Film-Tech Forum ARCHIVE: Baffle wall question

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Posted by Steven Smith (Member # 8350) on 06-12-2015, 02:11 PM:

Hey guys, I'm not really sure what section this should go in as I don't see a construction type of section. Sorry if it's in the wrong one.

So I have a (L)21'6"x(W)16x(H)10'4" theater room(new construction). It's new construction. I have a foyer with a ramp that leads to the back riser and 2nd row, the riser steps down 16" to the front row. The rows will be at 12' and 17'.

I plan on building a baffle wall 18" off the back wall. I have 3 QSC 2150s for the front stage. I have a few questions about the baffle wall. I'm using 1.5" ductboard (OC 703) to line the front wall and face of baffle.

Should i put a slight angle on the baffle for the L/R speakers to help with imaging or just aim all speakers straight forward? Does the baffle have to be air tight around the speakers or can I just fill any small gaps with fiberglass board? The screen will be 165" 16:9 so all 3 will easily fit behind the acoustic screen.

Posted by Mark Gulbrandsen (Member # 72) on 06-12-2015, 03:06 PM:

Ductboard is NOT really the right thing to line a baffle wall with. Two sheets minimum of 5/8 gypsum covered with the correct Johns Manville lining that comes in sheets or rolls. Three sheets of 5/8" gypsum is even better. Johns Manville fuzzy glass stuff that gets all over you!
^Accept no substitutes... and the stuff can be difficult to find in some places.

Mark

Posted by Steven Smith (Member # 8350) on 06-12-2015, 03:26 PM:

Thanks for the info on the Gypsum. That answers that question.

The ductboard is OC703 compressed fiberglass. It has the same coefficient properties as linacoutic etc even down to 250hz. It's very reasonably priced and I can get it locally. http://www.bobgolds.com/AbsorptionCoefficients.htm

As for the wall, should I toe in the side baffles slightly? How sealed does it have to be around the speakers?

Also if I remember correctly Mark you've installed a few 2150s and liked them. Were they installed in baffle walls?

Posted by Mark Gulbrandsen (Member # 72) on 06-13-2015, 08:56 AM:

I'd have to see a whole room drawing to see if you should curve the wall or angle the sides. It's not always necessary or desirable.

None of my 2150's were in baffle walls but they were all in smallish theaters with less then a 50 foot throw.

Is the OC703 black or yellow? If it's yellow that may be a problem. If you can get it in black you should. The linacoustic is black.

Mark

Posted by Steven Smith (Member # 8350) on 06-13-2015, 09:27 PM:

Thanks for offering to give me some advice Mark. Yep the duct board is black on the facing.

As for the layout I don't think this forum will allow me to post it. If you don't mind clicking on a link, I linked my build thread with the layout and pics of the room all in the first post. Thanks.

[ 07-13-2015, 04:35 PM: Message edited by: Brad Miller ]

Posted by Mark Gulbrandsen (Member # 72) on 06-14-2015, 02:53 PM:

I don't think your room is wide enough to worry about angling the sides ofvthe baffle. Aim through the HF driver port with a laser to a point about three feet above the rear center seat when aimimg the speakers. Stereo imging should be very good

Mark

Posted by Steven Smith (Member # 8350) on 06-14-2015, 05:06 PM:

So your saying build a flat baffle wall and install the speakers with no toe in and aim 3' above the back row... Correct? You said center seat but that would require toe in and the speakers have to be flush with the wall.. Correct? I may not be following you, I was under the impression that with a baffle wall the speakers had to flush to the wall and sealed to the wall with no gaps around the edges.

Also... Do you think the benefits of a baffle wall are big enough to make it a good idea? If the benefits are small heck I'll just put them behind the screen without building a full out baffle wall.

Posted by Mike Blakesley (Member # 26) on 06-14-2015, 11:06 PM:

It makes a difference. We went to a baffle wall when we changed out our speakers (went from 2-way Peavey to 3-way QSC). I guess I don't know how much of the difference is attributable to the wall and how much to the speakers, but it's a lot better since we added the wall. Better dialog intelligibility, for one thing, and a much tighter sound.

I guess I don't know how much difference a baffle wall would make in a room that small though.

Posted by Brad Miller (Member # 2) on 06-14-2015, 11:31 PM:

Anyone serious about quality sound will put in a baffle wall. It ALWAYS helps. I wouldn't have put one in my screening room if it didn't make much of a difference.

Posted by Steven Smith (Member # 8350) on 06-15-2015, 11:25 AM:

I'm thinking of using mdf for the wall facing covered in the duct board. So how big should the gap be between wall and speaker? 1/4"? Do I need to fill the gap? Speaker to the edge of mdf or acoustic materials edge?

Posted by Steven Smith (Member # 8350) on 06-15-2015, 06:17 PM:

Thanks guys for the nudge to stay with the baffle by the way, I'm sure I'd regret it if I didn't do it. Any info on my questions related to gaps and how sealed it needs to be around the speakers or any tips would be greatly appreciated.

Posted by Mark Gulbrandsen (Member # 72) on 06-15-2015, 10:55 PM:

MDF is a really bad idea. Its not dead enough and fireproof mdf is gonna cost a fortune. All in all its the wrong stuff. 5/8" drywall only.The wall itself needs to be very dead and non resonant.

Posted by Louis Bornwasser (Member # 3063) on 06-16-2015, 08:58 AM:

The best baffle wall is marble faced with duct liner (fuzzy side out). The closest I have come is a guy in Michigan who had about 50,000 record albums on industrial shelving on both the back wall and also just behind the screen. True "bookshelf" speakers, just industrial sized speakers AND bookshelf. What you are looking for is MASS.

According to JBL designers, Mark Gander and John Eargle: "NO speaker ever made will meet its spec without a sound wall." JBL has a lift that puts their speakers in a roof hatch in plane with the finished roof. A microphone is mounted on a small tower above it.

Posted by Steven Smith (Member # 8350) on 06-16-2015, 10:40 AM:

OK, materials are being delivered today. I changed to order to 5/8 sheetrock. Double layers.

My biggest remaining question is how to handle the gaps between speaker and baffle.

Posted by Mark Gulbrandsen (Member # 72) on 06-16-2015, 07:04 PM:

Put the studs up and frame out the opening where the speakers go. A small platform on the rear is usually good enough for each speaker.Can't really tell you where to put the sub at because I donno if you are building on top of a stage or on a flat floor. If on a flat floor about 12" up from the floor is good for the bottom of the sub.

Then put one layer of drywall vertical and the next layer horizontal. Three is THX spec usually. I have done walls with two layers, especially where the weight of the wall wold become worrisome. It is recommended to tape each layer... Depending on how good a taper you are so have fun with that! Then cover the entire wall with duct liner and cut the openings for the speakers but leave several inches of duct liner around the hole. When you slide the speaker cabinet in the duct liner will push it's way back around the speaker and seal the hole up around three sides of the cabinet. Fill in behind the mid-high frequency unit with another sheet of the stuff as needed to make it nice and neat and closed off.

The following pictures should give you some idea... This one took about 5 days to build and get the screen back up. This wall got two layers of 5/8" drywall because we ere concerned about all the weight on the stage. I believe the theater owner did have the county architect inspect under the stage before this went in so we were ok on weight.

Mark

Posted by Steven Smith (Member # 8350) on 06-16-2015, 09:43 PM:

You're the man Mark.... Thanks! The stage is going to be 12" tall and the subs be one in each of the four corners. Two behind the screen and two in alcoves in the back corner. I have them labeled on my plans in my link above .

Being the subs are in the corners and 6'tall. Can I just frame around them and make their baffle even with the baffle wall? If so I'm guessing I should run the acoustic material across the sub baffles as well.

Posted by Mark Gulbrandsen (Member # 72) on 06-17-2015, 01:11 PM:

Humm, not sure about doing that with the subs. While it may be perfectly ok it is not where THX would locate them. In the systems I have done they are always front and center about 12" above floor. Placing the subs next to each other nets you ~1.5 db more output per pair of subs. So you may want to reconsider that as grouping the four subs together may improve efficiency by as much as 3db.

Mark

Posted by Steven Smith (Member # 8350) on 06-17-2015, 01:32 PM:

I did a ton of research on multiple subs as I'm building alcoves I can't really move them after I install them, so I'm kinda trusting the studies done by others. I linked a study done by Harmon for multiples complete with responses and comparisons.

It may not be the best spot but for rooms as small as mine it looks to be a good spot with a smooth response and room gain. If you look two subs at wall midpoints also gives a smooth response which would be similar to what you described.

The corner option can also give as much as 6db gain per sub..of course that's optimal. But no matter what I'll have plenty of headroom well above reference.

I'll also be using an EQ on them and getting the phase tweaked and aligned with the mains.

http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White%20Papers/multsubs.pdf

Posted by Mark Gulbrandsen (Member # 72) on 06-17-2015, 10:08 PM:

One thing that concerns me placing them in the corners is standing waves.

Mark

Posted by Steven Smith (Member # 8350) on 07-13-2015, 03:50 PM:

Hey guys. So I have my Baffle wall built and treated with 1". My screen will be built with 1x4s and covered with acoustic screen and will hange against the wall using french cleats. Can I mount the screen directly to the baffle wall? I have two vertical braces between the main and L/R speakers. With these 1x4s cause any issues if they are mounted against the baffle wall acoustic material?

Here's pics in case anyone wants to see what I'm working with.

[ 07-13-2015, 04:32 PM: Message edited by: Brad Miller ]

Posted by Brad Miller (Member # 2) on 07-13-2015, 04:33 PM:

Steven, your link was removed because it breaks the archive. Please upload any pictures relevant to the discussion directly into this thread. When you go to make a new post, there is a text link "click here to upload an image" that you may use. (An example is how Mark G posted his pictures on page 1. This way they are there forever for future people reading this thread.)

Please also upload your drawing from your page 1 link. Thanks.

Posted by Mark Gulbrandsen (Member # 72) on 07-13-2015, 05:24 PM:

Typically the screen frame is not mounted to the baffle wall in any way, although a brace here and there may penetrate the wall to connect it to the back wall of the building. Angle iron and springs are the best frame material and best way of hanging a screen. This way the frame is more minimalist and it won't bend or sag over time. The springs allow the screen to breathe with temperature changes maintaining a more constant tension...

Mark

Posted by Steven Smith (Member # 8350) on 07-13-2015, 08:27 PM:

OK, thanks I will try to upload a pic when I get to a pc.

As for the frame I've built half a dozen and I have a method that results in a very solid frame and the way I mount it sagging is almost impossible.

My question is acoustic in nature. Will it hurt to have vertical pieces of 1x4 centered between the center and mains and to the outsides of the mains against the baffle wall or should they float in front of the baffle and it touch it.

Posted by Marco Giustini (Member # 4544) on 07-14-2015, 03:25 PM:

Hello Steven,

Thanks for the JBL doc, very interesting. I found that the hard way in my small home theatre; it's nice to have more literature on the matter.

Posted by Steven Smith (Member # 8350) on 07-15-2015, 06:37 AM:

Thanks, I'm glad I could contribute something useful! Here's some pics of the room. I just finished covering my panels. My phone is horrible in low light.

And here's the baffle wall. Double layers of sheetrock covered with 1" ductboard.

So mounting the screen..Any issues with securing the frame to the baffle wall? Should I space it off the wall a few inches? If so how much? I've put a lot of work into the room acoustically and don't want to make any big errors.

Posted by Marco Giustini (Member # 4544) on 07-15-2015, 12:48 PM:

Wow. And... Wow! [Smile]

The screen should be pretty close to the screen but you want to leave a little space or you will see the LF 'soundwaves' from the stage speakers. I'd say 3 inches? You have a sound wall so it shouldn't really matter too much, HF are going to be absorbed by the rockwool.

Posted by Steven Smith (Member # 8350) on 07-15-2015, 01:19 PM:

So I guess I get your seal of approval?? [Big Grin]

3" isn't too bad. I'll just block it off the baffle wall and kind float it then. I may try 2" and measure. I want to keep as much of that edge on the top of the stage visible.

I'm also going to throw some 1" ductboard over the sub baffles to continue the baffle wall to the side walls.

Yes I did that professional mess of a plan myself.

Posted by Gerald Dente (Member # 8927) on 07-16-2015, 07:31 AM:

Room is looking great. 2" kinda close to the screen? I was told 3-4" minimum if I was to have subs behind it. Do your subs left and right go behind the screen as well or is it just the QSC's?

Posted by Steven Smith (Member # 8350) on 07-16-2015, 09:10 AM:

The good thing is I can measure with and without the screen and adjust the distance even after the build is done. Quick and easy. Without measurements it's all speculation anyway as any issue that could exist may not be audible or measurable.

The baffle wall is 12' wide the screen is just a tad wider.. (170" diagonal 16:9). The screen will extend over past the edge of the subs but not so far that it's to the cones. The father I pull the screen from the wall the more the side bracing of the screen will get into the path of the mains.

I'll hang the projector and screen this weekend and I'll post some pics.

Posted by Gerald Dente (Member # 8927) on 07-16-2015, 11:14 AM:

Good luck, I always hated mounting my projector. Although I kinda did it backwards and put the screen up first then the projector second.

Posted by Mark Gulbrandsen (Member # 72) on 07-16-2015, 04:19 PM:

Best to aim the HF units before you put up the screen. Then you can see how far out the screen actually needs to be. You want the HF horns almost touching the back of the screen if that's possible to cut down on reflections. If that's the proper rock wool it has very specific absorbition characteristics and it won't completely absorb multiple reflections. The room has to be treated too.

Mark

Posted by Steven Smith (Member # 8350) on 07-17-2015, 10:08 AM:

Thanks Mark, that's the kind of priceless info I need. I'll put them in place this weekend. As you can see below the room is well treated but not overly so (It isn't letting me post pics for some reason). All first reflections are taken care of. As for the ductboard it's basically identical in absorption properties as acousticliner it's just a rigid version (OC 703), it'll put a spanking on frequencies down to 200 hz or so.

The speakers sit up pretty high so it shouldn't require too much angle to get them the couple of feet over my head (on back row) you recommended earlier.

Posted by Jarod Reddig (Member # 6363) on 07-27-2015, 02:35 PM:

With my big JBL horns it took a considerable amount of toe-in to get the best imaging across all my seats. Make sure to do lots of trial and error with that before finalizing.