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Sub placement under stage behind stairs

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Tom Young:
I should have been more thorough in my initial response.

The closet needs to enclose the subwoofer and be joined to the stage front opening through which the subwoofer projects forward energy. Simply enclosing the subwoofer on 5 sides with gaps around it does nothing to keep sound from diffracting around and into the understage area. The wavelengths of the frequencies produced by the subwoofer are such that they pretty much all bend around the enclosure (diffract). This is basically what George was addressing.

Such constructions need to be far more than 1/2" plywood. Your subwoofer itself is likely to be braced 3/4" plywood or MDF. I recommend at least one layer of 3/4 void-free plywood with substantial 2x4 bracing. A more flexible (less rigid) enclosure will result in resonances and diaphragmatic behavior which will do no good to the sound quality of the subwoofers. Make the closet slightly larger than the subwoofer and stuff (pack) fiberglass into the sides and top gaps, plus a neoprene pad on the bottom to keep it from walking *and* to decouple it from the box/closet and floor. Ensure that the closet is firmly anchored (and sealed) to the stage and to the floor.

Make the cable passage airtight (putty it up).

Blake Ross:
Thanks for the expanded reply Tom. we may not have enough room under the stage for that much structure but I won't know until the carpenters get in there and look.
If we don't have enough room, could we remove the stage floor trusses in the area of the sub closet and then design the sub closet heavy enough to become the structural support for the floor, or would we have too much stage vibration even with the sub closet air gaps stuffed full of fiberglass and solidly attached to the church floor (wood truss floor with full basement underneath 1962) with the stage weight on it?
It will be some work, but if gets the subs out of our way and delivers the sound we will put in the time to do it right.
Thanks again for your input!

George S Dougherty:
Another thought, run a trial with the subs on the stage in the position where they'd end up below the stage.  You want to be at least 8ft from any major boundaries to avoid major cancellation issues in the subwoofer's passband.  Make sure you won't have any suckouts or noticable response issues before you go to all that effort.

I'd take some effort to decouple the cabinet enclosure from the stage floor if it will become part of the structure.  There are isolation membranes for studio construction that may work.

Tom Young:
I cannot advise about stage structure modifications. You need someone with proper credentials to determine that this would be safe.

Whatever you do, make sure the subwoofer is decoupled from the stage and stage structure. It is OK for the subwoofer closet to be affixed to the stage or stage structure provided the closet is well-built and the subwoofer is decoupled from the closet, which it would be if it sits on a neoprene pad and has stuffed fibergalss around it.

Blake Ross:
Thanks George and Tom. We did run the subs as near to their stage position as possible and had acceptable results. The nearest wooden pews are 7 feet away and the main center aisle the subs line up with is 4 feet across.
I wasn't worried about structural safety as we have people to deal with those issues.  The concern was with keeping the stage floor uncoupled from the closet. From your responses its now apparent we will need to take the time to ensure the materials used will isolate the closet as much as possible and minimize any sound interaction with the stage floor.
I'll be out of the country for 3 weeks, so hopefully when I return the carpenters will have had time to get under the stage and do some measuring and we can make a decision to go with the sub closet or look elsewhere in the church to place them.
Thanks so much for all your input, you gentleman certainly have added much needed information we had not considered on our quest to locate our subs   .

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