BiB for Fostex FE168 Sigma: anything better? - Page 2 - diyAudio
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Old 12th December 2006, 01:24 PM   #11
ronc is offline ronc  United States
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Yep on the Austin principal. I designed those for Tony (layertone), he built them on a CNC out of layers.

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Old 27th April 2008, 09:27 AM   #12
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Scottmoose,

Headed to the local hardware today and they had pre cut MDF panels that looked good for a BiB build but they were only 16mm thick.

Is this going to be too thin and flexy? My previous horn build for the FE168 had a fair wad of panel honk and that was built in 25mm MDF with lots of internal bracing and panels so I"m concerned that 16mm will sound terrible.

Can do chipboard too if folks think that would sound better than mdf...

Suggestions?
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Old 27th April 2008, 10:05 AM   #13
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3/4in BB ply.
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Old 27th April 2008, 10:24 AM   #14
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Hmm,

Baltic Birch is a bit tricky to come by here in the Antipodes...

Fair nuff though, I'll see what I can do about tracking down some decent quality 3/4 inch ply.

My lungs could do without a dose of mdf anyhow...
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Old 27th April 2008, 04:57 PM   #15
GM is offline GM  United States
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Quote:
Originally posted by DrewP
Hmm,

Baltic Birch is a bit tricky to come by here in the Antipodes...

Got access to 19 mm or thicker marine grade plywood? Since the 'FRD' website got hacked, I can't view RC's design, but I imagine it generates much more acoustic pressure than a BIB due to a higher compression ratio/low flare factor, so won't tend to 'sing along' nearly as much.

Another option is well braced HDF.

WRT MDF or similar, its MOE (modulus of elasticity, i.e. its stiffness spec) varies quite a bit around the world and often between factories within a company and even batches for various reasons whereas plywood was fairly consistent for a given grade/thickness/number of lams (or at least this was still the case last I checked with Georgia-Pacific back 6-7? years ago), so by increasing the thickness of lower MOE materials we can arrive at a ~equivalent to BB ply if you're willing to build a box around a box.

For example, the last MOE specs I have for 3/4" MDF and marine grade ply were 527k and 1.8 m psi and since stiffness increases at the cube of thickness, the MDF needs to be at least:

t2 = t1*(m1/m2)^0.33 = (0.75*(1800 k/527 k))^0.33 = ~1.125" thick

where:

t1 = reference thickness
t2 = equivalent thickness
m1 = MOE of t1
m2 = MOE of t2

In this scenario though, it's better IMO to do the outer box in a furniture grade hardwood if the budget allows and/or there's any potential WAF issues with monolithic structures in her 'space' and in each of these cases, bracing requirements will in theory be limited to one below the driver and cross bracing on the mouth side to tie all four walls together to keep them from potentially 'breathing'.

BTW, there's been some minor advances in BIB design, so FWIW I recommend going ahead and using T.C.'s original line length/folded height and adjust the dims per either the formulas or one of the provided spreadsheets, adjusting Qts as required for any added series resistance in the signal chain such as a high output impedance amp, etc.: http://www.zillaspeak.com/bib-howtobuild.asp

GM
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Old 27th April 2008, 06:26 PM   #16
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Thanks for keeping things fresh Scott! I will try to post these newer, modified dims for these two great Fostex drivers on the the BIB site.

Overall performance between the older 168s and the 165k is a matter of personal taste. They each have a slightly different midrange presentation. 168s handles more power and stays together (less congested) at higher volumes. But the 165k has a different enough sound that some people may prefer it overall at lower and medium volume levels. Both AMAZING drivers imo!

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Excellent. Lovely unit. Revised dimensions for the non 'E' model 168Sigma - old style with wizzer:

Line length 128in
Zdriver 27.75in
Sl=75in^2.

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Just for you mate: a Godzilla shortened BIB for the FF225K. 136in line for a 68in tall box. W = 10in, D = 14.5in (including a 0.75in thick internal baffle). Zdriver 29.5in.
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Old 27th April 2008, 10:50 PM   #17
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Many thanks all for the input.

I'm afraid the early arrival/presence of a little one in the house 2 and a half years ago threw a spanner in the works with regard to audio projects but lately I've got to and got the Garrard 401 up and running and I'm keen to set up a single source vinyl rig in the living room so the FE168 Sigmas will be coming off the open baffles that they've been sitting on for the last half a decade and into some proper enclosures at last.
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