Mini IB?

Discussion in 'Home Theater Projects' started by Ned, Oct 17, 2004.

  1. Ned

    Ned Supporting Actor

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    I have a rec-room that is 13'x21'x7'. There is no way to use an adjacent room as the IB, so I'm wondering if I could just make a really big enclosure at the screen end of the room. So as an example, 1.5' depth x 13' x 7' would give me 136.5 cubic feet or about 3900 litres.

    From what I've read, you want VAS x 10 for an IB. So a Tumult would need 160x10= 1600 litres. Could I then mount 2 Tumults in this space and get the full benefit of an IB? Will this give me 115db+ from say 15hz and up? I want the best sound quality and high SPL, is the Tumult the best IB solution here?
     
  2. Rich Kraus

    Rich Kraus Stunt Coordinator

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    yes.

    it doesn't have to be an attic [​IMG]

    i think obi has a rig (area behind built in RPTV) using a couple of 12's just about like what you describes (less the allmighty tumults [​IMG] )
     
  3. Greg Monfort

    Greg Monfort Supporting Actor

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    Close enough anyway. I have periodically suggested doing them this way, using it as a platform for either the speakers, etc., or as a seating riser, but AFAIK, to date no one has done it besides me. Yes, if you build it rigid/well damped enough, and with room gain it should be somewhat more if it doesn't 'bleed out' through the room's construction.

    WRT SQ, this is pretty much moot WRT high displacement drivers XO'd
     
  4. ThomasW

    ThomasW Cinematographer

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    Given the depth of the driver and the excursion of a Tumult, I suggest making it deeper than 18"
     
  5. Greg Monfort

    Greg Monfort Supporting Actor

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    Hey TW,

    I'm curious, have you done any measurements or has DW made any comments? FWIW, Altec recommended a 1.5*eff. diameter minimum, with no consideration for excursion, which puts the Tumult
     
  6. Greg Monfort

    Greg Monfort Supporting Actor

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    :b
     
  7. ThomasW

    ThomasW Cinematographer

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    Hi GM,

    Haven't spoken to Dan at all regarding the Tumult. I did run into him at the RMAF where he was looking quite dapper modeling the new line of Adire polo shirts, and inviting people to ride in the new Adire limo at WCES. :wink:

    My experience with stand alone enclosures indicates that deeper cabinets do better when low bass is being produced from a high excursion drives ie Stryke HE-15/BP1503. So I imagine that translates to IB use as well.

    Just doesn't seem logical to have a pair of drivers with 68mm P-P Xmax, sitting 18" from a wall. If the rear wall is concrete, the wave will head back toward the drivers. If it's stud wall construction the wall itself will vibrate. I'm not sure which is worse.
     
  8. Jason Dalton

    Jason Dalton Stunt Coordinator

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    How about an 18" deep space with a "standard" two driver transition box/manifold? That way the driver rear wave will fire into the long dimension of the "box".
     
  9. ThomasW

    ThomasW Cinematographer

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    If you look at the IB gallery you'll see several designs where the drivers are quite close to a concrete wall. That isn't recommended, the owners however are pleased with the performance.
     
  10. Ned

    Ned Supporting Actor

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    What if I built it so there was a 1-2' section cut out from the middle vertically. Then mount 1 driver in each "IB" and have them face each other, with their backs facing the enclosure.
     
  11. ThomasW

    ThomasW Cinematographer

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    Sorry but I can't visualize what you're describing [​IMG]
     
  12. Mark Seaton

    Mark Seaton Supporting Actor

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    Hi Ned,

    The opposed mounting to cancel out net forces is my preferred method. Were it me, I would probably look at 4-8 Tempests or maybe Part Express's 15" IB driver. The Tempest offers a nice Qts flexibility when driving one coil, both, or resistively loading the other coil. If you will have EQ available and will be measuring the response, this matters much less, but I would use say 8 drivers and drive one coil each. The only issue with the Tumult being used in an IB is the power required to make use of the excursion. I know that others have also found good results with the AV15 from Stryke and the larger BluePrint drivers. The Maelstrom from Adire is also an interesting option, but the Tempest does have the most bang/efficiency per buck going for it.

    Good Luck!
     
  13. Ned

    Ned Supporting Actor

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    [​IMG]

    The tempests have a 310L VAS so that would double my mini-IB size. 4 tempests would need something like a 4' cavity depth to get 3000L each. What kind of performance would I get from just 2 tempests though? I'm after 115-120db from ~15hz and up.

    The only space this rec-room could vent to is the 1st floor [​IMG] I don't think that would go over well.
     
  14. Chris Brock

    Chris Brock Second Unit

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    honestly if you are talking about a price in the range of two tumults, then i woudl strongly suggest you look into going with 4 of these 15" units.......

    http://www.acoustic-visions.com/~aco...ers/blueprint/

    you could get 4 for alomst the same price as 2 tumults. 1 of them has almost 30 liters smaller vas then a tumult and 4 of them would give you almost 6 liters more displacement then 2 tumults. Get 4 of those and give them around 500wrms each and you would be in bass heaven.
     
  15. Mark Seaton

    Mark Seaton Supporting Actor

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    Hi Ned,

    What you depict is what I had understood you to mean and what I meant by opposed mounting. You want to structurally connect the two baffles above and below the woofers to take maximum benefit. Basically you are bracing one woofer against the other. This way, the main source of vibration is air pressure, not from trying to support the driver.

    So far as "required volume," don't sweat it. If you want, model it out in Adire's free version of LspCAD with the Tempests. An IB is a sealed box large enough to not significantly affect the response of the driver. At anything more than 4x the Vas the response changes very little with increased volume. Even at much less volume, remember that with a sealed system volume displacement is key. If you were to put a gun to my head and say "You NEED to hit 115dB or more from 15Hz up at the seats," I would probably put in 8 Tempests, 4 in each side cavity, two each side at the top of the channel, two at the bottom. Add an some form of DSP based EQ and you are set. Power it with a stereo amp capable of ~500W/ch at 4 Ohms (1000W total = 125W each). With 8 voice coils the power compression is nil, and a room your size you should be able to get the response to extend flat down to single digits with EQ. I would suggest making sure you EQ has shelf filter capabilities.

    Some may say the space is too small for that many drivers, but I would point out that after EQ, I doubt you could find any option for the same price which would be close. The only other option which looked in the vicinity would be the BP1803, but I suspect you would need more power and more EQ. You also have the benefit that for much of your listening, the drivers will barely be moving. A pair of Tumults, 4 Tempests, or a pair of BP1803s would be a nice value option, but I suspect they would be at their limits in reaching the goals you described.
     
  16. ThomasW

    ThomasW Cinematographer

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    BP-1803/1804's are no longer available.

    Instead of cutting and making the space smaller, I'd mount the drivers in a box that stuckout from the wall into the listening area. Have the rear of the box open into the space behind the wall.

    This would address the overall depth issue. If you were clever you could design a mount so that the drivers forces were in opposition using a variation of the Linkwitz Phoenix concept
     
  17. Greg Monfort

    Greg Monfort Supporting Actor

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    Hey TW,


    But that's the point I was trying to make, in the BW where it's really pumping there is no wave reflecting back per se, it's like there's a relatively weak 'spring' acting against the driver in such an acoustically large cab. Now if the sealed cab is acoustically small, the 'spring' will be much stronger so it will take more power per unit of excursion, again regardless of driver orientation, though in this high an aspect ratio, there will be some minor blips in the FR if the drivers aren't located at ~the midpoint.

    Vented OTOH is a whole different story due to it being a 1/4WL resonator once you move away from a golden or acoustic ratio cab (~uniform particle density). It's only been recently AFAIK that there's been a modeling program to sim the effects of a high aspect ratio cab and the impact that the driver and vent location has on its FR, so if you're still building vented designs I highly recommend MJK's ported MathCad worksheet, it's quite a bit more accurate than any other program I'm familiar WRT 'tower', Sonotube, tapered, etc., designs: http://www.quarter-wave.com/

    WRT stud wall construction, for sure you have to brace it to a fare-thee-well. The first IB I did was a false wall ~150ft^3 one in a neighbor's concrete block/slab basement using an obscene amount of 2 x 4 triangulation and skinned with 3/4" marine grade ply/plasterboard double wall. He joked that if he'd known how strong it needed to be, he would have just built it out of solid concrete blocks and saved some $$/sweat. Maybe I overdid it, especially since it housed only four ~5mm Xmax 15"ers, but I had no desire to re-do if it wasn't 'dead'.

    Jason,

    Making a bipole is a good plan to ~cancel out rocking vibrations, especially with high excursion drivers, but with a ~21ft 1/2WL, comb filtering between them will begin in ernest around 565ft/21ft = 27Hz, much too low for an XO point in all but the most extreme HT audio systems.

    Ned,

    You could do it that way as long as they are tied together to make them a bipole.

    Anyway, for a variety of reasons I normally just answer Qs rather than suggest alternate designs, but since MS opened up the driver selection, I agree with him in that a bunch of cheaper, yet relatively high SQ drivers can be had for the price of two Tumults, so a sealed horizontal 'infinite' line array of low Fs, Qts drivers is my suggestion, EQ'd with a Linkwitz Transform to adjust the cab's Qtc, corner frequency to ~match the room's gain curve. Since these don't need to be driven nearly as hard, even with a lower Qts driver, SQ is improved WRT damping, non-linear distortions, and flatter aneochoic response due to a lower effective Le. Not to mention the amp doesn't need to be as powerful/expensive, or for a given price, more SQ can be purchased.

    At a glance, six Peerless 830500 limited to half power should yield near huge straight basshorn performance in-room, with more SPL capability than you and your house can stand.

    GM
     

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