# Concrete subs home theatre



## Graydon (Jun 28, 2007)

Hi guys,

I've been reading this forum for quite a few months learning as much as I can. I started building a hometheatre in my home last spring. It started abotu two years ago when I poured the foundation for an additon to my home. I formed the cabinets for my main left and right speakers and sub horns into the foundation. Now two years later, I have the room well on the way. it is functionally operational in a sort of test mode. I am working out a few bugs and checking systems to make sure I have everything I need and that I haven't screwed something up royally before I do the final push to finish the room. So, in that light, I'd love to hear your comments and advise. What can I do to improve it? What is obviously a mistake and should be fixed before going forward? Finally, if anyone is in the Fenton, Michigan area, please feel free to invite yourself over to check it out in person.

The room is going to function as a studio/jam room for me and my friends to play live music and rehearse. For that reason, I have a 16ch mixer in my AV rack and four mic input panels spaced around the room each with 4 dual XLR/phone jack inputs. 

Secondly, it will also function as a home theatre room. it is wired for 7 channels of surround but no separate sub because the main left and right speakers go down to 15 Hz. They actually are capable of making 85dB at 10Hz, but that is a bit ridiculous. That said, I am using a Behringer Digital crossover for the main L&R speakers so I could split up the drivers into two 2-way speakers and two subs. The subs would be fed the .1 channel from the surround pre/pro, acting more like a conventional surround system except it would effectively be a 7.2 since we would have two subs.

There's a 109" diag SeymourAV screen with electric actuation that covers some of the 18" drivers but not the midranges or tweeters. The center channel is going to be directly behind the screen.

There are lots of photos in my blog which is here... http://gdsamps.wordpress.com

I welcome any comments you might make. I'll be happy to go into detail on any aspect of the room that might interest you.

Thanks,


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## John Simpson (May 10, 2007)

Welcome Graydon!

I read that bit about your main speakers extending down to 10-15 Hz and thought you might be wishful-thinking, then I went to your blog. Hah! That's some speaker setup you have there... very nicely done.

In terms of audio, it looks like you have the equipment pegged. The sound treatment of the room would be the next big thing, and no doubt you'll want to do a lot of measurements and calibrations to get your curve as flat as possible.

Decor of the room will be interesting -- any thoughts there?

Good luck with the testing! Remember, measure twice, cut once!


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## Graydon (Jun 28, 2007)

Thanks, John.

I measured right at the mouth of the horn, with a decent quality dB meter. I used a signal generator going through my Denon Prepro. I actually started a sweep from about 100Hz at 70dB and gradually lowered the frequency while watching the dB meter. It was pretty smooth down to about 15Hz. Then it really dropped off as I went down to 10Hz. Then at 10Hz, just for grins, I started raising the volume to see what volume I couldget before the amps started to clip. I got up to 85dB and the big Crown K2 started flashing its lights. My pant legs were flappin' like crazy!

The room is prepped for 2" O-C 703 on both of the long walls and the ceiling. The rear wall is entirely a bass trap as are the walls on either side of the hallway that goes out beside and behind the AV rack. The O-C 703 will be set out from the concrete 1.5 inches due to the way the panels are being held on by the 2x4s. I plan to use hardwood on the floor with maybe some area rugs if floor reflections are a problem. I don't want the room to be too dead but I do want to control the sound so nothing jumps out at you.

I plan to use some sort of fabric over the O-C panels probably in black or dark blue. There will be 3" wide boards on top of the 1x2s between each panel which will hold hte panels in place. Those boards I plan to make out of brazilian cherry with a chamfer on each edge. I might try putting LEDs in the bottom of the lowest brazilian trim board to shine on the floor for ambience light and to see where you're walking. The floors will be matching brazilian cherry. So, the walls will have three long horizontal strips of cherry tieing it all together while the ceiling will have them going laterally. I'll finish around the speakers and the racks with wood trim probably painted black so as not to distract from the movie.

OK, got it, 'measure twice cut once.' Green side up. :doh: Thanks.


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## MatrixDweller (Jul 24, 2007)

Holy mackerel those are some huge speakers you've got there. 

I could see with them being in a sealed "concrete" cabinet that the baffle on the front would be flexing and vibrating a lot. There would probably be some air leaking in and out around where the baffle attaches to the wall unless you've caulked it tight. 

If they are air tight I could see their accuracy suffering unless the second woofer is passive.


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## Graydon (Jun 28, 2007)

MatrixDweller said:


> Holy mackerel those are some huge speakers you've got there.
> 
> I could see with them being in a sealed "concrete" cabinet that the baffle on the front would be flexing and vibrating a lot. There would probably be some air leaking in and out around where the baffle attaches to the wall unless you've caulked it tight.
> 
> If they are air tight I could see their accuracy suffering unless the second woofer is passive.


The baffle is made of two layers of 3/4 plywood laminated together. There is a 2x6 frame around the entire perimeter of the baffle which is attached to the concrete via Tapcon screws (lots and lots of them) and sealed with construction adhesive. Then, the baffle itself is sealed to the 2x6s with closed cell foam. The baffle is then screwed to the 2x6 frame with long screws every 6". There are no major air leaks in the baffle-cabinet interface.

There is also a steel pipe going from the middle of the baffle back to the back wall of the concrete to prevent the baffle from flexing. That helped alot.

Both woofers are driven. The cabinets are horn loaded via the horns outboard of each cabinet. I tuned the horns to 18Hz. Granted, they are short and not the most ideal proportion but I think they help.

The inside of the cabinet is almost completely stuffed with fibreglass.

When the depth charges hit in U-571, my shirt flaps in the breeze sitting in the middle of the room.


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