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Venue remodel: Sub bunkers?

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dave briar:
Ive reviewed as many of the stage-design threads Ive 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 its 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 Im 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 Ive 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.

Caleb Dueck:

--- Quote from: dave briar on June 19, 2021, 03:09:47 PM ---  Any thoughts on the ramifications,  sonic performance, and overall advisability would be greatly appreciated.

--- End quote ---

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.

dave briar:

--- Quote from: Caleb Dueck on June 19, 2021, 09:41:27 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. 

--- End quote ---
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?

--- End quote ---
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). 

--- End quote ---
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.

--- End quote ---
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!

Doug Fowler:

--- Quote from: dave briar on June 20, 2021, 03:49:14 PM ---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!

--- End quote ---

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.

dave briar:

--- Quote from: Doug Fowler on June 21, 2021, 09:34:25 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.

--- End quote ---
Yea Im 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 Im 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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