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THX baffle walls – design, build and benefits

What is a THX baffle wall and why do I want one in my home theater?! This article explains what a baffle wall is, covers the acoustic and audio benefits and finally provides tips on how to design and build them.

A baffle wall is essentially a false wall into which the screen speakers (i.e. left, center and right) are mounted. It is a core component of a THX certified cinema.

“Unique to every THX Certified Cinema is the THX “baffle wall.” If you were to peel away the screen at any THX Certified Cinema, you will see a massive wall of speakers housed in an acoustic baffle. The baffle wall is approximately the same size as the screen, providing a solid, smooth and uninterrupted surface to distribute sound throughout the auditorium. It produces a large sound image and accurately tracks sound elements with the onscreen action. This makes panning shots and off-screen sounds more believable and natural, helping to pull audiences into the storyline. Without a baffle wall, sound is uncontrolled – producing a weak, uneven image.”

Acoustical and sound quality benefits of baffle walls

Baffle walls have a couple of major acoustical benefits which translate into sound quality benefits:

  • No speaker boundary interference from the front wall behind the speakers, since the speakers are mounted flush with the surface.
  • Increased low frequency output. The baffle wall essentially removes the ‘baffle step‘ which happens when the speaker radiation transitions from half space to full space as the frequencies exceed those which can be controlled by the baffle. If your speakers are designed for flush mounting in a baffle wall, as Procella speakers are, then you gain 6dB of headroom in terms of the ability of the speaker (or sub) to reproduce reference levels.
  • Reduced diffraction. There is little to no diffraction as the front baffle of the speaker is flush with the wall.

From a sound quality perspective these things mean that the sound tracks cleanly from left to right with no jumps, we have more headroom at low frequencies and we have better bass free from boundary interference suckouts.

Note that very few speakers are designed to be baffle wall mounted – most are designed to be used in free space and hence incorporate baffle step compensation circuits or are otherwise designed to counteract baffle step losses. When placed into a baffle wall the frequency at which the baffle step occurs is moved significantly downwards, to 80Hz or lower, which results in a bass boost. It is possible to equalize out this boost using a low shelf filter.

 

Visual and aesthetic benefits of baffle walls

baffle wall allows you to hide the screen speakers and front wall located subwoofers so that they are no longer visible. This provides for a very neat and visually attractive installation.

If shallow format speakers and subwoofers are selected baffle walls can be made very shallow. The baffle wall in our demo room uses Procella Audio speakers and subwoofers which allow the baffle wall to be only 8″ deep.

 

How we design and build baffle walls

  • Speaker’s acoustical center optimally placed. Typically this means 1/2 to 5/8ths of screen height but will vary depending on the vertical off axis response of the speakers in question as we want the relationship between speakers and audience to be within the angular range where the speakers sound good.
  • Multi-layer construction using constrained layer damping techniques. A proper baffle wall is at least 1.5″ thick and comprised of multiple layers of material, primarily to stop the baffle wall from resonating and becoming a giant speaker. This might happen if thin material were used as the speakers sit in the wall and hence transfer energy to it.
  • Absorbent covering. The front of the baffle wall behind the screen should be covered with a 1″ layer of light and sound to absorb light transmitted through the screen and sound reflected back from the screen to the wall.
  • Speakers and subs decoupled from the wall using appropriate isolation pads. These are used to prevent transfer of energy into the wall from the speakers.
  • Void between the baffle wall and structural wall filled with absorbent material to prevent resonances developing in the space. In wide or tall rooms cutouts in the baffle wall can be made to allow the area behind the wall to function as an effective bass trap.

We have written an in depth case study of the design and build of our Demo Room baffle wall which explains the steps in the process and how it fits in the overall theater design process.

38 thoughts on “THX baffle walls – design, build and benefits”

  1. Hello.

    Thank you for the tipps.

    Can you tell me what material are the three layer damping construction?`

    best regards

    Michael

    1. Hi Mike, thanks for your comment!

      The wall does not have an STC rating because it is not designed for sound isolation. The wall is built within the outer wall of your room. It’s the outer wall that does the sound isolation. The baffle wall has holes in it for speakers and subwoofers so doesn’t offer any sound isolation.

      Hope that makes sense!

  2. I understand that you need absorptive materials on the front of the baffle wall to absorb light that leaks through the acoustically transparent screen. I also understand that this material needs to be sound absorbent if you use a perforated vinyl screen, but do you still need the baffle wall to be sound absorbent if you use a woven textile screen? A woven screen should have a lot less audio reflections from the back of the screen compared to a perforated vinyl screen.

    Also, if you clad the entire front wall (the baffle wall) with a thin layer of sound absorptive material, won’t this cause other acoustical problems? You will get very non-linear dampening of the sound. You will have a lot of dampening in the high frequencies and very little in the mids and lows. Won’t this be a problem?

    I also have the impression that the old hi-fi concept of dead-end / live-end is not the best acoustical solutions anymore. It is better to place the absorption on strategic places elsewhere in the room. Based on this, why would you still want to use a baffle wall in a home theater?

    Please comment.

    1. Hi Lars thanks for reading and leaving a thoughtful comment! The material should be sound absorbent regardless of whether the screen is woven or perforated, since even woven screens reflect sound back towards the baffle wall.

      You are right on point that cladding the whole front of the wall with a thin absorber is generally bad, since you only absorb the highest frequencies. A better approach is to use thick absorbers around the speaker and leave the upper and lower parts of the wall outside the screen reflective, or add diffusors. If you think about it from the perspective of the surround speakers the front wall is an major reflection point. However this needs to be balanced with practicality, and dealing with thick absorbers and diffusors on a screen wall can be challenging.

      The point of a baffle wall is to reduce diffraction, increase the low frequency output of the speaker and prevent the sound energy from wrapping around the speaker baffle and bouncing off the front wall [aka speaker boundary interference or SBIR]. All the top studios still have baffle mounted speakers (they call them soffit mounted speakers), and you will see that they often clad the wall in stone for diffusion.

  3. Hello great info here. Obviously the LCR should be mounted directly behind my micro perf screen but my question is can the front 2 subs be built into the baffle wall below the screen. Or do they need to be built in so they they travel the the micro perfs as well? I hope that makes sense. Also how high should you mount the main drivers on the LCR’s? ear/ listening level ? thanks in advance. jj

    1. Hi JJ

      Thanks for reading!

      Rule of thumb for LCRs is acoustical center (typically between midrange and high frequency driver) to be at 5/8ths of screen height. This comes from commercial cinema best practices.

      A baffle wall is a great place to hide subwoofers. As long as what covers the subs is acoustically transparent then they do not need to be behind the screen. In fact putting subs directly behind a screen can sometimes cause problems as subs can move a lot of air and this can actually cause the screen surface to vibrate!

      Nyal

  4. Lars says: "I also have the impression that the old hi-fi concept of dead-end / live-end is not the best acoustical solutions anymore."

    • Still likely my favorite solution I’ve heard!
    1. The live end / dead end (LEDE) approach was designed for two channel monitoring in professional recording studios. LEDE was never conceptualized to provide an ideal acoustic environment for multichannel reproduction. For surround speakers you don’t want the front of the room to be completely dead and the back end completely live, you need in my opinion a more balanced approach in order to achieve great surround sound envelopment and effects localization.

  5. What are your thoughts on the tradeoffs between the benefits of baffle walls and the benefits of broadband or low bass absorption on the front wall, especially for small room applications (1000-1800 sqft)? For the same 8" depth of a baffle wall, you could also treat the entire front wall with 8" of treatment, providing huge amounts of absorption to quite low frequencies. You would also eliminate SBIR (right?). You are then only left with the benefits of effeciency and lack of diffraction when considering a baffle wall. Add in the fact that small rooms need significantely more bass treatment(?), and also lack the wall real estate of a larger room for bass absorption/trapping, and I would lean towards the opinion that baffle walls only make sense in rather large listening venues/rooms, especially considering effeciency is much more important for said venues. What are your thoughts?

    1. Hi Brian,

      Thanks for your message. A very good point!

      1000-1800 sq.ft would be a very large home theater, do you mean cubic ft?

      Small rooms definitely need all the help they can get with bass absorption, so your position is a valid one. Most speakers need more than 8" of depth, so in reality the face of the speakers is going to be further into the room and the SBIR frequency lower. SBIR is hard to kill with even very deep absorbers. One option is to incorporate bass trapping into the baffle wall. I’ve done a few walls like this recently, where there are essentially cut outs in the baffle wall to expose deep bass trapping behind.

      There is a relationship between how large the baffle is around the speaker and the lowest frequency to which it controls sound waves. Below this frequency the sound wraps around the baffle. So one option is just to build large baffles around the speaker and leave the rest of the wall open. Or you can build a continuous baffle wall and then have cut outs towards the ceiling or side walls to expose deep bass trapping behind.

      Thanks,

      Nyal

  6. Thank you for unveiling the mystery of the THX Baffle wall. More importantly thank you for expounding on the knowledge and science behind it.

    I am in the process of building my home theater. I decided to go with a AT screen so that I can maximize the screen size. Further research led me to your website. I would like your opinion on implementing a thx style baffle wall in my build. My LCR speakers are CLearwave Dynamics 4TSE and 4TCC They are rear ported. I know that since these speakers were not designed for in wall use their may be some complications implementing them into a baffle wall. If they can not Ill just place them behind the screen.

  7. Hi, great article. I see you recommend placing the HF 1/2 to 5/8 of the way up the screen. My question is what if 1/2-5/8 up the screen is significantly over ear level? I have heard to try and keep the tweeters (or compression driver in my case) around ear level. I am using JTR Triple 12’s as my LCR and they will be standing on top of Seaton Submersives which puts the CD at about 45" off the ground. I was also going to try and incorporate a 10" high front stage but that is then putting the CD at 55" high and seating level will just be on a couch on the ground, not elevated by a platform or anything. I could lower the screen to accommodate for the -5/8 height but there will also be a secondary elevated row on a platform behind the main listening position. If I lower the screen too much then that increases the height I have to make the riser so they can view all parts of the screen. Any advice would be appreciated!

    1. Hi Devon

      Thanks for reading the article and posting a comment! There are a couple of home theater design principles that are at play here.

      1) Keep speakers within their operating window
      2) Tie on screen action to sound

      In more detail:
      1) Most speakers have a narrow vertical ‘operating window’ where the frequency response does not deviate significantly from the on axis sound. That’s why you see the advice about trying to listen on the speaker axis. The only real way to know how wide the operating window is is to take measurements. I’m not sure if there is an official definition of operating window but I’d say you want to position the speakers vertically so that the response is within 3dB of the on axis sound. Most speakers will only have 5-10 degrees of vertical operating window, so placement is constrained.

      The only real way to know is to measure your speakers vertical off axis response and use CAD modeling or trigonometry to figure out the vertical angle from head height / listening position to the speakers.

      2) The ventriloquism effect is strong if you are watching video with accompanying audio. 5/8ths screen height is the commercial cinema recommendation. If you move off the 1/2 to 5/8 recommendation then you are starting to compromise tracking but still your adaptation to angular differences in video/audio location is good up to about 15 degrees, so theoretically there is some good flexibility with speaker positioning.

  8. Hi Nyal,

    thanks for the great posts and answering the questions.

    I have a rather annoying question. Does it make sense at all in constructing a baffle wall if I use 2 THX down fire sub-woofers?

    Thanks.

    Ibrahim

    1. Yes definitely, you can build a partial baffle wall around the LCR speakers and then leave the bottom part of the wall open where the down-firing subs will be. Another option is to turn the subs on their sides so they fire forwards.

  9. Great article!

    I do have a question, though. The article’s “How we design and buld baffle walls” section specifies that the “Void between the baffle wall and structural wall filled with absorbent material to prevent resonances developing in the space.”

    Could you be more specific about the type of materials which can/should be used for this and how full the space needs to be? If a baffle wall is deeper for some reason, such as if the front structural wall is not flat, would the entire space need to be filled with absorbent material? In my case, there is a recessed section of the front wall about eight feet wide and two feet deep, right in the middle. I am trying to decide whether I should just wall that space up, or if some other treatment might be effective.

    Thanks in advance!
    Kevin

    1. The answer, as usual, is “it depends”.

      In a solid front baffle wall the filling is there to prevent resonances. Even a thinnish layer of pink fluffy (R13) will do the job. If you have a baffle wall design which incorporates open areas to allow the space behind the wall to function as a bass trap then what and how you fill the space with is critical. In this application the type of material needs to be selected based on the depth it needs to fill. Over a certain thickness even low density pink fluffy fiberglass becomes internally reflective (as in you increase the depth but absorption DECREASES). That effect is due to GFR or gas flow resistivity. A good technique for deep cavities is to hang sheets of semi-rigid insulation from the ceiling, spaced with air gaps. With these they should be strategically positioned in areas of low pressure, high velocity.

  10. Great article.

    Can you clarify th placement of the surface of the absorption versus the plane of the flush mounted speakers and the plane of the baffle wall? Are they all flush causing a need to recess the absorption?

    Thanks!

    1. Ideally front of speaker baffle should be flush with absorption. Baffle should not be significantly recessed into absorption.

  11. Great article,

    “Speakers and subs decoupled from the wall using appropriate isolation pads”
    What pads ?
    Speakers decoupled, are the platforms that support speakers screwed into the wall?

    Thanks in advance!
    Cédric

    1. http://i.imgur.com/YBwqrZ4.jpgou dont want any vibration from the speakers transferred to the wall.
      Smalers speakers can be mounted on the beams but largers speakers need more support and one way to fix that is with a platform.
      My own solution was to build a box thats 5 mm larger then the speakers (inc damping feets).
      Then i used cinder blocks to get correct height (inc damping) and mounted the boxes in to the wall.
      Since i have 0.6met off space behind the wall that was realy the only solution.
      http://i.imgur.com/MuDjikE.jpg?1
      http://i.imgur.com/nuiXq7M.jpg
      http://i.imgur.com/YBwqrZ4.jpg

    2. Hi Louis

      Platform to support speakers is part of the baffle wall. For speaker isolation you can use a number of things including rigid fiberglass (OC705), high density foam (Auralex Platfoam), Auralex MoPads, etc.

      Thanks,

      Nyal

      1. Hi,

        I have built a baffle wall but made some mistakes. I screwed the speaker with the baffle wall (vibrations), now i want to rebuild the Baffle Wall. I Decouple the speakers and i make a multi layer construction.
        Now my Question:
        In the moment i have only a 0.8″ mdf Layer. I can make a three Layer (Multi-layer construction) construction. First Layer 0.8″ mdf, second Layer 0.5″ gypsum Board , third Layer 0.5″ gypsum Board. Icant make gypsum, mdf, gypsum. Can i do this?
        In Austria i dont have Green Glue, what can i take?

        Here you can see my Baffle Wall:
        http://www.heimkinoverein.de/forum/4-heimkinobau-und-ausstattung/14135-planung-und-bau-von-alpis-cinema?start=448#42053

        Thanks, Horst

        1. I have forgotten to write, the vibrations are not great (very little/light) but easy to hear in certain scenes.
          Sorry for my bad english.

        2. MDF–>Gypsum–>Gypsum will be fine. Not that much different to the way I normally do it.

          If you don’t have GG you can use construction adhesive. Again not as good but probably 80%. You may find some local version of GG (damping compound used for sound isolation) which you can substitute.

    1. It’s there to provide a different set of material properties than gypsum board, to control resonances when used in the sandwich with the gypsum board.

  12. Nyla
    What’s your thought on ideal spacing between the baffle wall and AT screen? Does it depend on speakers? thanks!

      1. In what capacity?

        Procella make very good speakers. They sound good, use one of the workable high frequency transducer designs for high SPL home theater (compression driver), have great form factors, are sealed, offer a fully timbre matched solution for spatial audio and are reasonably priced.

    1. Depends on the screen. If you have a woven screen then you may only need a few inches. If it’s a perforated then you might need 18″. Of course there are variances between the various woven and perforated materials, that’s where it helps to have measured the screen at different distances from the speaker, and at various off axis angles to determine what a good minimum distance is.

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