Room Modes in a 9x29 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 19.4 Hz, set by the 29 ft length.
• Between 19.4 Hz and 38.8 Hz there are no modes at all, so notes in that 19.4 Hz gap will sound thinner than the bass around them.
• Mode density drops in the 25 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 : 1.13 : 3.63) fall outside the Bolt area because the room is long and narrow for its height, so modes bunch up along the length.
• Below about 165 Hz (the Schroeder frequency) individual modes shape the sound; above it, reflections and reverb matter more.

Between 19.4 Hz and 38.8 Hz there is no mode to reinforce anything, a gap of 19.4 Hz. Notes that fall in that gap sound thinner and quieter than notes just above or below it.

If you use this room for movies, that gap 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.

This room's ratio (1 : 1.13 : 3.63) is outside the Bolt area: the room is long and narrow for its height, which bunches modes up along the length. Treatment can still get it sounding good, but the shape is not helping as much as it could.

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.
2. At 25.0 Hz the quarter wavelength is about 11.3 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. Avoid the exact middle of the 29 ft length for your seat or mix position; try around 11 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 165 Hz; above that, general room reverb matters more than any single mode.

Every mode below 200 Hz

Below are all 80 modes this room produces under 200 Hz: 15 axial, 40 tangential, 25 oblique. Axial modes, which only involve one pair of opposite surfaces, ring the loudest. Tangential modes (four surfaces) come next, and oblique modes (all six surfaces) are the weakest of the three.

Questions about 9x29 rooms

Is a 9 by 29 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 60/100 because there is a gap of 19.4 Hz between 19.4 Hz and 38.8 Hz with no mode in between. Treatment will make a real difference here.

What is the best corner for bass traps in a 9x29 room?

Start in the four floor-to-ceiling corners; every mode in this room peaks there. The 25.0 Hz mode itself would need about 11.3 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 25.0 Hz.

What size subwoofer does a 9x29 room need?

Subwoofer size is less about this room's 261 sq ft and more about the output headroom you want; room size mainly changes where the 80 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 does bass sound thin in spots in a 9 by 29 ft room?

Here it comes down to this: there is a gap of 19.4 Hz between 19.4 Hz and 38.8 Hz with no mode in between. 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 9x29 room?

Corner bass traps come first, as thick as you can fit (6 to 12 in) plus a membrane trap tuned near 25.0 Hz, since that note's own quarter wavelength (about 11.3 ft) is too deep for any panel. After that, reposition your seat near 11 ft from the front wall and verify with REW below 165 Hz.

Similar room sizes

• 10x29 ft, 8 ft ceiling
• 9x28 ft, 8 ft ceiling
• 9x30 ft, 8 ft ceiling