Terry Cain's BIB -why does it work and does anyone have those Fostex Craft Handbooks? - Page 201 - diyAudio
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Old 18th December 2006, 09:06 PM   #2001
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Would the shape be more like this...


http://www.zillaspeak.com/bib-twofold.asp
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Old 18th December 2006, 09:17 PM   #2002
stroop is offline stroop  Germany
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Yeah! But make the third length to side rather than the rear, it'll make it a little shallower but also little wider. Waddayareckon!?
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Old 18th December 2006, 09:20 PM   #2003
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Both sound good to me. Jeff -that's spot on to what I was thinking.
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Old 18th December 2006, 09:20 PM   #2004
stroop is offline stroop  Germany
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like this form the front

! ! /
! ! /
! ! /
! ! /
!...../

sad i know....

edit; drat... didn't work....
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Old 18th December 2006, 11:22 PM   #2005
holdent is offline holdent  Canada
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Wow - I can't get over how many posts can occur over 24 hrs!

I'd like to get back to the question Scottmoose posed to GM (?) earlier:
"(for example, Sl=100in^2, and the possible difference between 10in x 10in WxD, and 5in x 20in WxD). He didn't think it'd be significant as the CSA down the line doesn't change (assuming no folds of course) between the two of them. Any thoughts you could add on that one?"

I've asked the question before about the 1.4142 W x D ratio. I understand that it may be the best ratio in a MLTL but can't figure out how it applies to a BIB. You can acheive a 1.4142 W X D ratio at the mouth of a BIB but the ratio can't be maintained in the rest of the BIB (2 walls are parallel and 2 wall aren't). Am I missing the boat here or are you only referring to the mouth?
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Old 19th December 2006, 08:31 AM   #2006
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Neither. It's not really the best ratio in an MLTL BTW -I'd rather go with golden ratio for that, though the damping makes it less significant in these cabinets than in, say, a reflex box.

You've missed the fact that I asked Greg and Martin different things -my fault for not being particularly clear. What I asked Martin was about the angle of flare rate in a TQWT assuming no folds, if there might be potentially a change in the harmonic resonances (which he reckons it won't), and how best to model this in MathCad. My question to Greg was in re the WxD ratio specifically WRT the BIB -turns out when you draw a bend out to scale, assuming two parallel walls, & the sloping internal baffle equidistant from front, rear and floor, whatever the other two walls angle is, it comes out to sqrt (2), or the 1:1.4142. I paraphrase from Greg here as he has a gift of explanation I lack, shall we say.
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Old 19th December 2006, 11:24 PM   #2007
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Quote:
Originally posted by Scottmoose
The 1:1.4142 [i.e. sqrt (2) ] Greg established is to preserve that constant cross section for a folded line with parallel walls -the bend is the bit that can mess it up a bit. For expansion at the bend to remain even, the aformentioned WxD ratio is necessary to ensure the path is equidistant from all walls, or there's a small deviation at that point, which will affect the performance of the line.
Thanks, Scott. I can't say it's obvious to me, but I'll chew on it for a while. Make a few sketches when I get the time.

Quote:
Originally posted by GM
Agreed, up to point, as on several occasions I've noted that down low the room will dominate, but higher up where the driver dominates, increasing aspect ratios will affect cone modulation, though selective damping can overcome this.
By increasing aspect ratios do you mean narrower and deeper? Any intuitive idea why that happens?

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Old 20th December 2006, 04:23 AM   #2008
GM is offline GM  United States
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Quote:
Originally posted by Dave Cigna
By increasing aspect ratios do you mean narrower and deeper? Any intuitive idea why that happens?
Greets!

Correct. More factual than intuitive. Think about it, there's this supersonic A/C pulse 'train' vibrating along the entire length of the pipe that's powerful enough to shoot out the terminus a considerable distance before moving back into it, so if we insert a discontinuity in its expansion, it causes a shockwave (standing wave or eigenmode) back to the point where it started, so every other cycle it's going to hit the back of the driver hard enough to modulate it at that frequency. Since this shockwave will have its own harmonics, the driver will be modulated by a series of frequencies at varying amplitudes, just like it does when it gets reflections from an undamped rear wall right behind the driver, only a lot more intense because the fundamental will be a lot lower in frequency due to the typically much longer pathlength.

It's primarily for this reason you need a big filter chamber for simple stepped pipe BLHs, to cushion/damp this contant bombardment. With longer/smoother bend horns a smaller filter chamber can be used, though the trade-off is more out of phase mids exiting the mouth to comb filter with the driver's output, trading one type of comb filtering distortion for another, so it's up to the designer to choose what proportions of these two types sounds best and lay it out accordingly and why I long ago concluded that the best BLH is either none, or at most, BW limited to mid-bass/lower mids duty ('scoop' bins).

As always though, YMMV.

GM
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Old 20th December 2006, 07:09 AM   #2009
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GM, I see the light!!!! That was clearing up a lot, at least for me.
Thanks!
Peter
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Old 20th December 2006, 08:49 AM   #2010
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Thanks as always Greg.
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