We recently finished a large theater project in a basement where in the screen wall was furred out from the concrete foundation wall. Initially the overall depth measured 6 inches, wherefore the in-wall Triad bronze subs would have fit fine. But after getting into this deeper it turned out that the concrete wall was very irregular and the boxes would not fit flush. Since we were covering the speakers with fabric panels anyway I though why not just bolt the boxes to the concrete which we did with a moisture barrier and blue screws through the inside back of the boxes.

After we got everything hooked (using a pair of Triads DSP amps for the subs) up we gave it a try and wow, I have not in my 32 years in the business heard and felt bass that good coming from a pair of boxes of that size. Punchy and tight does not even begin to describe the experience.

The oddest thing was that normally as we all know when you walk about the room you expect to have nulls and so forth. but with this arrangement nulls almost seemed non existent.

I have installed a lot of these subs so I am very familiar with their normal performance. It seems that what is going on is the passive harmonics that resonate off the box itself which would normally counter some of the woofers front firing energy because of phase cancellation is nearly non existent when dampened by the ultra low resonance concrete.

I always wanted to make a sub woofer enclosure out of concrete, but hey I would be more than happy with this solution. If you ever get the chance do this, it will blow your socks off.

I suspect a lot of things converged to make this sound so good. One of them definitely was the concrete keeping the boxes from bouncing around and thus radiating sound from the cabinet, but that doesn't explain the great bass acoustics.

You hit the jackpot on this one, no matter what!

If you don't understand the science behind room acoustics, you might as well be performing Vudo. So break out the dolls and hope for the same luck on your next install.
 

Oh come on John.
Grant (who also worked on this) and I both have gone to acoustics school as well. I understand room acoustics and for all practical purposes the room should have had some real issues since it was almost a square box. and as you know there is only so much you can do with absorptive materials. I am sure the ceiling being partially decoupled from the rest of the structure had something to do with it as well.

I have poured a lot of time and effort into engineered rooms in the past that did not sound this good. I think Ernie is right it was simply one of those jack pot installs where everything just came together perfectly.

I'd say by mechanically coupling the boxes to the inert concrete wall, you COULd be getting a little bass reinforcement from the wall itself...? Glad to hear it worked out. Stop by our shop sometime, David.

I am going with "reduced energy loss from the sub cabinet due to the coupleing to the concrete".
You can not have a box tha is too stiff.

I agree with Brent.


I don't know. John also noticed that what you wrote wasn't very acousticianish.


You really would have had issues! Squares have zero height!


To my unschooled way of thinking, that would help it not seem, acoustically, to be a box with a square base, or a cube, depending on what you really meant.


All of which means the voodoo dolls aren't any worse idea than anything else. Although stopping cabinet vibrations by making the cabinets have to move the concrete was the ideal thing to do.

I've got a slim4 in a pool house that is installed the exact same way - screwed directly to the concrete. Can't wait to hear it, firing up that zone tomorrow!!!

Yes I need to take the time to come by soon :-)

Well, the concrete foundation walls with the many tons of fill packed against the other side make for a high mass for coupling that ridiculously braced cabinet so the baffle is as stationary as possible.

Certainly better than mounting in the 2-by stick framing and a sheet or two of drywall.

Erskine, Yates, Grimani, and others recommend filling the stage with sand to increase the mass of that platform if subs are going to be placed on it. You just applied the same principle to an in-wall application. The only issue may be the penetration and the pathways that exist on the backside of the wall that could effect the isolation of the project.

+1 Yates designed our theater. Did a nice job. Sounds pretty good.