Question about FRK on bass traps for 12 x 9 x 8h room

Hi boyd. Glad you want to treat your space.

Whereabouts is 'locally'? I'm in the market for some rigid insulation and would like to compare vendors. Do you mind sharing the cost per square foot and at what thickness?

Bass is on the difficult end of the acoustic treatment spectrum. You need a lot of insulation to treat bass. How much depends but 8"-thick rigid like you have should be pretty effective down to 63Hz.

By '3#', I assume you mean JM 813 which, at 125Hz with 2" thickness, will only absorb at a 0.08 rating. That's practically nothing. Above 1kHz, you'll be golden but even acoustic guitar in standard tuning goes to 83Hz.

While you are right to consider aesthetics, making the panels smaller will further reduce your surface area coverage unless you compensate with more panels. If this is a mixed-use room, you mentioned recording and mixing, then you will certainly want ample coverage. Besides a reflection-free mixing zone, you also need to have spaced panels like you're planning to deal with flutter echo and ringing.

The greatest improvement you can make to your plan is to reconsider the 2" thickness and think about something thicker. 4" for rigid or 1' for fluffy has been what people with experience recommend as a minimum. Four 4" panels is a good start for low end but eight 4" thick or even sixteen 8" thick will give you a better result.

While this might seem excessive for a small room, the opposite is true: small rooms need just about as much as a slightly larger room. The natural resonance of the room, it would be misguided to consider it 'ambience', will doubtlessly imbed an undesirable and virtually irremovable tone into your recordings. Your goal is best set on making the room 'neutral'; neither adding or subtracting from the qualities of your source. In other words, not so 'live' that you hear a boxy ring and not so damped that your high frequencies are disproportionately dampened. The latter is certainly not a realistic concern for most people's efforts; few go that far.

FRK can remain on panels which don't serve to absorb non-bass frequencies. In other words, having FRK on corner panels is ok but probably less useful in your reflection zones.

Oh, also, if I'm reading the JM 814 spec sheet properly, it looks like 2" of 814 absorbs 0.24 @ 125Hz. The 1" 813 is the one with the 0.08 rating.

I haven't put the fabric on these boxes yet and also don't have them permanently mounted in my room. I did put two 4" thick ones in the corners (so there's a total of 8" (with a small 0.5" gap in between because of my frame) and it does cut down more of the bass issue, for sure. I may have to re-think how I do this. Maybe I'll turn those into 8" thick bass by fastening them together. Originally I planned to hang two of the 4" thick panels above on the ceiling with a decent air gap, but maybe those on the ceiling will now be the 2" thick ones with a good air gap.

I finished building and installing this round of panels. I ended up with 15 of them. Four of them are bass traps for the corners, 3 on the ceiling, and the rest on the walls. On the side of my room that has a closet I have two panels that I'm gonna have to build stands for so I can easily move them as needed. I don't have a reference mic, but the room definitely sounds better. I don't think I've perfectly trapped all the bass, but I'll have to experiment with recording techniques and positions in the room with my guitar and bass (which were the two instruments causing issues) and see how it goes. I'd show a picture but I haven't yet figured out how to do that on here.