THX Style Baffle Walls for AT Screens - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 181 Old 02-07-11, 04:54 AM Thread Starter
Mark Techer
 
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THX Style Baffle Walls for AT Screens

I did a quick search and didn't find anything, so I apologize in advance if there is a thread or discussion on this topic already.

So does anyone have or are thinking of making a baffle wall for their AT screen? I have always wanted an AT screen, possibly dating back to when a projector didn't even seem a viable option. I enjoy the cinema and I actually really like the way sound cue actually match the on screen visuals in my system. I made my own wall back in 2007 and would not go back to anything less. If I was to alter anything, I would only make it larger.

So basically, the baffle wall directs all of the sound into the listening space. It is treated to prevent HF comb filtering as whilst the AT screens are "acoustically transparent", HFs can and do reflect off the back of the screen and bounce off the baffle wall itself. Adding sound absorbing panels to this wall will minimize the unwanted effect of comb filters.

My wall is 'faceted' meaning that the wall is essentially a 'reflection dish'. I've used a medium density convoluted foam over 100% of the surface. The reason for the shape was to allow the L and R speakers to be toed in slightly. It works unbelievably well with precise sound imaging. You will notice the L and R speakers are quite close and this was done to place them just inside the 1.78:1 area. The baffle walls of real dubbing stages and cinemas also have the L and R speakers just inside the 1.85:1 masking. Because if tbis fact, I essentially hear what the sound engineers heard during the mix and everyone that has visited my room have given nothing but positive feedback about the sound stage and how well it matches the visuals.

The first cinema baffle wall I got to see was back the early 1990's when a cinema in Townsville was retrofitting one of their cinemas. The owners claimed it was to meet the requirements for THX certification (though the cinema would not actually apply for THX certifiaction due to cost). All I got to see then was the huge cutouts (AKA speaker cubby holes that Procella said not to build BTW) in the front wall. The treatments had not yet been installed, so I had to sneak a peak from another cinema to see what they used.

In 2002, during a visit to Sound-firm (THX certified dubbing stage in Sydney's Fox Studios) I was shown behind the baffle wall and it made me want to add one to my own home cinema now discovering how simple the baffle was actually was.

Last year during my visit to CEDIA in the US, I did the "THX Baffle Wall" course that was being run by Procella Audio at the CEDIA expo. It was rewarding to discover that what I had done was pretty much correct. The difference is that my own "wall" is only as big as the screen and is mounted inside a rig that can be moved around. The screen walls Procella described are true walls from floor to ceiling.

I am currently also helping a forum member build his own full height wall. I assisted him in placing the pre-cut MDF pannels in place today. He'll be using 3 M&K LCRs.
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post #2 of 181 Old 02-07-11, 04:01 PM
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Re: THX Style Baffle Walls for AT Screens

I don't have an AT screen, but I'm planning one in the future - and I've read quite a bit about baffle walls.

It's been talked about a little bit on AVSForum:
http://www.avsforum.com/avs-vb/showthread.php?t=1280395
http://www.avsforum.com/avs-vb/showthread.php?t=1291697

And I know I've seen a couple other threads out there on baffle wall construction.

The two concerns that I've read are:
1. The baffle wall has more of an impact on some speakers than others (I believe that polar plots are very helpful in determining just how much will change). Some speakers are designed for baffle walls, ie Seaton Catalysts (and some M&Ks if I remember right??)
2. It takes some engineering to determine how MUCH absorption you need on the front wall.

Some of the responses that I've read are:
1. Baffle walls should be engineered - so you have the correct absorption (ie, pay someone to design a wall)
2. Build a solid (MDF, etc) wall - and treat with 1"
3. Baffle walls aren't necessary for home use - and do more harm than good.

I'm definitely interested in how it works out for you.
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post #3 of 181 Old 02-07-11, 04:05 PM
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Re: THX Style Baffle Walls for AT Screens

Quote:
Mark Techer wrote: View Post

Last year during my visit to CEDIA in the US, I did the "THX Baffle Wall" course that was being run by Procella Audio at the CEDIA expo. It was rewarding to discover that what I had done was pretty much correct. The difference is that my own "wall" is only as big as the screen and is mounted inside a rig that can be moved around. The screen walls Procella described are true walls from floor to ceiling.
Do you have any secrets that you can share?

My last post was pretty much a brain dump - but it sounds like you know what you're doing when it comes to baffle walls!
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post #4 of 181 Old 02-07-11, 05:40 PM Thread Starter
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Re: THX Style Baffle Walls for AT Screens

Quote:
Zeitgeist wrote: View Post
I don't have an AT screen, but I'm planning one in the future - and I've read quite a bit about baffle walls.

It's been talked about a little bit on AVSForum:
http://www.avsforum.com/avs-vb/showthread.php?t=1280395
http://www.avsforum.com/avs-vb/showthread.php?t=1291697

And I know I've seen a couple other threads out there on baffle wall construction.
Like everything in HT, there will be those for and against the concept. It should be considered a specialist thing because the wall itself needs to be constructed properly. The book that Procella handed out with their course notes talked more about their speakers than the actual wall. The idea is about harnessing 100% of the energy from the speaker and directing that into the room.

Quote:
The two concerns that I've read are:
1. The baffle wall has more of an impact on some speakers than others (I believe that polar plots are very helpful in determining just how much will change). Some speakers are designed for baffle walls, ie Seaton Catalysts (and some M&Ks if I remember right??)
2. It takes some engineering to determine how MUCH absorption you need on the front wall.
The answer to both is a yes.

1. The baffle wall extends the baffle making it infinite. Some speakers simply won't work in a baffle wall, IE bi-poles that require the rear lobe to be reflected off the wall behind the speaker. In a good baffle wall design, that sound is lost and only the sound from the front of the speaker is used.

2. Tom Holman discovered the need to treat the baffle wall when designing the first THX dubbing stage by simply (so I read) placing his head between the screen and the wall (cinema screens have some distance there) and he could hear excessive HFs that were not audible on the other side of the AT screen due to phase issues of the reflected sound. The solution was to absorb the reflections. Cinema AT screens have much larger holes than HT AT screens, so this is probably not as bad in the home anyway due to the smaller hole size.

Quote:
Some of the responses that I've read are:
1. Baffle walls should be engineered - so you have the correct absorption (ie, pay someone to design a wall)
2. Build a solid (MDF, etc) wall - and treat with 1"
3. Baffle walls aren't necessary for home use - and do more harm than good.

I'm definitely interested in how it works out for you.
1. The general rule seems to be 1" of foam treatments. The exact density however could be scientifically calculated. From what I understand, the baffle wall is about directing sound into the main room and addressing the already mentioned HFs, not controlling bass response. You still need bass traps in a small room.

2. I've used 16mm MDF. It was suggested that 18mm be the minimum. I think in my case, the three panels that make the wall are just under 1000mm high by 800mm wide, so the panels themselves are thick enough based on the surface area.

3. So much has to so with the type of speaker being used and the fact that the space behind the baffle wall is lost. You have to design a room with the extra depth if doing this.

My room is small and I've allowed 600mm for the screen and baffle wall. I can actually get in behind mine to service the speakers, so I could use a speaker than fires sound both forward and backwards as the sound can radiate over and under the screen in my case. The full wall behind my baffle wall is also treated as well. This was done during construction of the room. What I wanted was to have the first 1/3rd of the room to be dead and I demo this by clapping my hands loudly right in front of the screen. The only sound you hear is the clap, no slap echoes at that part of the room and as a result, I hear only the sounds in the mix. The other 2/3rds of the room is a different story where I wanted half live and half dead to give the surrounds some spaciousness. I think I have achieved quite a good result considering the low budget I had for this room at the time.

What I did find was the imaging improved. When I was running the three LCRs under my screen (tilted up), I used to pack allot of foam between the speakers and that seemed to improve things. Once they were installed in the baffle, they sounded very different in that they were cleaner and more directional. For me, that was 2 bonus points and why I didn't scrap the project in 2007. Material costs were not high, so it was not a case of keeping it because it cost too much to throw away. It didn't. I've kept is because it worked.
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post #5 of 181 Old 02-07-11, 07:32 PM
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Re: THX Style Baffle Walls for AT Screens

Nice to see such an articulate informative post about baffle walls.

As you can tell from my post... I was merely regurgitating information that I've read on other baffle wall threads... which contained more discussion than actual information.

Quote:
Mark Techer wrote: View Post
1. The baffle wall extends the baffle making it infinite. Some speakers simply won't work in a baffle wall, IE bi-poles that require the rear lobe to be reflected off the wall behind the speaker. In a good baffle wall design, that sound is lost and only the sound from the front of the speaker is used.
Bipoles not working with a baffle wall seems like common sense... Is there some other speaker design that doesn't as well? I've seen it mentioned plenty of places - where it's stated that a baffle wall will do more harm than good... but then nothing else is really mentioned. Maybe it's my limited knowledge of speaker design, but I don't quite get how making the baffle infinite can really *drastically* change a a speakers performance. Change it enough to need EQ - but from what I understand, speakers behind an AT screen need some EQ anyway.


Quote:
Mark Techer wrote: View Post
2. Tom Holman discovered the need to treat the baffle wall when designing the first THX dubbing stage by simply (so I read) placing his head between the screen and the wall (cinema screens have some distance there) and he could hear excessive HFs that were not audible on the other side of the AT screen due to phase issues of the reflected sound. The solution was to absorb the reflections. Cinema AT screens have much larger holes than HT AT screens, so this is probably not as bad in the home anyway due to the smaller hole size.
Never new the history behind the need for baffle walls. That's quite interesting. I've read plenty of mentions about higher SPL due to the baffle - and the importance of that for filling a big theater - but first time I've read about excessive HF being the original reason.

Quote:
Mark Techer wrote: View Post
1. The general rule seems to be 1" of foam treatments. The exact density however could be scientifically calculated. From what I understand, the baffle wall is about directing sound into the main room and addressing the already mentioned HFs, not controlling bass response. You still need bass traps in a small room.
1" seems to be what I've read in most installs... and the couple THX baffle walls that I've seen first hand. In a commercial theater there is a couple feet (well some anyway) between the baffle wall and the screen. Any comments on what you do/have done with regard to distance between screen and baffle? In a commercial theater - it comes out to be a substantial volume between the screen and baffle.

Quote:
Mark Techer wrote: View Post
What I did find was the imaging improved. When I was running the three LCRs under my screen (tilted up), I used to pack allot of foam between the speakers and that seemed to improve things. Once they were installed in the baffle, they sounded very different in that they were cleaner and more directional. For me, that was 2 bonus points and why I didn't scrap the project in 2007. Material costs were not high, so it was not a case of keeping it because it cost too much to throw away. It didn't. I've kept is because it worked.
Glad to hear about your results and that you're happy with it.
I feel like I've been rambling - I'm just excited to see a post about baffle walls - from someone who actually built their own.

FINALLY figured out where I recognize your name from...CAVX... duhhH!

Last edited by Zeitgeist; 02-07-11 at 08:34 PM.
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post #6 of 181 Old 02-07-11, 11:26 PM Thread Starter
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Re: THX Style Baffle Walls for AT Screens

Quote:
Zeitgeist wrote: View Post
Nice to see such an articulate informative post about baffle walls.

As you can tell from my post... I was merely regurgitating information that I've read on other baffle wall threads... which contained more discussion than actual information.
I was surprised and a little annoyed (when building mine) that there is not more information readily available. Maybe more people would build them if they knew how. I am lucky I got to see the actual THX baffle wall in a THX certified facility.


Quote:
Bipoles not working with a baffle wall seems like common sense... Is there some other speaker design that doesn't as well? I've seen it mentioned plenty of places - where it's stated that a baffle wall will do more harm than good... but then nothing else is really mentioned.
My speakers are custom made and therefore were designed with the baffle wall in mind well in advance of actually building the wall or the current room. I would suggest that any direct radiating sealed speaker will be OK. If there is a port or vent, then it needs to fire out the front. If it fires out the back (behind the wall), it will not be heard in the room and the speakers may even sound 'thin' as a result.

Quote:
Maybe it's my limited knowledge of speaker design, but I don't quite get how making the baffle infinite can really *drastically* change a a speakers performance. Change it enough to need EQ - but from what I understand, speakers behind an AT screen need some EQ anyway.
Think of it as having the wrong sized box for a particular driver. In my case, the baffle wall has done more good and why I continue to use it. The biggest challenge for me was that I'm the sort of guy that would take grills off speakers and here I was about to place a permanent grill in front of the LCRs. I ended up using what is commercially known as SmX fabric (sourced from a different supplier) and I was amazed with the fact that I could not hear a difference when I first tested out the fabric in front of my speakers. In the end, I hired a RTA to prove to myself my ears were not lying.

Quote:
Never new the history behind the need for baffle walls. That's quite interesting. I've read plenty of mentions about higher SPL due to the baffle - and the importance of that for filling a big theater - but first time I've read about excessive HF being the original reason.
So according to Procella (and other sources), you do get a gain from the baffle wall compared to using the same speakers in the same size room without one and that was certainly the goal of engineers building the first THX dubbing stage. As it turns out, there was other issues that came as a result. There was a (You-Tube?) video with Tom Holman actually telling the story about his discovery and the need to threat the wall.
Quote:
1" seems to be what I've read in most installs... and the couple THX baffle walls that I've seen first hand. In a commercial theater there is a couple feet (well some anyway) between the baffle wall and the screen. Any comments on what you do/have done with regard to distance between screen and baffle? In a commercial theater - it comes out to be a substantial volume between the screen and baffle.
Because 1" seems to be the standard, I am guessing they are not too fussed about below 800~600Hz. I've actually now made 4 (three for clients) walls now and 1" was all that I ever used.

Spacing was one thing that came up for me during my build too. At the time, it was said that you needed at least 6" (150mm) between the speakers and the screen fabric. "SmX beta testers" were reporting being able to have less than 1" (25mm) spacing and no adverse effects. I originally designed my wall to have the screen 6" away, however as I curved the screen, I lost much of that space and probably have about 2" max if I am lucky. I think the spacing issue is fabric dependent, much like the need to eq.

Quote:
Glad to hear about your results and that you're happy with it.
I feel like I've been rambling - I'm just excited to see a post about baffle walls - from someone who actually built their own.

FINALLY figured out where I recognize your name from...CAVX... duhhH!
Yes I am CAVX I've had my baffle wall in place now since at least 2007 so a few years now. I do like the end results and as I said in a previous post, if I was to change anything, it would be the size of the wall/screen/room. My current room is basically a 1/10th down scale of a commercial cinema. It has been designed to give me a 36 degree viewing angle from the back of the room to the edges of the Scope screen. The LCR speaker locations are horizontally placed to match (as close as practical) that of a dubbing stage.
I would not change any of that if I was to do it all again.

Like a commercial cinema, even my LCRs are now active and I have to say, there is nothing that quite delivers the dynamics of active speakers. They actually deliver a punch and beg to be played at 00dB reference. Of course, whilst I like it loud, I do live in suburbia, so have to be mindful of those around me, especially at night. I would say that the baffle wall has helped here by directing the sound into the room. Standing outside of the room a few weeks back (playing Avatar at 00dB) and there was not much over a dull rumble.
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post #7 of 181 Old 02-08-11, 12:20 AM
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Re: THX Style Baffle Walls for AT Screens

I have a hard time visualizing this baffle wall. Is this basically a 'false' wall with recesses for the speakers, that is then treated?

Edit:
I found this image, which is what I'd think it looks like. How is that done behind the wall? The cavity behind the baffle wall and the room wall would be resonating and creating a mess in the LF, I'd think?


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post #8 of 181 Old 02-08-11, 12:41 AM Thread Starter
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Re: THX Style Baffle Walls for AT Screens

Quote:
atledreier wrote: View Post
I have a hard time visualizing this baffle wall. Is this basically a 'false' wall with recesses for the speakers, that is then treated?
Pretty much. The construction for this wall on a commercial scale would have to be the same as any other acoustically treated wall so that it doesn't vibrate, rattle etc.

Quote:
Edit:
I found this image, which is what I'd think it looks like. How is that done behind the wall? The cavity behind the baffle wall and the room wall would be resonating and creating a mess in the LF, I'd think?
The image in you link is of a system that features SDDS (5 screen channels) 8CH system. Your question about what is behind the wall is interesting as even the THX baffle wall I got to see behind didn't appear to have anything special in there. It was not like it was stuffed with fiber glass or anything like that. I'm sure you could do that in the smaller environment of the home theatre. Behind my wall is treated, but only because I did the treatment in the room before installing my screen.

I can't seem to attach images here although I have been able to in the anamorphic projection forum
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post #9 of 181 Old 02-08-11, 07:35 AM
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Re: THX Style Baffle Walls for AT Screens

Quote:
atledreier wrote: View Post
I have a hard time visualizing this baffle wall. Is this basically a 'false' wall with recesses for the speakers, that is then treated?

Edit:
I found this image, which is what I'd think it looks like. How is that done behind the wall? The cavity behind the baffle wall and the room wall would be resonating and creating a mess in the LF, I'd think?
If you mean solid MDF/drywall - when you say "false" - then yes.

I'm sure Mark will give a better answer than I can - but I believe what I've read regarding THX's recommendation - it's that the cavity should be totally dead and stuffed with with acoustic fiberglass/foam -- so that way you kill any LF. I saw a graphic once that illustrated that...perhaps by JBL. I'll see if I can dig it up.

Edit: Now that I've said that........ I'm having a hard time finding examples to support that..

In case you are interested, more baffle walls:
http://forum.ecoustics.com/bbs/messages/3/308259.html


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Last edited by Zeitgeist; 02-08-11 at 07:49 AM.
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post #10 of 181 Old 02-08-11, 08:05 AM
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Re: THX Style Baffle Walls for AT Screens

Quote:
Mark Techer wrote: View Post
The image in you link is of a system that features SDDS (5 screen channels) 8CH system. Your question about what is behind the wall is interesting as even the THX baffle wall I got to see behind didn't appear to have anything special in there. It was not like it was stuffed with fiber glass or anything like that. I'm sure you could do that in the smaller environment of the home theatre. Behind my wall is treated, but only because I did the treatment in the room before installing my screen.
When I got a chance to see behind a couple THX baffle walls - There was 1" fiberglass on the real wall behind the baffle wall. But other than that --- nothing..There were perhaps 4-5 feet between the baffle and real wall.

Maybe I'm wrong about what I read about making the area behind the baffle wall totally dead by filling with foam/fiber.


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