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Author Topic: Sub placement under stage behind stairs  (Read 23942 times)

Blake Ross

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Sub placement under stage behind stairs
« on: May 27, 2010, 02:48:16 pm »

Hi, we're looking for some help finding a place for our subs on or around the church stage and I'm hoping others out there have some thoughts for us. The church has been upgrading its sound system to support its growing musical theatre ministry. We recently bought a pair each of Yorkville U15's and USC1's. The U15's will be flown, but we're having trouble finding a home for the subs. The 19.25 inch wide subs would fit under the stage, but the stage has 3 seven inch steps attached, so the center clustered subs would be baffled by the risers and any vertical supports needed.
The stage space is at a premium with a choir loft, organ pit and pulpit competing for space in a 40' wide area. The subs need to be placed so they will not interfere with the actors which only leaves under or above.

Is the understage/behind the stairs a viable option for us, or should we be looking elsewhere in the church for placement? The USc1 is not designed to be flown, but it could be rigged for sidewall placement. The subs would then be up and out of the way on one of the laminate beams. This would mean less design and carpentry, but we give up the benefits of having the subs center clustered.

We would appreciate your ideas on the pros and cons of placing them under-stage, or any other placement you think we should look at. Thanks!

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Tom Young

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Re: Sub placement under stage behind stairs
« Reply #1 on: May 27, 2010, 07:03:14 pm »

Provided the architectural/structural elements in front of the under-stage subs do not represent a whole lot of acoustic obstruction, you should be fine.

Ideally you would build a solid "closet" for the subs to help contain some of the diffracted energy so it does not as freely enter and bounce around in the understage and eventually find its way out the front (and therefore is mislaigned with the direct energy).

Be sure that the stage floor is fairly substantial (no pun....) in its construction. Subs beneath a flimsy floor can cause vibrations which are mechanically transferred through the mic stands and on into the mic's. Perhaps worse would be if the subs are in physical contact with the stage, or its support structure.

One more suggestion:  if you play the system loud, the subs will "walk" (move around) over time. You can prevent this by placing neoprene pad material below them (between them and the floor).

HTH-
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Tom Young, Church Sound section moderator
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Blake Ross

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Re: Sub placement under stage behind stairs
« Reply #2 on: May 27, 2010, 09:27:58 pm »

Thanks Tom, I hadn't given any thought to containing the sound using a 'closet' for the subs - great idea, as is the neoprene mat underneath to help keep the subs at home. I don't foresee the system being played loud enough for sub movement but I've certainly been wrong before...
Should a sub closet be built pretty tight to the subs (2 inches clearance all around?) or is more clearance than that a better idea? I'm envisioning 1/2 inch plywood glued together in a box, screwed down to the floor with the front open for the grills and enough of a hole at the rear to feed in a speaker cable.
Please let me know if this is what you had in mind, thanks!
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George S Dougherty

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Re: Sub placement under stage behind stairs
« Reply #3 on: May 28, 2010, 12:15:20 am »

Tight to the subs is fine, leave yourself room to maneuver and drag them out if necessary.  The ideal is for them to be airtight to the stairs and room so the pressure isn't being lost behind the subs under the stage.  
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Blake Ross

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Re: Sub placement under stage behind stairs
« Reply #4 on: May 28, 2010, 08:34:59 am »

Thanks George and Tom. Getting these subs out of the way under the stage will be a huge help for us.
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Tom Young

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Re: Sub placement under stage behind stairs
« Reply #5 on: June 02, 2010, 08:19:23 pm »

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).
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Tom Young, Church Sound section moderator
Electroacoustic Design Services
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Tel: 203.888.6217
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Blake Ross

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Re: Sub placement under stage behind stairs
« Reply #6 on: June 03, 2010, 01:45:03 pm »

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!
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George S Dougherty

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Re: Sub placement under stage behind stairs
« Reply #7 on: June 03, 2010, 03:51:19 pm »

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.
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Tom Young

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Re: Sub placement under stage behind stairs
« Reply #8 on: June 03, 2010, 05:37:54 pm »

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.
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Tom Young, Church Sound section moderator
Electroacoustic Design Services
Oxford CT
Tel: 203.888.6217
Email: dbspl@earthlink.net
www.dbspl.com

Blake Ross

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Re: Sub placement under stage behind stairs
« Reply #9 on: June 04, 2010, 11:07:23 am »

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  Razz .
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Blake Ross

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Re: Sub placement under stage behind stairs
« Reply #10 on: February 23, 2011, 11:24:29 pm »

Hi Myles, mixed reviews but a lot of compromises occurred in my absence, as I was away during construction.
The carpenters/designers/many others involved decided not to build a closet under the stage, but rather built a proscenium in the middle of the steps at stage front that became the sub closet.
Unfortunately with the angle wall design they chose,  the closet was too small to put the subs end to end, so they are offset, with one sub in front of the other with a 4 inch overlap. I guess that's one way to introduce a cardioid sub design minus the knowledge of how to align it Smile.
The subs are only used for our musical plays a couple of times a year, and never with high SPL. We have no praise band.
The subs are now out of the way and out of sight, and much of the audience is in the sweetspot for sound.
The misalignment of subs/flown tops is noticeable to me in other parts of the church but I think that centre clustered subs always have that compromise.
I will grab a picture for you tomorrow so this post might make more sense.
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