Where to Place Sound Absorbing Panels?

Discussion in 'Home Theater Projects' started by Larry Talbot, Jun 22, 2003.

  1. Larry Talbot

    Larry Talbot Second Unit

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    I am finally about to build my own sound absorbing panels and am seeking advice on where to place them. I know that every room is different and there is no magic formula, but I would greatly appreciate some basic guidelines.
    I have a 5.1 set up in a small squarish room, and will have four 2x4 panels. I understand that panels on the side and rear walls should be mounted at ear level and below, while the front wall should be from floor to ceiling. With only four panels to start, I have to be judicious as to where to place them...For instance, I can not cover the front wall from floor to ceiling as I will only have one panel per wall. Given that, should I center the front wall panel, placing it directly behind my TV and center speaker, even though that would leave the left and right fronts with bare wall space behind them? Where should the panels on the side walls go? Dead center, or closer to the front speakers? Which wall is most important when dealing with reflections?

    I would greatly appreciate any advice anyone has to give!
     
  2. Joel X

    Joel X Stunt Coordinator

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    If you think of your walls as mirrors, put the dampers anywhere you can see speakers. Also, don't forget to make dampers that handle as much of the spectrum as possible (Read: thicker the better and space off the wall as much as you can). Enjoy!
     
  3. Larry Talbot

    Larry Talbot Second Unit

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    Thanks for the advice, Joel!
     
  4. Brad Wood

    Brad Wood Stunt Coordinator

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    Larry,
    You want to place the absorption at the point of first reflection in relation to the listening position. Common practice when treating a studio control room or a home theater is to sit at the listening position, and have a friend drag a mirror along the sidewall or the ceiling. When you can see your loudspeaker in the mirror from the listening position, that is where you should put the treatment. Good luck!
     
  5. Larry Talbot

    Larry Talbot Second Unit

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    Thanks for the information Brad! If I read your response correctly, it sounds like the first point of reflection would be somewhere on the side walls? And I should slide a mirror along those walls untill I see the front of the speakers reflected in the mirror?

    What about the front of the room, where my TV is located, and the back of the room, where I have my theater seats? How important is it to have acoustic panels located on the back wall behind the theater seats, does anyone know?
     
  6. Dave Milne

    Dave Milne Supporting Actor

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    Larry,
    Taming the first reflection is important, as others have pointed out. This typically involves treatment on the side walls, but could include ceiling. I also use absorption behind the speakers (assuming the speakers are placed somewhat out in the room... as they typically need to be for best imaging). This approach loosely follows the LEDE "live end, dead end" concept whereby the "speaker end" of the room is relatively dead and the "listening end" is relatively live.

    A balance of diffusive surfaces should be included in addition to the absorptive and reflective. Diffusion can be achieved by QRD (quadratic residue diffusor), skyline panels or even a bookshelf full of books with irregular depth. Check out Auralex website for a nice collection of reasonably-priced acoustic treatment materials. Visit the "Auralex University" from their website for good infomation on treatment. I believe they also offer a free consultation service. Another option is to use RPGs "room optimizer" software.

    Do a net search on "acoustic treatment" and you will probably find lots of other stuff. If you are serious about room treatment, you may want to hire an expert like Michael Green
     
  7. Brad Wood

    Brad Wood Stunt Coordinator

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    Larry,
    First reflections can occur on the ceiling, side walls, and floor. Basically anywhere there is a reflective surface. You are correct, use a small mirror, and have someone drag it along the ceiling or walls until you see the speakers in the mirror, and place your panel there.

    As Dave pointed out, the LEDE is one school of thought in room treatment, and there are many others. I personally like to treat the first reflection with absorption and put diffusion behind the speakers and at the back wall to make a more live room. That's just my taste.

    If you have a small amount of panels to start, I would treat the first reflections and maybe a couple on the back wall directly on-axis with the speaker. I don't like to put absorption behind the speakers, because unless you really know what you're doing, your homemade absorber panels won't be absorbing anything below 1K, and there really isn't much information above that behind a loudspeaker. When you can afford it, I would also look into some bass traps. I find that bass traps will give you some of the best bang for your buck in acoustical treatment in tightening the low end. You could build one yourself, (tuning them is a bit difficult) or you could buy a premade one from Auralex or RPG.

    If you have access, visit as many custom home theater showrooms as you can. They should have a wide variety of rooms and acoustical treatments. That way you can hear live rooms, dead rooms, or rooms somewhere in the middle. As always, it comes down to what sounds good to your ear.
     
  8. Larry Talbot

    Larry Talbot Second Unit

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    Dave, Brad, thanks for the additional responses!

    When using the mirror technique along the side walls to find reflections, I have read that you are supposed to look for the reflection of the opposite speaker, instead of the one closest to you. Is this right?
    And, either way, I assume you want to see the front of the speaker reflected in the mirror, as opposed to just the sides?

    Brad, you wrote: "I don't like to put absorption behind the speakers, because unless you really know what you're doing, your homemade absorber panels won't be absorbing anything below 1K, and there really isn't much information above that behind a loudspeaker."

    The panels I'm building (they should be done today) are following the design listed here:

    http://www.angelfire.com/sports/RCca...usticpanel.htm

    According to the author, they should "absorb sound waves from the high frequencies, to upper bass notes near 80hz (depending on materials, size, and placement)."

    Assuming that is so, would that change your advice about putting them behind my main speakers?

    I'm using Klipsch RBF bookshelves, by the way, set to small with a KSW12 sub (the weakest link.) I'll let you all know how the panels work out, once I've experimented with them.

    Thanks again for all your responses!

    I forgot I had one more question. I've read that such panels should be placed ear level and below on the side and back walls. But Chris Tsutsui, the man who created the design linked above, has his panels mounted vertically, rather than horizontally, on the side walls (putting them way above ear level). Any thoughts on which way may or may not be correct?
     
  9. RichardHOS

    RichardHOS Second Unit

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    If you're setting up the room acoustics primarily for the benefit of two channel (stereo) listening, then following the above advice with treating primarily first reflection points and following the Live-end/Dead-end approach is appropriate.

    If you're concerned more with home-theater and/or multichannel music listening, then you should damp all walls at ear level, not just the first reflection points. The best use of your panels in this case would be to mount them horizontally, centered at ear level, and make a "ring" around the room. With only four panels to start with, I'd place one on each wall making sure that the early reflection points are covered (being the points where you see the reflection of either speaker's baffle. If you can also cover the early reflection points from side/rear speakers, then you have a good start.

    When you make more panels (assuming they are needed), finish covering the front wall (behind the main speakers), and then add more to side and rear walls (again, around ear level) until the decay time of the room is around .35 to .5 seconds for each frequency band.
     
  10. Larry Talbot

    Larry Talbot Second Unit

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    "If you're concerned more with home-theater and/or multichannel music listening, then you should damp all walls at ear level, not just the first reflection points. The best use of your panels in this case would be to mount them horizontally, centered at ear level, and make a "ring" around the room."

    Richard, thanks for the input! I'm happy for all the suggestions I can get. Having finally finished my panels early this morning, I've been experimenting with them in various places, and am about to try your suggestion. Two questions: some say panels should be mounted at ear height and below, meaning the top of the panels would be at ear level. You think centering the panels at ear level would be better? That would leave roughly a foot of uncovered wall space beneath the panels.
    Second, and most important from my point of view, my three main speakers sit in front of my 27 inch TV. If I mount a panel horizontally on the front wall, it will mostly be behind my TV. Will it still effectively absorb sound from that position?

    Thanks again for taking the time to respond.
     
  11. RichardHOS

    RichardHOS Second Unit

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    For multichannel setups, damping all walls at ear level is the first priority. After that, the additional damping is added as needed to achieve the desired decay time. In my opinion, placing that additional material can be done in a couple of logical ways:

    (1) Keep adding the extra damping symmetrically above and below the ear-level line, meaning the "ring" stays centered at ear level.
    (2) Treat the early reflection points with additional damping in the vertical direction (floor to ceiling possibly) in just those local areas.
    (3) Add the extra damping from the "ring" to the floor, as needed.

    I think which is appropriate probably depends on your room characteristics, and will take some measurements and calculation of necessary sabins of absorption at each frequency band to properly determine. For example, say you started with a 24" tall ring around the room centered at ear level (avg. 42"). After measurements, you figured that you needed an additional 12" tall "ring" of damping added to the room. I would be inclined to split that, 6" above and 6" below, so that the ring stayed centered. However, if you calculated that you needed an additional 36", then I'd be inclined to fill in all the way to the floor, and put whatever is left (6" in this case) at the top of the ring.

    I also think the decision of whether to damp the early reflection points additionally in the vertical direction is dependant on your use of the room. Stick with the ring approach for a pure multichannel environment. If you don't have a good surround processing mode (Logic7, Trifield, ProLogicII), and will listen in straight 2-channel mode fairly often, then you might bias more damping to the early reflection points and keep the ring height to the minimum. That will allow more reflection area near ear level for the front generated sound to bounce to the back of the room and generate some room ambience. Used in this manner, you should apply diffusion panels to the rear of the room to ensure that the "bounced around" sound makes it back to ear level, otherwise it will sound really dead. That would be less than optimal for multichannel though, so it is a compromise.
     
  12. ericLee

    ericLee Stunt Coordinator

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    What if you have tripoles or dipoles rears?
     
  13. Larry Talbot

    Larry Talbot Second Unit

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    thanks for the detailed information, Richard. I'm still not sure whether I should mount an absorbtion panel on the front wall behind my TV. My front left, center and right speaker are all in front of my 27 inch TV. Does it make sense to center a panel on that front wall, when it will mostly be behind the TV? There are only about seven inches between the back of the TV and that wall.
     
  14. RichardHOS

    RichardHOS Second Unit

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    Seeing as how an 8' panel is considerably wider than a 27" TV, I'd still be inclined to mount the panel behind the TV. Unless I misunderstood something. [​IMG]
     
  15. Larry Talbot

    Larry Talbot Second Unit

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    No, you didn't, I made a mistake. I should have said 2x4, NOT 2x8. Whoops. I'm still going to try your suggestion anyway, as soon as I can figure out how to mount my panel behind the TV...
    So far, the four panels have definitely improved the sound of my theater, particularly, for some reason, in the area of bass. Bass just sounds incredible now, I get bass in scenes where I never noticed it before, and the whole room shakes. But it doesn't seem any louder outside the room, which I'm sure my neighbors are thankful for.
    The front left and right have also improved a great deal, with subtle sounds in familiar scenes I never noticed before.
    Vocals in the center channel have improved as well, but they are also a little too quiet, just slightly indistinct, with some loss of clarity. At first I thought I just needed to rebalance my center speaker's db level, but I checked and it hits the same mark on the ratshack meter as all the other speakers, so now I'm not sure what's going on. I think maybe the bass is simply overwhelming the sound of the center. I'm going to try to drop the bass a little bit and see if that improves vocal clarity...
     
  16. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    FWIW, the panels in my room are mounted horizontally on the sides. The panels on the front wall are mounted vertically mostly because they would not fit horizontally. I didn't have to worry about placement because the room is only 11'x10'. The difference the panels add to such a small room is astounding though. (I too noticed upper bass improvement)

    I do not have a panel mounted on my back wall because there's a closet filled with clothes. I also do not have anything on my ceiling. If I do put one on the ceiling It will likely be a compressed fiberglass panel covered with cloth.

    BTW, I'm glad the panels helped Larry.

    Eric LEE. I have Vifa Dipoles in my room situated 1-2 feet in front of the acoustic panels. They still sound fine and the dipoles lose some of that "reflective" type sound that I hear. I also tried the dipoles in a room without treatments and they mostly sound more lively. I'd still have to say that it simply comes down to taste. Personally, I'm a fan of a more acoustically dead room than a reflective one which is why I kept the panels in.

    Dipoles create extra reflections, and since the panels absorb a great deal of mids and highs, you may hear more bass.
     
  17. Larry Talbot

    Larry Talbot Second Unit

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    "BTW, I'm glad the panels helped Larry."

    They helped tremendously, Chris. Without them, in its current location (I've recently moved, and there was no where else it could go), my theater was virtually worthless.
    Now it sounds better than it ever has before.
     

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