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[dave briar]

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.

[Caleb Dueck]

  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.

[dave briar]

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...

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;-)

  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.

 
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!

[Doug Fowler]

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.

[dave briar]

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?

[Doug Fowler]

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.

[Tim McCulloch]

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

[dave briar]

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.

[Ivan Beaver]

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.

[dave briar]

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.

[ProSoundWeb Community]