x21 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 187.6 Hz, since that note's own quarter wavelength (about 1.5 ft) is too deep for any panel. From there, move your seat to about 8 ft from the front wall, then measure with REW below 141 Hz to see what still needs work.

Similar room sizes

• 14x21 ft, 9 ft ceiling
• 16x21 ft, 9 ft ceiling
• 15x20 ft, 9 ft ceiling
• 15x22 ft, 9 ft ceiling
• 15x21 ft, 8 ft ceiling