Tempest and the Linkwits Transform

Discussion in 'Archived Threads 2001-2004' started by Jack Gilvey, Mar 20, 2001.

  1. Jack Gilvey

    Jack Gilvey Producer

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    Well, my dual-Tempest IB didn't work out due to construction problems (old plaster walls don't take well to big holes and a lot of shaking), so I've been looking into the possibility of throwing them into a 7-8 cu.ft sealed enclosure, and Linkwitz-transforming the system to a .707 Qtc and an f3 of 16Hz. Hoping for many of the benefits of an IB, but maybe more output due to the driver protection of the sealed box. I also found this discussion of the GD benefits of pushing the system bandwidth lower very interesting. I've never seen anyone post about having done this here,though. I'm thinking in terms of something like the QSC RMX 1450 ([email protected] ohms) to provide power.
    Anyway, I've been playing with a couple spreadsheets, and it looks like I'd need around 10db of gain at max with my proposed initial Qtc. Am I better off with using a larger box,lowering the F3 slightly, and using less gain, but losing some driver protection (from the smaller box)? Or is an initial Qtc of 7.07 an ideal starting point?
    The spreadsheets call for an input of the F(sc) of the original system, which I can't find in the Tempest 0.7Q .box file for LSPCad. I've been using the f3.
    Based on the control functions, and the graph of the applied correction, the Marchand Bassis appears to be a variable version of the transform circuit. Is this a fair assessment? Would there be a great advantage to designing the circuit specifically for this sub?
    Thanks for any info.
    Jack
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  2. Mark Seaton

    Mark Seaton Supporting Actor

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    If you were going to get a true Linkwitz transform circuit as is show on the Linkwitzlab web site, I would say there is no advantage to a box any bigger than about 6 cu.ft. given the 450W available per driver. Going to 8 cu.ft. only saves you about 1.5dB of gain at 20Hz, but even in the 6cu.ft. box you should still be able to reach Xmax with the available power. So there's really no benefit. The thing to remember is that the transform does in fact alter the transient response to that of the corrected box Q and fc. A slightly smaller box will also be easier to make resonance free, and take up less space. Many people seem to miss the point that the initial box Q will not affect the corrected response, other than determining the power required for a given excursion & output at different frequencies. So you are really sizing the box to the excursion and power limits of the system, the response doesn't matter much.
    The only way to conceptualize the fact that the transient response is in fact changing is to consider the response of the box itself at resonance. With an input of flat frequency response, the system returns more energy at resonance than in the passband for a high Q system. After correction, the input is no longer flat in frequency response, and at resonance, less power is delivered relative to the passband, so the output is the same, and similarly less power in will take less time to "decay". This description skates over a couple issues, but the point of resonance damping is accurate.
    If you do place the 2 Tempests in the same box, the best bet would be to mount the drivers on opposing faces of the box if possible. This way you only have to worry about making the box rigid, as it won't have any net forces to make it dance across the floor.
    Regards,
    Mark Seaton
     
  3. Jack Gilvey

    Jack Gilvey Producer

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    Thanks a bunch,Mark. As far as opposing drivers, that's what I had planned. One at each end of a length of 18" Sonotube. 6 cu.ft. sounds good to me [​IMG], I just didn't want it so small that I'd be needing just too much gain.
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  4. Jack Gilvey

    Jack Gilvey Producer

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    If we assume enough available gain (I could go with the larger QSC,[email protected], if it helps)then my choice of enclosure size would be based on maximizing the Tempests' power handling and excursion capabilities (what size would best "protect" the drivers?)
    I can't get the "excursion" plots to work when I change the specs to those of the Tempest in the spreadsheet I'm using, but I'd love to choose an extremely low f3 of about 10, and a Q of .577 or so. I'm interested in lowering the GD in the audible range as low as possible. How would this work, again, assuming scads of available gain?
    Is it possible to model the circuit with LSPCad using the component values taken from the spreadsheet?
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  5. Mark Seaton

    Mark Seaton Supporting Actor

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    Jack,
    Same as with any sealed subwoofer, your output is limited by the total displacement of the drivers. As you push the system response lower, your maximum output drops at the same time. Remember that for a fixed max displacment, output ability drops at a rate of 12dB/oct. Obviously at some higher frequency power becomes the limitation. You have to consider your room size, distance from the sub, and how loud you like to listen and decide how much output is "enough" for you. With a pair of Tempests, and anechoic F3 below 16Hz would be a waste in my oppinion. Something like a Q of 0.5-0.65 and fc of 15-19Hz should mate nicely to your room, and still get you some decent output. Of course if you like it really loud, I wouldn't set the fc below 18-20Hz.
    Mark Seaton
     
  6. Jack Gilvey

    Jack Gilvey Producer

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    Thanks again,Mark.
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