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Old 4th May 2006, 12:28 AM   #541
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Quote:
Originally posted by Scottmoose


One of the reasons I used to post specific WxD dimensions to ensure that sort of thing didn't happen. And if it did, you had someone else's face to put on the voodoo doll you stick pins in.

Take heed of this warning prospective BIB builders: just make sure what dims you choose allow the driver to fit! Saves so much grief...

Bet they'll still sound good though when they're finished!
The FR125 is pretty deep, I don't think the interference could be avoided with any dimensional changes. Not a problem really, just a few minutes more with the router, and it doesn't have to look good.
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Old 4th May 2006, 02:17 AM   #542
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Quote:
Originally posted by loninappleton
I had the same problem of figuring out dimensioning but
I had an idea. Maybe someone can program a mini-
calculator to put in the height for instance and spit out
the length and depth.

It's just a guess, but I think Greg and the others want us
to calculate the dims from the area of the terminus (horn mouth)
and length of the fold.

I am working up the nerve to let the crew at HD or someplace
do the panel cuts..............
Greets!

I haven't been able to keep up with this thread and at this point I can only recall that some of the posts I scanned WRT folding I didn't really agree with, though within reason some error isn't going to be audible, or at least not distractingly so. I mean these things are fundamentally flawed unless coupled to a corner just so to continue its expansion and the room chops it up pretty good no matter what, but thankfully our hearing acuity begins rolling off below ~1 kHz, so in general we're pleased with the results if the average peak response is ~flat in-room.

That said, to minimize line length and expansion errors when folding, there is one set of width/depth dims that preserves both 'close enough' for all but the most anal retentive, err perfectionist.

Ignoring the internal baffle thickness in the bend and the slight loss of length where it comes to a point, if the mouth area is proportioned so that the depth is SQRT(2) longer than the width, then both the expansion and line length is preserved if it's divided in two. If you want even more correct, then put angled baffle boards in the corners and fill the cavities with concrete, but all this does besides making it heavier is that it will cause even more HF out of the mouth that will need attenuating.

For example, a 55"^2 mouth requires:

depth = SQRT(55*SQRT(2)) = ~8.82"
width = 55/~8.82 = ~6.24"
or can be done by using the reciprocal of SQRT(2):
width = 1/SQRT(2)*~8.82 = ~6.24"

So in theory these dims are superior to TC's nominal 11" x 5" not only because they come close to preserving the straight pipe's dims, but it also now has a good acoustic ratio, though without building/comparing the two, your guess is as good as mine as to whether it's audible and to what degree (if any) one is better sounding. I imagine it would be tough to tell with smaller CSA pipes, but larger pipes would definitely need more acoustic power robbing damping (been there, done that). Since it's better to have more Vb than less (within reason), always round up the numbers if needed.

Once you move away from this ratio in either direction, then preserving the simmed values gets increasingly complex since you can no longer just divide the length in two, so I'll leave those to the math whizzes to work out.

If HD in your area is like in mine, you're braver than me to trust them with any cuts other than +/- 1/4" accuracy coarse ripping.

GM
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Old 4th May 2006, 02:46 AM   #543
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Use a good circular saw with a high quality 40 tooth carbide blade ( CMT, Forrest, etc. ) and a saw board. You get straight, true cuts every time without fail....it even works with miter cuts.
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Old 4th May 2006, 03:57 AM   #544
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Quote:
Originally posted by mcgsxr
Hey lousymusician, how are those FR125's breaking in?

I have a set just sitting on a shelf, that I might see my way clear to boxing up... that design looks interesting!

I think I have sorted out the sizing, having scrubbed this entire thread, but could you clarify that for me?

Total H
Total D
Total W

etc

Thanks!

Almost tempted to try my b200's in one of these... but I really like them in OB, warts and all!
At the moment, they aren't breaking in at all. I'm trying to get the cabinets finished up. It'll take a few days.

My dimensions are:

Height = 60" (that's internal, the bottoms aren't on yet)

Width = 7-5/8" outside (6-1/8" inside)

Depth = 11-1/4" outside (the mouth is 9" deep)

These dimensions were strictly a matter of convenience. 60" is 1/2 of a 10 foot board. 11-1/4" is the width of a nominal 1 x 12 plank, so my sides were a 10 foot 1 x 12, cut in half, and used full width. I ripped 1 x 8 stock down to 6-1/8 for the front, back and inner baffe to make the mouth area come out to 55 in^2.

So, I 'accidentally' wound up within 1/8" of GM's dimensions:

Quote:
For example, a 55"^2 mouth requires:

depth = SQRT(55*SQRT(2)) = ~8.82"
width = 55/~8.82 = ~6.24"
Sometimes life just works out right!

Bill
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Old 4th May 2006, 05:02 AM   #545
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Greets!

True, just not in this case unfortunately. 11.25/6.125 = ~1.837x, which is close enough to 2x to cause considerable summing of resonances.

GM
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Old 4th May 2006, 05:47 AM   #546
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Quote:
Originally posted by giantstairs
hey lolina,

i'm certainly not one of the experts here but there appears to be very little to calculate. the height is just half the line length of the driver you choose -- 85" (42.5") for the fe103 or 97" (48.5") for the fe126e.

in the fe103 the driver gets mounted 25.5" from the bottom of the speaker (42.5 - 17).

what you have to decide is which combo of width and depth are acceptable to you (and can fit your driver). just make sure the product of the 2 is equal to the Sm.

for example, the fe103 (the dims for less bass) could be 5" x 6.6", or 4.5" x 7.3", etc. for more bass you could use 7" x 7", or 5" x 10", or 6" x 8", etc. just make sure the driver fits. depending on the size of the driver magnet you may need to use a combo of less width and more depth to make sure the piece in the middle allows it to be mounted.

well, this is my understanding at least. i hope others will chime in if i am wrong.

good luck and let us know how it turns out!
Lolina, Ha.

I'm Lon and I'm in Appleton, WI, USA.

Something clicked when you said half the line length
so I understand that much now (being pretty slow on the
uptake.) So as not to have to cut stuff, what driver has
a line length of 144 in to keep my 72 in untrimmed
board length? Nothing too fancy.
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Old 4th May 2006, 05:55 AM   #547
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Quote:
Originally posted by GM


Greets!


[snip]



If HD in your area is like in mine, you're braver than me to trust them with any cuts other than +/- 1/4" accuracy coarse ripping.

GM

HDs are all the same. A good heads up. I went to a Stock lumber yard
and the gave me an estimate on birch ply plus cuts at $1 each.

The veneer core was more than the fiberboard core but what is
the thinking regards construction? Which can be worked better with
a router to do chamfers and such? The weight and density of these should be about the same.
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Old 4th May 2006, 03:08 PM   #548
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Scott sent me two sims for the Fostex 168S. One is for a straight pipe the other for a BIB. Clearly the pipe is smoother but it's also less efficient and doesnt go quite as deep.

Can someone better explain what's happening from these sims?

Thanks,
Godzilla

PS... both cabinets are just over 5' tall and take up approx the same footprint (pipe is approx 9 x 10 footprint, BIB is approx 9 x 11.5)
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Old 4th May 2006, 03:29 PM   #549
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Greets!

Unless the veneer core is very dense (BB, Apple, or marine ply) it's probably not void free, so the fiberboard core will probably machine better if its at least medium density since it won't be prone to splintering or having spurious resonances, though practice safe routing since the dust is to some extent toxic.

GM
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Old 4th May 2006, 03:35 PM   #550
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Greets!

Not sure what you're asking. I mean the pipe is mass loaded to get a max flat alignment by damping pipe resonances and the horn to get max gain with the attendant undamped pipe resonances. As the driver is moved towards the horn's mouth it will become more like the mass loaded pipe, but at the expense of gain. No 'free lunch here'.

BTW, how did you do the FR overlay?

TIA,

GM
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