Placement of an infinite baffle subwoofer.

Discussion in 'Home Theater Projects' started by Eric_Schriver, Jun 7, 2003.

  1. Eric_Schriver

    Eric_Schriver Auditioning

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    Hey all,

    I'm designing my own home theater room for the basement, and up until now, things were moving along smoothly. I am planning to install an ib subwoofer into the side wall, relatively close to the screen. This is where I get stuck. I have come to the conclusion that there is no way I can test for sweet spots as you would a normal subwoofer, as I have to make the wall framing in that area slightly wider and with more support to accommodate the baffle board and to withstand the forces of four AV-15 drivers. By placing subwoofer without testing, am I running a huge risk? If so, how can I make calculations or good estimates on where to place it? Also note that I will be installing a box that will allow me to adjust the phase of the drivers if need be. I will also attach my current plans so someone might be able to make an educated guess.

    Here are my plans as they stand.
    [​IMG]
     
  2. Pete Mazz

    Pete Mazz Supporting Actor

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    Can you make a test box using just one of the drivers. Perhaps make the IB manifold but before cutting all the hole, make it with a temp baffle. This would allow you to measure possible locations. If height of manifold on the wall is flexible, test different distances from floor, as well.

    A BFD is another option if it turns out that all possible locations really don't work.

    Pete
     
  3. Rick Guynn

    Rick Guynn Second Unit

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    Actually, I owuld say that a BFD (or other parametric eq) is pretty much a requirement for nearly all sub setups unless you just happen to have a 'golden' room. You can get by without it in alot of situations, but it you really want the smoothest repsonse curve or simply wan to setup a house curve of some sort, they are great.

    Anyway.. Eric, is there any particular reason you want to line up the AV15s in that fashion? Have you considered mounting them into a cube-manifold arrangement? It would creat a smaller hole in the wall and alot less stress on the structure.

    As to finding the seet spot, truly difficult to do without some bass source. The only way I could think of to make a really good 'guess' would be to buy and use the CARA software to figure it out for you. Just remember that if you try to do some temporary box for one of the 15s to find your sweet spot, you will need to make a base (no pun intended [​IMG] ) measurement of the response of that box out in the open to compare your in-room response to. Also, if you have the flexibility to move your furniture around a little (~1-2ft in each direction) that can mitigate a fe sub mode problems right there.


    Good Luck,
    RG
     
  4. Eric_Schriver

    Eric_Schriver Auditioning

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    Rick,

    Basically I was just designing the system from Mike Knapp's IB sub he has on his site. Personally, I think the four linear drivers on a baffle board would look pretty sweet if you added the aluminum cones to the mix. Plus, I didn't want to risk making the manifold too small and have a lot of unwanted noise or too large and have a giant unexplained hole in the wall. I plan on using 4x4's where the sub would be mounted in the wall in order to brace the sub. Any opinions on this or am I just asking to shatter my foundation? And if you think I should go with the manifold, where can I find a DIY website that will guide me through designing that? Thanks for the replys so far.
     
  5. Rick Guynn

    Rick Guynn Second Unit

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    IB Gallery Link

    Check out the link above for a couple of pictures of 4-driver manifolds. Don't overestimate the necessary opening size. It would likely have to be much smaller than geometry of the manifold qould require in order to cause any 'chuffing'.

    I admit having 4 drivers looks cool on the wall [​IMG] And if you have the WAF, real estate, and are willing to do the reinforcements, go for it [​IMG]

    Good Luck,
    RG
     
  6. Rich Kraus

    Rich Kraus Stunt Coordinator

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    you mentioned that the HT is in a basement. where is the IB manifold array venting to?
     
  7. Eric_Schriver

    Eric_Schriver Auditioning

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    Rich,

    The basement is only going to be partially finished (that is going to be the only room for now actually). It will be venting into the remainder of the basement, which has plenty of room to accomodate the size needs. Also, I don't think I am going to bother with the manifold array. I'm just going to mount a baffle board in the wall.
     
  8. Pete Mazz

    Pete Mazz Supporting Actor

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    If you really want the line of drivers, I would try to isolate the frame holding the baffle from the rest of the wall. Make a seperate self contained frame and run the wall just to it during construction. Stay away from 4x4s, as they're inherently unstable and difficult to work with. I would use MDF or ply and build up layers to get the strength you need. Worry more about a deeper frame than thicker, as the front to back forces will dominate what will be going on.

    Pete
     
  9. Greg Monfort

    Greg Monfort Supporting Actor

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    Rooms are 1/2WL resonators so the axial modes that most affect the sound can be calculated with the assumption that it's well sealed and rigid, IOW like a huge sealed cab. If it's leaky then it can be modeled as a 1/4WL Aperiodic cab.

    If stuck in a corner you get max modes in all directions, which can be both good and bad, but they average out to a net gain and ensures that there will be some efficiency gain across its whole BW at the listening position even if it's in a room null.

    Once you start moving down a side wall you move in/out of the room's peaks/nulls harmonic modes so you indeed run the risk of winding up with 'sub' par performance if the sub is in one null and the listening position is in a peak, or vice versa.

    Anyway, with no dim from the array's centerline to the corner I can only speculate. For example, at 3ft, this isn't far enough down the side wall to be in a lower harmonic, but equates to ~60hz before the corner begins loading, so above this point there will be standing waves between the array and corner. If an 80Hz 4th order is used then this portion of the BW will be rolling off so it may not be audibly obvious unless they're strong enough to modulate the left main more than the right one. For sure the standing waves off the opposite wall are going to be modulating both the sub and mains though, like what happens when 'stereo' subs are used.

    With all the displacement available, EQ/sufficient power should make up for any infrasonic shortcomings of the less than ideal location. At 4th order, reversing the sub's polarity WRT the mains should be all the phase adjustment required since they're ~on the same plane.

    GM
     

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