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Venue remodel: Sub bunkers?

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dave briar:

--- Quote from: Ivan Beaver on June 24, 2021, 08:10:51 AM ---Most people don't have the luxury of being able to bring a fork lift into the venue.

How would you get it out from under the stage?  Remember that the bunker needs to be airtight (or pretty close) in order to prevent leakage from getting under the stage.  That usually means attached to the stage itself, with a good seal against the floor, and the front of the stage. 

Were you also planning on removing the stage and front in order to get the bunker out?

--- End quote ---

Hmm, good questions indeed.  Let me start off by saying that construction is at least a month off and that I am totally open to suggestions/guidance in the matter.  That said, here is what I have been thinking.

Fundamental design goal: The bunker should be mechanically unconnected from the stage.  (yes/no?)

I envision the four-side bunker sitting directly on the concrete floor with a single layer of "sill seal" (1/4" thick closed-cell polyethylene foam meant for sealing any gap between the bottom plate on residential walls and the concrete foundation) glued to the bottom edge sealing the cavity at the floor.  The stage itself will be framed with 2x4" stud walls running up-down stage on 12" centers decked with at least two layers of 3/4" HDF (and possibly with either drywall or cement-backer board in-between for added mass?) all glued and screwed and then carpeted.  I envision there being ~2" clearance between the top of the bunker and the bottom of the stage deck allowing the bunker to be lifted and then moved out from under the deck as needed.

Routing of cables does present a question.  Our MDF/cement-board bunkers outdoors have a 1" hole drilled near the back to allow cables to pass through. As there are only two ~1/4" cables routed through each I'll bet these "woof" pretty substantially when driven.  Question: How critical would it be to seal a similar hole around the single 12ga cable that will be feeding our indoor subs?  Several options for that come to mind -- compression fittings etc.  An alternative would be to skip a hole entirely and just route the cable forward along one side of the sub then turn the corner at the front edge of the bunker.  This would of course not look great at the front but I believe we intend to cover the front of each-or-both sub bays with a decorative expanded-metal grate similar to our outdoor setup (I'll get pics later today and post them).

Your point about providing an airtight seal between the front of the bunkers and the front of the stage is something I had not previously considered.  If the front of the bunker is essentially flush with the front of the stage is a seal really that critical?  Related: I believe the intent is to face the front of the non-sub-bay part of the stage with beer kegs similar to what we have currently (another pic coming later today).  To accommodate the kegs the stage framing would stop 12-16" back from the downstage lip with the stage deck extending over and resting on the kegs.  If full kegs (of water!) would be better acoustically that would certainly be possible as well.

I truly appreciate any thoughts/perspective/help provided.

Caleb Dueck:
For sealing the back of the cavity - a single gang deep box, surface mounted, with the cable opening caulked closed to the box - really helps.  The just install a single gang tech plate with an NL4. 

Studs for the cavity walls/ceiling - forgot to note, typically I call out 8" on center, to keep the amount of sheetrock flex to a minimum. 

It doesn't hurt to put some acoustical absorption between the entire stage joists to soak up whatever sound is under there.  Mineral wool batts work great and aren't expensive, aim for the 7.5" thick ones, and put them so they're towards the bottom of the joists so they have air above them.  This isn't due to subwoofers per se, just to mop up whatever remaining gak is bouncing around under there.

A layer of some sort of decoupling between the floor layers, like felt, also helps keep the stage surface from transmitting sound like a giant drum head.

The top of the sub cavity can be used as a load bearing surface, if there isn't space for a cavity top, gap, and then joists/floor.  Between the 2x4's that are8" OC - also use standard P1000 type Unistrut and washers/shims between the studs since they are load bearing.  The studs allow the sheetrock underneath to be screwed to it, while the strut supports the weight of the stage.

The under-stage area and the audience area should be separate (sealed). 

Something else I've done if you want it extra decoupled - add a retaining wall around the cavity, and fill it with sand bags, with the bags touching the cavity walls/ceiling.  That's overkill but works really well!

dave briar:
Update (overdue):  So with the concept of a concrete sub bunker embraced by all of the powers that be we proceeded to price out what the remaining 25’x15’ stage framed 12” o.c. and decked with three layers of screwed-and-glued 3/4” MDF would cost and were more than a little taken back by the material prices we were quoted. As I had kept stressing how desirable a rigid/dense stage deck would be the owner said well if concrete is good for isolating the subs why don’t we just pour the entire stage?  As I mentioned upthread, these guys are very comfortable working with concrete. So that’s what we did. Built all the forms and installed rebar over three full days and then poured everything in one step on the fourth. Six 6” support walls running up/downstage with a 4” slab for the deck — basically like a 28” high parking garage. After curing we removed every last piece of form so we didn’t have to install sprinklers as would have been required by the city. We’ve done a dozen or so shows since and every last performer has gushed praise on how quiet and clean sounding the new stage is. Or course a new Nexo PA doesn’t hurt either. Thanks to all who offered their thoughts above.


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