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Thread: Semi-trapezoidal 2-way

  1. #1
    Senior Member John W's Avatar
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    Semi-trapezoidal 2-way

    I have parts on hand for a quick and dirty variant. A pair of 2344a horns with 2425j drivers, a pair of LE14H white cone woofers and a pair of NL200T3 crossovers (probably need rebuilt down the road).
    The box plan I’m kicking around is sort of a trapezoid. I have a few questions:
    • Any major design flaws in the box?
    • Has anyone cut large triangles on a table saw?
    • My plan right now is to paint these black, and am considering using the texturized rubber stuff they sell for coating pickup truck beds. Does anyone have experience with this stuff? Or, what is the “DuraFlex” finish that JBL Pro uses?
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    Quote Originally Posted by John W
    * Any major design flaws in the box?
    - I'd make the overall cubic volume bigger for a le14h. I feel 2.2 cu' is on the small side.
    - 3 cu' up to 4.2 cu' is what I would personally look to build .

    - Thin and tall baffles such as in your drawing won't support as much bass reproduction as something twice as wide ( for instance ).
    - So, be prepared to muck about with the crossovers' LC values to buildin some baffle-step compensation ( which should likely be quite different from that of the 200t3 ). This will translate into using a larger inductor than the stock crossover ( in the lowpass section )
    - Ever notice how JBL enclosures are wider than what would be considered hip by todays design standards ? There's a reason they have a larger frontal circumference than what most of us would prefer t visually give over .

    Quote Originally Posted by John W
    * Has anyone cut large triangles on a table saw?
    - no, I can't help offer any insights there .

    Quote Originally Posted by John W
    * My plan right now is to paint these black, and am considering using the texturized rubber stuff they sell for coating pickup truck beds. Does anyone have experience with this stuff? Or, what is the “DuraFlex” finish that JBL Pro uses?
    - There is now latex water-based stuff , that offers a similar durability to the older "truck-liner" / oil-based stuff .

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    Senior Member Don Mascali's Avatar
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    I use a piece of 3/4" aluminum angle and some clamps as a straight edge for a circular saw.

    Those angles will be weird for cuts like that.
    4406, 4412A, L100, L100t3 (3 pair), L1, L7, 4645C, 4660A, 4695B, SR4735 and various DIY JBL Pro loaded systems.

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    Senior Member John W's Avatar
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    Thanks Earl,
    I have a lot to learn on this stuff. I realized they where a little small and narrow. I am willing to compromise a little low-end for size, though not too much. I guess I will do some more modeling here.
    I did a quick search and found an artical that discusses what you are talking about with diffraction loss.
    http://www.trueaudio.com/st_diff1.htm
    I still need to play around with the numbers on this, but one thing I did notice when I went back and looked at the L200T:
    http://manuals.harman.com/JBL/HOM/Te...200t3%20ts.pdf
    is that the baffle on these is the same width as mine, which on the surface seems like a good thing.

  5. #5
    Senior Member edgewound's Avatar
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    Narrow baffles give a wider soundstage, better power response, and more 3 dimensional imaging, while reducing cabinet reflections.

    Some of these issues are addressed in the literature of the upcoming Project Array Series

    I think your design looks great...just make sure the volume is optimal for the LE14
    Edgewound...JBL Pro Authorized...since 1988
    Upland Loudspeaker Service, Upland, CA

  6. #6
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by Earl K
    - Thin and tall baffles such as in your drawing won't support as much bass reproduction as something twice as wide ( for instance ).
    Yes and no. The improvements in imaging and the lowering of midrange coloration due to a narrower baffle far out weight the minimal change in bass response. IMO.

    I agree with Don. When I have to do shapes like this I use an aluminum extrusion and circular saw. It is still a real pain, and calculating the compound miters is a trick. I've never gotten them right the first time.


    Widget

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    Hello
    If you have a large table area on your saw you could clamp guide rails down at the angles you need and that would make the cuts very repeatable. The guide clamped directly to the plywood and a circular saw always works to. As was mentioned the compound angles will be real tricky. Do you have access to a radial arm saw, one of those are real handy for compound angle cuts.

    Watch your fingers and be ready to make some scrap plywood as you tweek the plywood cuts.


    Mike Caldwell

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    Quote Originally Posted by Widget
    Yes and no. The improvements in imaging and the lowering of midrange coloration due to a narrower baffle far out weight the minimal change in bass response. IMO.
    - John , I quickly fired that first post off before going shopping .
    - I really should have started with a statement that I very much like your design ! In fact, I follow all your building endeavours with great interest because your "fit & finish" is spot on.

    - Now, I realize that by first trotting out the negatives , I've likely given the wrong impression . Sorry for that .

    - I agree that the positives of low diffraction baffle-design far outweighs any apparent loss of bass in the balance of midrange to bass voicing .

    - As first mentioned, a "voicing" rebalance can be addressed/restored through altering the LC components within the crossover ( the lowpass portion ) . I do believe that this rebalancing exercise is more empirically based than pure science .

    - The need for varying degrees of "bafflestep" compensation ( in a crossover ) is something that I don't think has been chewed over much in this forum ( which is ironic , since nearly everything else under the sun has been talked about at least twice )



  9. #9
    RIP 2011 Zilch's Avatar
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    BB6P suggests your volume (red) is a little small for optimum "Extended Bass" tuning. Heavy fill to gain virtual volume doesn't help much. Bumping it up to 3 cuft. gets you to the BB6P recommendation (black) for LE14H. Note the additional ~2.5 dB at 30 Hz.

    Consider flaring out the sides toward the bottom per my Z2 design to gain that volume. This also makes them non-parallel all the way down, and gives the boxes more of a "base."

    http://audioheritage.org/vbulletin/s...7&postcount=52

    For Hamilton's L200t3 clones, we went even larger, 3.5 cuft., if I recall.

    http://audioheritage.org/vbulletin/s...070&#post82070

    With the Z1 prototype, I was able to do the side bend with a saw kerf from the inside. It's actually not much of an angle; filling the kerf with structural adhesive and backing it with a full-length mitered brace might be easier than trying to piece together the sides....
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  10. #10
    Senior Member John W's Avatar
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    Quote Originally Posted by Mike Caldwell
    Hello
    If you have a large table area on your saw you could clamp guide rails down at the angles you need and that would make the cuts very repeatable. The guide clamped directly to the plywood and a circular saw always works to. As was mentioned the compound angles will be real tricky. Do you have access to a radial arm saw, one of those are real handy for compound angle cuts.
    No radial arm saw, unfortunately. I have a long piece of bar stock that I think I will screw onto the sheet and then place this in the crosscut slot on the table to make the angles. I think if you do it the other way around and make the angle on the table you tend to create a lot more sawdust and smoke than you may expect.

  11. #11
    Senior Member John W's Avatar
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    Thanks for the plot Zilch, and everyone elses input. I think I will extend the back out a little and keep the parallel sides.

  12. #12
    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by John W
    Thanks for the plot Zilch, and everyone elses input. I think I will extend the back out a little and keep the parallel sides.
    Yes, if you can get a bit more depth, you can bump up the port diameter to 2.5" or 3" (which will be longer) also. Your vent velocities get pretty high with those 2" ports at higher SPL's. Once you settle on a final volume, I can run it again and get you preliminary port dimensions.

    [I can hear your project already. It's gonna be GOOD! ]

  13. #13
    Senior Member John W's Avatar
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    Another good idea to increase the port size.
    I have been using WinISD to model the ports. My latest estimate when bumping the back out a couple inches is about 2.8 ft3, or 79 liters.
    For 2 - 2.5 in ports at 30hz that would be about 8.6 inches long, by WinISD calculation. Is that what you get?
    I plan on using a fair amount of stuffing, and I know my volume estimation could be way off. I will have to tune these manually once everything is in place.
    Next step is a cardboard mockup to try and get a handle on the angles involved.

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    Quote Originally Posted by John W
    No radial arm saw, unfortunately. I have a long piece of bar stock that I think I will screw onto the sheet and then place this in the crosscut slot on the table to make the angles. I think if you do it the other way around and make the angle on the table you tend to create a lot more sawdust and smoke than you may expect.

    My brain to keyboard connection was a little off in what I was trying convey there but you already had a plan!

    Extension wings and a deeper back extension makes handling a 4 x 8 sheet a lot easier if you don't have something like that set up already.

    Mike Caldwell

  15. #15
    Senior Member John W's Avatar
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    Quote Originally Posted by Mike Caldwell
    My brain to keyboard connection was a little off in what I was trying convey there but you already had a plan!

    Extension wings and a deeper back extension makes handling a 4 x 8 sheet a lot easier if you don't have something like that set up already.

    Mike Caldwell
    You got that right. 4x8 mdf is brutal. My toenail is only about 1/2 grown after loosing it from dropping a sheet on my toe about 6 months ago.

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