Axiom Home Page Forums General Discussion Home Theater Possible Bass Trap in Wall Cavity

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 bottom of which will be approx. 8" from floor. 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,

Photo thumnails below (not sure if this will embed properly):

Do you have a bass resonance issue? I'm more inclined to explore other options such as re-running a room optimization software, moving the subwoofer, adding a second subwoofer, or even just moving the crossover frequencies to a lower setting to see if that helps. I'd rather add room dampening or panels if there's an issue that can't be addressed with the tools you already have.

Bass traps are unlike other acoustic treatments in that they need to be membrane-like to work. They're more like hollow boxes than absorptive foam that you can stick on the wall. I jury rigged bass traps in one of my apartments; it was the ugliest thing ever, because each one was about 12 cu ft, and I had 8 of them in one room. It totally worked, but don't do what I did.

Thank you for your response Hambrabi,

It has been a while since I have had my setup in operation, back when I did, I recall having a fairly large dip at and around 40-60hz at the MLP (I have a UMIK 1 mic, with REW software, and use a minidsp 2x4HD for subwoofer corrections)

I currently have three subs, one at front wall (approx. 1/4 from right wall under center channel), one at the rear right corner of room, and one near field behind MLP.

I was also thinking of adding a fourth one of two possible locations (no other corner is possible given doorways/bar cabinetry), those positions either being about middle of room at back wall, or the left wall about 1/3rd from front wall (by the stair case).

Ultimately, once the renovation is complete, the issue I was having at the 40-60hz area, may be totally different given that I have changed sitting location by a couple of feet, added a drop ceiling, changed the flooring material and added a subfloor. I will definitely need to remeasure and dial in once everything is completed, but I figured if this could potentially help, it would be best to plan for it before everything was built.

Having said that, it seems like it is just too unscientific, and is unlikely to add any real-world benefit.

Our room acoustics expert, Trevor, will tend to your needs after his vacation.

Thank you Mojo,

I look forward to Trevor's input!

Following this one closely. I have similar basement space with this idea in mind of an x.x.x channel theatre.

Very cool project Remus. Sorry about the flood. That’s never fun. But now you’re rebuilding. Nice work.

Thank you Kodiak,

Yes it was definitely no fun when it first happened, we were planning to renovate in a couple years, so we just expedited the project.

I have learned a lot about my home in the process, and have added some prevantative measures that will hopefully greatly reduce/eliminate the chance of it ever happening again.

I also posted in audioholics forum, and have had a number of responses there, that you may be interested in.

See here: https://forums.audioholics.com/forums/threads/possible-bass-trap-in-wall-cavity.126157/#post-1579113

Thanks for the audioholics link. I’ll check it out.

I should also second mojo’s accolades of Trevor as the local subject matter expert.

Cheers!

I do have a question. Why not put a (*gasp*) fireplace where the fireplace goes? If it's wood, you could always switch it to the convenience of natural gas or wood pellets. I don't think it's a liability unless it's hardly used, and there's certainly beautiful fireplaces and stoves that could fit.

I'm also confused as to why the firewall has electrical outlets on them, isn't the whole point of the wall to be fireproof?

We most definitely are putting a fireplace there! The question was whether or not to add ports below the fireplace that would reach into the hollow cavity behind the fireplace, making it act as a helmholtz resonator.

The original fireplace was wood, but converted to natural gas by the previous owner. The cavity you see will remain hollow behind new framining which will be built with stack ledgestone/brick (approx. 6ft wide, 7ft tall, 14" depth). The gas fireplace will be installed in the new to be built brick wall (which will be at the same depth as cabinetry which will be built and installed under each of the windows).

The electrical you see will be above the new mantle and be used power xmas lights etc. We also need 120v ac behind/wired into the gas fireplace to operate the lighting/fan. The pre-existing outlet you may have seen in the cavity, would not have been original to the home when the fireplace was wood, but was added by a previous owner when they converted to gas (probably installed in the 80s before I was born lol).

Not sure if I explained it sufficiently, but once everything is completed it will definitely be up to code/fireproof, with no concern re: location of electrical. I grabbed a couple random photos from the internet for context: https://postimg.cc/gallery/zH7LDpw

Of course we will not be putting a tv up above the mantle, as our projector faces a different wall, but will have an outlet there.