Possible Bass Trap in Wall Cavity

P

PFranco

Audiophyte
Hello everyone,

I am reaching out to see if my idea of using an old fireplace wall cavity as a make-shift bass trap is worth considering or if it will be unlikely to have any meaningful benefits. A summary of the room (currently under construction - we had a flood several months back):

Approximately 23ft x 24ft x 7ft6". Not a dedicated area, is open to rest of home via stairway. 7.3.4 setup with the following:
- Axiom M80, VP180, QS8s, QS10hps, two DIY sealed 18" subs, one PSA S3601, four RSL C34E MKII atmos.

The flooring will end up being a floating subfloor (dri-core insularmor) with LVP plank on top, area rug between the MLP (two seats) and the front LCR. There is an HVAC channel that runs through the middle of the ceiling, which will be drywalled, and on either side there will be a drop ceiling using USG/CGC Mars acoustical panels (2ft x 2ft) - 4" space between drop ceiling and ceiling joists.

Projector and screen (acoustically transparent) will be ceiling mounted. I do not currently have any room treatments but am planning to buy three 2ftx3ftx2" panels for front wall just behind LCR, and one 32"x4ftx2" panel behind the MLP on back wall. I am also considering two 4ft high (16" x 16" depth) bass traps to be placed on floor in two of the four room corners - behind the right channel speaker, and on the right wall behind MLP (this one will be in front of a MDF cabinet door, but it will be opened infrequently). The other two corners will not be possible to place bass traps due to a door and bar cabinetry.

MLP is about 2/3s front wall, and 1/3 from back wall.

Due to room limitations, the right front speaker is placed very close to built in painted MDF cabinetry (not yet on-site), the left speaker is fairly open to the other half of the living space. Other than the front and rear wall abosrption, I am unsure how else I can easily add any other type of treatments to the room (absorption or reflection) but am open to any and all suggestions.

Attached to this link: https://postimg.cc/gallery/qp1ncQq , you will find various photos of the current state of the basement, and a rough diagram made using GIK Acoustics free online room builder utility (all green items are speakers, the big radiators are supposed to be subwoofers - they did not have a subwoofer icon to place in the room), I also added an old photo of the room for additional context.

On to the actual question: At the mid-point of the right wall in the room (fairly close to MLP) will be a 6ft wide x 7ft high x 14" deep stack ledgestone wall with a gas insert fireplace. The distance from the floor to the bottom of the fireplace will be approx. 8". Behind this brick wall and framing will be a further cavity where the old fireplace was vented (approx. 6ft high, 3ft wide, 16" deep).

If I were to fill both of these cavities with a combination of something like owen's corning 703/705 and fluffy pink insulation, and then have two or three false vents (say 4" x 8" each) near the floor (under the fireplace), could it allow for sound to pass behind the brick wall and get absorbed by the insulation, thereby acting as a very large bass trap? Sort of how a large seating riser is sometimes dual purposed as a bass trap?

Any thoughts, insights, suggestions, questions are welcome.

Thank you in advance,

Edit: Added thumbnails below










 
j_garcia

j_garcia

Audioholic Jedi
Bass traps would ideally be in the corners, so not sure how much benefit it would give you inside a fireplace.
 
P

PFranco

Audiophyte
Bass traps would ideally be in the corners, so not sure how much benefit it would give you inside a fireplace.
Thank you for your response.

I was thinking of also running an RCA/XLR cable to the wall directly opposed to the fireplace, as a potential 4th subwoofer placement in the future, that may possibly do more for evening out the response. It would be around 1/3 of the way from the front wall (beside the staircase)
 
highfigh

highfigh

Seriously, I have no life.
Acoustics and 'make shift' don't really go together well.
 
P

PFranco

Audiophyte
Acoustics and 'make shift' don't really go together well.
That is fair. I am thinking now what I may do, is for the cabinetry that is going in, I may have them leave the very corner door as not MDF, and have them make an acoustically transparent door instead, and then fill that one cabinet with absorbent material, to get at least some bass absorption in that corner of the room.

All of this said, I truly won't know where I am at until the room is completed, I did measurements originally via Rew, and did have a deep null at MLP I believe around 40-60hz. Of course with so many things changing (flooring, ceiling, seating position by a couple feet, wall to wall cabinets instead of brick fireplace etc. ) I won't know where I stand until it is all done and I can take measurements.
 
highfigh

highfigh

Seriously, I have no life.
That is fair. I am thinking now what I may do, is for the cabinetry that is going in, I may have them leave the very corner door as not MDF, and have them make an acoustically transparent door instead, and then fill that one cabinet with absorbent material, to get at least some bass absorption in that corner of the room.

All of this said, I truly won't know where I am at until the room is completed, I did measurements originally via Rew, and did have a deep null at MLP I believe around 40-60hz. Of course with so many things changing (flooring, ceiling, seating position by a couple feet, wall to wall cabinets instead of brick fireplace etc. ) I won't know where I stand until it is all done and I can take measurements.
 
TLS Guy

TLS Guy

Seriously, I have no life.
That is fair. I am thinking now what I may do, is for the cabinetry that is going in, I may have them leave the very corner door as not MDF, and have them make an acoustically transparent door instead, and then fill that one cabinet with absorbent material, to get at least some bass absorption in that corner of the room.

All of this said, I truly won't know where I am at until the room is completed, I did measurements originally via Rew, and did have a deep null at MLP I believe around 40-60hz. Of course with so many things changing (flooring, ceiling, seating position by a couple feet, wall to wall cabinets instead of brick fireplace etc. ) I won't know where I stand until it is all done and I can take measurements.
The problem is that room is virtually square with a fairly low ceiling. The dimension ratios are awful, and that room will always be an acoustic problem.
 
P

PFranco

Audiophyte
Appreciate the link, that is exactly what I was thinking, did not know there was a name/calculation for it. I will play around with it a bit, but it probably will not be worth persuing further, doing some quick calc's, i'd probably need multiple ports to make it practical re: total port volume needed.

Any other comments (from anyone) are still appreciated, even when it comes to my initial idea of placing absorbers on the front and rear walls.

The problem is that room is virtually square with a fairly low ceiling. The dimension ratios are awful, and that room will always be an acoustic problem.
You are not wrong!!

Oh how I'd dream of being able to put up a wall to cut off a portion of the room, and make a truly dedicated space.

Luckily, I have not been to any other dedicated home theater space, so do not truly know what I am missing, but do know that I have serious issues, of which I will never be able to fully resolve, without changing the room structure.

Thank you kindly!
 
Eppie

Eppie

Audioholic Ninja
@PFranco we all have budgets and room limitations that we need to work within. I have low ceilings too but I'm not about to dig out the basement to fix it. You've gathered by now that the fireplace won't make a suitable bass trap. Without the aid of a trained professional you are best off to finish the room and take some fresh measurements to see what needs to be addressed. Some companies like GIK Acoustics in GA offer free advice on planning for room treatments and they make a good product too. Absorbers for mids and highs can be fashioned DIY but bass traps are trickier as the wavelength is long and they operate differently. They usually employ an energy absorbing membrane and I would lean more towards a commercial product. Work on sub placement first though. You should get the biggest benefit from experimenting with sub placement. Another option vs bass traps is to use a mini-DSP on your subs to flatten out the peaks (you can't EQ out nulls).
 
P

PFranco

Audiophyte
Thank you @Eppie , I appreciate everyone's comments.

I agree it is definitely more complicated than I originally thought, I will move forward with the completion of the room without any further thought into the idea of turning my fireplace cavity into a DIY Helmholtz resonator; definitely too complex with too much room for error. Further complicated by the fact that my room, while square, isn't even a sealed square, as the stairway opens to the remainder of the house.

I definitely am not one who will shy away for a good ol' sub-crawl and working with MiniDSP.

All I will focus on now, is running speaker wire/XLR/RCA to one additional location while the walls are ripped up, to maximize my future placement options.

Cheers,
 
j_garcia

j_garcia

Audioholic Jedi
Bass traps in the corners since the room is near square should help. Makeshift or bought, I am going to say it will help. Seating position is critical in a square-ish room, so see if there is a spot where the null is less. I had a square room before and it was a pain because I had to crank the sub higher than desired to get it to sound good at the MLP. Everywhere else it sounded hot, so I moved both the sub and the MLP lol. It did not eliminate the null completely, but it helped a lot.
 
P

PFranco

Audiophyte
Bass traps in the corners since the room is near square should help. Makeshift or bought, I am going to say it will help. Seating position is critical in a square-ish room, so see if there is a spot where the null is less. I had a square room before and it was a pain because I had to crank the sub higher than desired to get it to sound good at the MLP. Everywhere else it sounded hot, so I moved both the sub and the MLP lol. It did not eliminate the null completely, but it helped a lot.
Thank you for your comments, I look forward to when everything is complete and I can set the room up again. I will not have much flexibility in moving the MLP, but definitely have flexibility with sub locations. I think having at least one near-field will help.

I also have crowson tactile transducers under the MLP, which definitely helps, but I keep those for under 30hz duty only.
 

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