Semi-trapezoidal 2-way

[John W]

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?

[Earl K]

* 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 .

* Has anyone cut large triangles on a table saw?

- no, I can't help offer any insights there .

* 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 .

[Don Mascali]

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.

[John W]

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.

[edgewound]

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

[Mr. Widget]

- 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

[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.

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


Mike Caldwell

[John W]

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.

[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.

[Mike Caldwell]

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

[Earl K]

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 )

[Zilch]

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....

[4313B]

I can't find my documents on the 240Ti and 250Ti right now but I think they were ~ 3.5 cu ft @ ~ 30-32 Hz and 4.0 cubic feet @ ~ 28-30 Hz respectively. The Citation 7.4 box was ~ 3.4 cu ft @ 28-30 Hz. The 1400 Array box was ~ 3.2 cu ft @ ~ 30-32 Hz.

I currently have four LE14H boxes, two ~ 2.0 cu ft @ ~ 32-34 Hz as well as two stock 240Ti boxes. I personally prefer the 2.0 cu ft boxes due to their increased portability but that's my personal preference. They're "bookshelf" size as opposed to "floor standing" size. At any rate, you should see a pattern here - 1.5 to 5.0 cu ft tuned in the 28 to 34 Hz range - nothing has really changed since the 70's in that respect. I personally feel that the 250Ti box is the largest practical volume with the lowest practical tuning - that's just my opinion.

[speakerdave]

I would not recommend trying to make long diagonal cuts on a table saw. Use a straightedge guide clamped firmly in place. Since I have a jointer (and before that a Stanley Bedrock jointer plane) I've made my own out of wood in the past, but they tend to move in the weather and need to be retrued. I just bought a commercial one the other day. It's made of extruded aluminum about four inches wide and has three ridges, one at each edge and a third about half inch from one edge. It's an improvement over angle stock because it has a lower profile, and I've also found that angle stock is not necessarily straight. It comes in two pieces, each a little more than four feet long and they can be splice together end to end with a coupling piece provided. Very neat and very rigid in either configuration. Two small C-clamps included. Price was in the $16-17 range.

The tricky thing about making beveled cuts with this kind of rig and a portable circular saw is that the offset distance you need from the cut line to the guide changes with every change in the angle of tilt. Using scrap ply of the same thickness, adjust your saw, trying your bevels, until you have the tilt right and then clamp your guide to a piece of scrap; make a cut with the chosen tilt and measure the offset.

David

[John W]

These are well underway now.
I ended up making a hinged angle jig for the table saw to cut the triangles. This isn't my idea they make commercial versions of these. It worked very well.
The correct angle on the jig is definitely the most critical measurement. I layed out the cut full size on the panel, then established the angle on the jig. Using my cardboard mockup I also captured the angle needed to butt the panels at the correct angle for gluing. Width and height of the starting rectangle where a couple inches large to start, then I screwed the jig directly to the panel, set the fence and made the cut. Then, depending on what was already established, I would either cut the triangle to the exact width or height needed. If the jig angle is set correct, once you cut width, height follows. In practice, with all the angles involved be prepared to be somewhat flexible on one side of the triangle.
Another trick was that for the back side I needed compound angles equally centered, so I cut one side of both speakers, then used the cut-offs from these to establish the angle on the other side.