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Author Topic: Venue remodel: Sub bunkers?  (Read 5151 times)

dave briar

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Venue remodel: Sub bunkers?
« on: June 19, 2021, 03:09:47 PM »

I’ve reviewed as many of the stage-design threads I’ve been able to find here which included several references to using mass to reduce unwanted low-frequency build up when the subs are positioned under the front of the stage.  Matter of fact we installed a sandbag wall behind the four 18” subs under our outdoor stage with very positive results.

Now it’s time to design our new indoor stage which presently will include two 18” subs center clustered under the downstage lip. The venue owner is proposing building (forming/pouring) 2” thick concrete boxes to enclose the top, sides, and back of each of the subs. The subs themselves will be sitting on the concrete floor. I told him I’m pretty sure a bunker like that will greatly reduce the low-frequency energy coming off the top and back of the subs but, given all that I’ve learned here over the years, suspect that there may be other detrimental effects do reflections off the inside walls of the bunkers etc.

So suppose we enclose a Nexo LS18 in a concrete bunker with 1” clearance on the top, back, and sides.  Any thoughts on the ramifications,  sonic performance, and overall advisability would be greatly appreciated.
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Caleb Dueck

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Re: Venue remodel: Sub bunkers?
« Reply #1 on: June 19, 2021, 09:41:27 PM »

  Any thoughts on the ramifications,  sonic performance, and overall advisability would be greatly appreciated.

Bunkers should be a must for any subwoofers under a stage.  Meaning - you never want bass to 'bounce around' under a stage.  It's awful. 

The question for bunkers isn't 'if', it's 'what type'. 

Concrete (2" seems too thin) is one option, but how easy is it to tear out and redo when the sub models change?  Multi-layer glued and screwed sheetrock with extra studs and strut is another option that works well. 

If you decide to go with concrete - push for thicker, at least 4".  Also keep the space beside/above/etc as small as possible (an inch or less). 

If you decide to go the drywall route - let me know as I have CAD blocks of previous projects, that have worked very well. 

Another consideration - will you ever add subs in the future?  If so, plan for a delay arc of some sort rather than simply centered.
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #2 on: June 20, 2021, 03:49:14 PM »

Bunkers should be a must for any subwoofers under a stage.  Meaning - you never want bass to 'bounce around' under a stage.  It's awful. 
Trust me, I know.  The outdoor stage was just completed when I started working this venue eight years ago so I had no input on it whatsoever. 24" high framed with 2x4 stud walls 24"OC running up-downstage decked with glued and screwed 2x6 tongue and groove sitting on a concrete slab with a concrete wall defining stage right.  Four FBT 118sa subs under the front.  The low-frequency roar underfoot was absurd.  Adding gain to a floor tom made it take off -- almost literally.  I remember one band had an electric stand-up bass -- no body, just a tall fretboard -- that the player set down a little abruptly and the thump from the subs made it literally jump an inch off the stage before it fell back and triggered again.  Sounded reminiscent of a 50-cal on auto mode.  Rather humorous in its own morbid kind of way. I suggested enclosing each sub in a 3/4" MDF box.  Not much improvement.  Added 5/8" cement board on top of the MDF.  Better but still insufficient. Adding the sandbag wall behind the subs quieted all but the downstage 3' of the stage.  Sigh...

Quote
The question for bunkers isn't 'if', it's 'what type'. 

Concrete (2" seems too thin) is one option, but how easy is it to tear out and redo when the sub models change?
New bunkers could very likely be needed for new subs but moving them in and out will be easy using one of the fork lifts onsite.  By the way "decorative" concrete expertise is available from one of the owners who poured and finished the single-piece 30' bar/serving station surface. 4" thick by the way;-)

Quote
  Multi-layer glued and screwed sheetrock with extra studs and strut is another option that works well. 
If you decide to go with concrete - push for thicker, at least 4".  Also keep the space beside/above/etc as small as possible (an inch or less). 
I appreciate the input and understand how multiple layers of sheetrock could work but am curious how many layers you've used previously based on your suggestion that 2" concrete would be insufficient.

Quote
 
If you decide to go the drywall route - let me know as I have CAD blocks of previous projects, that have worked very well. 

Another consideration - will you ever add subs in the future?  If so, plan for a delay arc of some sort rather than simply centered.
Appreciate the offer of design specs and the advice on a delay arc.  I'm pretty confident that two LS18s will suffice for our little space.  Also center clustering has as much to do with keeping the subs from radiating up under the mezzanine immediately adjacent to the left edge of the stage as anything.
 
Thanks for your input!
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Doug Fowler

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Re: Venue remodel: Sub bunkers?
« Reply #3 on: June 21, 2021, 09:34:25 AM »

Trust me, I know.  The outdoor stage was just completed when I started working this venue eight years ago so I had no input on it whatsoever. 24" high framed with 2x4 stud walls 24"OC running up-downstage decked with glued and screwed 2x6 tongue and groove sitting on a concrete slab with a concrete wall defining stage right.  Four FBT 118sa subs under the front.  The low-frequency roar underfoot was absurd.  Adding gain to a floor tom made it take off -- almost literally.  I remember one band had an electric stand-up bass -- no body, just a tall fretboard -- that the player set down a little abruptly and the thump from the subs made it literally jump an inch off the stage before it fell back and triggered again.  Sounded reminiscent of a 50-cal on auto mode.  Rather humorous in its own morbid kind of way. I suggested enclosing each sub in a 3/4" MDF box.  Not much improvement.  Added 5/8" cement board on top of the MDF.  Better but still insufficient. Adding the sandbag wall behind the subs quieted all but the downstage 3' of the stage.  Sigh...
New bunkers could very likely be needed for new subs but moving them in and out will be easy using one of the fork lifts onsite.  By the way "decorative" concrete expertise is available from one of the owners who poured and finished the single-piece 30' bar/serving station surface. 4" thick by the way;-)
I appreciate the input and understand how multiple layers of sheetrock could work but am curious how many layers you've used previously based on your suggestion that 2" concrete would be insufficient.
Appreciate the offer of design specs and the advice on a delay arc.  I'm pretty confident that two LS18s will suffice for our little space.  Also center clustering has as much to do with keeping the subs from radiating up under the mezzanine immediately adjacent to the left edge of the stage as anything.
 
Thanks for your input!

The trick with sheetrock is to create pockets of air between the surfaces. For residential applications there are hanging/spacing devices which help accomplish this. This should be well documented out there somewhere.
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #4 on: June 21, 2021, 10:53:06 AM »

The trick with sheetrock is to create pockets of air between the surfaces. For residential applications there are hanging/spacing devices which help accomplish this. This should be well documented out there somewhere.
Yea I’m generally aware of that technique.  When we remodeled the basement here I hung the ceiling Sheetrock on sheet-metal brackets to decouple it from the floor joists above and as a carpenter (in a previous life) we built some interior walls using 2x6” top and bottom plates with alternating 2x4” studs so that the inside and outside Sheetrock only couple at the plates but I’m not sure how effective that would be for mitigating strong 30’ waveforms.  Abundant mass seems to work better at that no?
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Re: Venue remodel: Sub bunkers?
« Reply #5 on: June 21, 2021, 11:09:37 AM »

Yea I’m generally aware of that technique.  When we remodeled the basement here I hung the ceiling Sheetrock on sheet-metal brackets to decouple it from the floor joists above and as a carpenter (in a previous life) we built some interior walls using 2x6” top and bottom plates with alternating 2x4” studs so that the inside and outside Sheetrock only couple at the plates but I’m not sure how effective that would be for mitigating strong 30’ waveforms.  Abundant mass seems to work better at that no?

No idea. The SynAudCon listserv might have ideas.
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Tim McCulloch

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Re: Venue remodel: Sub bunkers?
« Reply #6 on: June 21, 2021, 11:20:32 AM »

A plug for The Master Handbook of Acoustics, by F. Alton Everest and Ken Pohlman (5th ed).
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #7 on: June 21, 2021, 01:02:51 PM »

A plug for The Master Handbook of Acoustics, by F. Alton Everest and Ken Pohlman (5th ed).
Sixth edition due in August. Couldn’t find a fifth edition but my questions are so basic I’m betting the forth edition (2000) will suffice — especially for only $6.99 delivered.
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Ivan Beaver

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Re: Venue remodel: Sub bunkers?
« Reply #8 on: June 21, 2021, 02:57:14 PM »

Concrete is nice, but as others have said, hard to redo if the subs change

Don't worry about a couple of inches.  You don't want them to tight, in case you need to pull them out for repair or whatever.  Be sure you can get your hands around them.

Think in terms of wavelength.  A couple of inches with wavelengths that are generally between 10-30' long will not be noticeable.

Don't worry about putting any "filling" to reduce those reflections either, you can't make it thick enough.

You might want to put a "stop" on the rear to keep them from moving back to far and damaging the connector plug.
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #9 on: June 21, 2021, 04:35:18 PM »

Concrete is nice, but as others have said, hard to redo if the subs change

Don't worry about a couple of inches.  You don't want them to tight, in case you need to pull them out for repair or whatever.  Be sure you can get your hands around them.

Think in terms of wavelength.  A couple of inches with wavelengths that are generally between 10-30' long will not be noticeable.

Don't worry about putting any "filling" to reduce those reflections either, you can't make it thick enough.

You might want to put a "stop" on the rear to keep them from moving back to far and damaging the connector plug.
Google says the density of drywall is 724kg/m3 whereas concrete is 2,400kg/m3 but no question Sheetrock would be easier to build and modify.  If it were me the Sheetrock option seems much more workable (I’ve never been enthralled with working concrete) but if the owner wants to indulge in some creative bunker construction that will work too.

Understood on the non-criticality of spacing between the sub and bunker.  Agrees with what I suspected. 

Good advice on putting a spacer behind to protect the connectors as well. I just yesterday had to replace a XLRM on one of the outdoor subs because it got yanked too hard pulling the sub out of its too-low-of-clearance MDF/cement-board box.  Also had to replace a second cable used to daisy chain that sub to the next in line as that cable got scrunched/run over as the sub was pushed back into the box exposing the shield.  Allowing a bit more space between the Nexo and its bunker will allow more carefully controlled routing of its 12 gauge cable.

Thanks Ivan.
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Helge A Bentsen

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Re: Venue remodel: Sub bunkers?
« Reply #10 on: June 21, 2021, 05:00:24 PM »

Depending of the amount of reduction needed, I've had pretty good results using MDF/drywall in a sandwich construction.
A inner shell of MDF with enough space for the sub(s), two layers of drywall and one outer layer of MDF with as little as possible physical connection to the rest of the stage. Easy to build and easy(ish) to modify later down the road. Also made the stage floor of the same sandwich type, put a pair of SB1000z under there and it worked well.


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Re: Venue remodel: Sub bunkers?
« Reply #11 on: June 21, 2021, 07:22:30 PM »

If sand worked before, what about using a CMU block wall that is internally filled with sand. Cap this with the drywall, or drywall-mdf sandwich.

CMU blocks are pretty easy to knock out with a sledge if you later need to reconfigure. I know years ago we would use similar construction in dance clubs to make mounting platforms for the turntables. This would decouple the TT's from all the vibrating wood that made up the DJ booth.
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #12 on: June 22, 2021, 12:54:42 AM »

Depending of the amount of reduction needed, I've had pretty good results using MDF/drywall in a sandwich construction.
A inner shell of MDF with enough space for the sub(s), two layers of drywall and one outer layer of MDF with as little as possible physical connection to the rest of the stage. Easy to build and easy(ish) to modify later down the road. Also made the stage floor of the same sandwich type, put a pair of SB1000z under there and it worked well.
I like the sandwich idea. No 2x4’s needed.  Bunker thickness is somewhat limited by the need to keep the stage as low as possible to maintain view lines from under the mezzanine.
Curious how the two layers of drywall in the stage deck are holding up?  I’ve been looking at using two layers of 3/4” HDF glued and screwed to the joists. Drywall would certainly add mass but drywall underfoot is not a “normal” application I’ve ever heard of. Just curious.
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #13 on: June 22, 2021, 01:08:07 AM »

If sand worked before, what about using a CMU block wall that is internally filled with sand. Cap this with the drywall, or drywall-mdf sandwich.

CMU blocks are pretty easy to knock out with a sledge if you later need to reconfigure. I know years ago we would use similar construction in dance clubs to make mounting platforms for the turntables. This would decouple the TT's from all the vibrating wood that made up the DJ booth.
That’s a pretty practical solution. There is sufficient room to the back and sides of the sub-cluster so CMU block could fit. Sand is cheap.  Building it during stage construction would be easy. Knocking it out and replacing it with the stage deck on would be more difficult.  Great idea though.
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Helge A Bentsen

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Re: Venue remodel: Sub bunkers?
« Reply #14 on: June 22, 2021, 02:59:37 AM »

I like the sandwich idea. No 2x4’s needed.  Bunker thickness is somewhat limited by the need to keep the stage as low as possible to maintain view lines from under the mezzanine.
Curious how the two layers of drywall in the stage deck are holding up?  I’ve been looking at using two layers of 3/4” HDF glued and screwed to the joists. Drywall would certainly add mass but drywall underfoot is not a “normal” application I’ve ever heard of. Just curious.

MDF forms the outer layers of the sandwich, so people are  performing on top of a solid surface. It's 1" MDF top/bottom with two layers of 3/4" drywall in-between, feels very solid to walk and perform on. Certainly  more  solid than most regular stage decks.  The sandwich floor is on top of 2x4" beams/legs not  connected to the sub bunkers. We used industrial carpet on top for less HF reflections from the floor and vacuum the  stage regularly.
I have no data on how effective this solution is, it was conceived as a cost effective solution using available materials and for ease of installation.
I've had several performers comment on how little sub there is up on stage after we did this compared to past experiences in the same room.
« Last Edit: June 22, 2021, 03:07:38 AM by Helge A Bentsen »
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Re: Venue remodel: Sub bunkers?
« Reply #15 on: June 22, 2021, 08:19:13 AM »

Google says the density of drywall is 724kg/m3 whereas concrete is 2,400kg/m3 but no question Sheetrock would be easier to build and modify.  If it were me the Sheetrock option seems much more workable (I’ve never been enthralled with working concrete) but if the owner wants to indulge in some creative bunker construction that will work too.


If you use sheetrock, it is advisable to use them double thick with alternating seams, glued and screwed.

That gets a lot closer to concrete density.

But hey, if he wants concrete, then just do that, but he must understand that he is pretty much stuck with the size for future sub choices.
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Dave Garoutte

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Re: Venue remodel: Sub bunkers?
« Reply #16 on: June 22, 2021, 12:34:40 PM »

You could do the MDF/sheetrock with a gap filled with sand.  The sand should be very absorbative due to the fact that it can move.
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #17 on: June 23, 2021, 10:04:12 AM »

MDF forms the outer layers of the sandwich, so people are  performing on top of a solid surface. It's 1" MDF top/bottom with two layers of 3/4" drywall in-between, feels very solid to walk and perform on. Certainly  more  solid than most regular stage decks.  The sandwich floor is on top of 2x4" beams/legs not  connected to the sub bunkers. We used industrial carpet on top for less HF reflections from the floor and vacuum the  stage regularly.
I have no data on how effective this solution is, it was conceived as a cost effective solution using available materials and for ease of installation.
I've had several performers comment on how little sub there is up on stage after we did this compared to past experiences in the same room.
Very interesting.  An alternative I have considered is adding a layer or two of 1/2” “cement-backer-board” (normally used behind ceramic tile in showers) between the two layers of HDF.  The cement board pencils out at ~1,200kg/m3 but drywall would no doubt be less expensive. Also planning on framing the stage on 12” centers — but that’s likely a discussion for a different thread.   Thanks for the details.
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #18 on: June 23, 2021, 10:14:56 AM »


But hey, if he wants concrete, then just do that, but he must understand that he is pretty much stuck with the size for future sub choices.
True that for sure but worse case would be bringing the fork lift, lifting the bunker out (it will not be attached to the floor — except by gravity), carrying it to the dump trailer, and building new ones. Modifying a sandwich construction bunker would certainly be more feasible if we need it smaller but making it larger would likely be easier to just build over as well.  But wise council nonetheless. Thanks.
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Ivan Beaver

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Re: Venue remodel: Sub bunkers?
« Reply #19 on: June 24, 2021, 08:10:51 AM »

True that for sure but worse case would be bringing the fork lift, lifting the bunker out (it will not be attached to the floor — except by gravity), carrying it to the dump trailer, and building new ones. Modifying a sandwich construction bunker would certainly be more feasible if we need it smaller but making it larger would likely be easier to just build over as well.  But wise council nonetheless. Thanks.
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?
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dave briar

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Re: Venue remodel: Sub bunkers?
« Reply #20 on: June 25, 2021, 01:46:47 PM »

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?

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.
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Caleb Dueck

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Re: Venue remodel: Sub bunkers?
« Reply #21 on: June 25, 2021, 06:30:48 PM »

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!
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Re: Venue remodel: Sub bunkers?
« Reply #22 on: December 04, 2021, 06:11:40 PM »

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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Re: Venue remodel: Sub bunkers?
« Reply #23 on: June 18, 2022, 03:04:22 AM »

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.

https://ibb.co/j8q0rK7
https://ibb.co/6bJdTFc
https://ibb.co/tMP3MjS
https://ibb.co/mHws2Sy

That is a really neat stage and I but it sounds great. Nice work!
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Re: Venue remodel: Sub bunkers?
« Reply #23 on: June 18, 2022, 03:04:22 AM »


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