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Old 2nd March 2007, 04:50 PM   #1
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Default BIB Calculations

I have been having a hard time finding how Sm and So are calculated for a BIB. Are these based off of the Thiel/Small perimeters?

I read that the line length is based on the Fs of the driver. Using a 1/2 wavelength at Fs as the line length, you can then get the height by dividing the line length by two. Is this the generally accepted procedure for calculating height?

Thank you for your help.
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Old 2nd March 2007, 05:49 PM   #2
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Yup. Combination of Vas, Qt and Re. I'm slowly working on a set of alignment tables for them. Time is the problem.

1/2 wavelength of Fs is ideal, though there's lattitude in that if needed. Don't go below 0.707Fs. Line length /2 will get you ~close enough to the correct height, though typically it'll need to be a little taller due to the curve around the base. If you search the main BIB thread, you'll find a couple of useful spreadsheets to help with that.
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Old 2nd March 2007, 06:02 PM   #3
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Thanks Scottmoose. I have been reading throught the Terry Cains BIB thread. There is a lot of information to injest there.
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Old 2nd March 2007, 06:26 PM   #4
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I know... how many pages is it up to now? I think it's the longest on the site / forum, though I could be wrong.

Until I finally get my tables done, and for anyone with a historical interest, if you want a quick ROT for finding the Sl of a BIB, then here's three to try.

1) Sl=4.25Sd. This is a quick alignment I hit upon when I first started playing with these cabinets. Providing Vas and Qt are reasonable (below about 0.4 and 50 litres) then this can be pretty effective.
2) Circumference of Sl = wavelength of Fc / 8. Horn enthusiasts the world over should recognise this one. Same caveat applies to this as the above BTW.
3) Courtesy of the old Altec formulas, then SWAGed by GM (his choice of phrase, not mine!) to a more common current definition, Am (Sm or Sl in current parlance) = 8Sd.

Any of these take us right back to the traditional cut & shut with approximate ROT approach to design. Not that that's automatically a bad thing -just that it's rather more hands on and won't allow you to hit a target alignment, or at least not particularly accurately. The Altec approach is probably the most varsatile, on the basis that within reason, bigger = better. You can always damp a big cabinet down. Squeezing more out of too small a box OTOH isn't such a happy proceedure. Small is not a phrase that instantly springs to mind though when thinking about this box...
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Old 2nd March 2007, 09:03 PM   #5
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Its at 245 pages and growing strong.

Thank you for your help again. I just want to get a few things clear. To start of with, I am only asking all these questions because I was thinking of building a BIB with the 6.5 inch FR from Hemp Acoustics. Last time I was on the BIB site from Godzilla, I did not see any calculations from you or GM or anybody using this speaker.

I am assuming Sl is the same as Sm that you where talking about in your previous reply? Fc would be target frequency of BIB?

And what about calculating the position down from the top the driver should be placed at? (So or Z like shown on how to build a BIB on Godzillas site)

Thanks again.
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Old 3rd March 2007, 10:30 AM   #6
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Yep, Sl, Sm & Am are all usually taken to mean the terminus (Sm is most often used for mass-loaded designs, Sl or Am for horns, though the latter is not so commonly seen as it used to be. Zdriver varies according to motor strength. However, you'll find it's usually within a couple of digits of 0.2 line length.

I was really, really hoping that nobody would notice those 6.5in hemp units. I've just done one, privately, and I can safely say it's not my favourite driver. Nice mid Q, very high Vas (for no discernable reason I can make out. The Audio Nirvana drivers are similar in sharing a high Vas for driver size. Unless someone can suggest a reason I can swallow, I write this off as sloppy engineering, or at least a lack of focus.) So they need a big cabinet -mid Q with high Vas always demands volume. Like this one:

Line = 138in
Zdriver 30in
Sl (wait for it -you'll love this one) 300in^2.

Even the Altec formulas modified to current parlance by GM, and taking Am=8Sd won't give anywhere near sufficient Vb for this unit. Best used in an ML box IMO. I prefer big cabinets, but from the point of view of sheer logic, a monster like this is excessive for what is not exactly a big driver & you can do better for less than 1/3 the size.
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File Type: gif 6.5in hemp.gif (6.9 KB, 441 views)
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Old 3rd March 2007, 03:04 PM   #7
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Thanks again Scottmoose. You are very helpfull and a great asset to the fullrange community.
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Old 3rd March 2007, 04:35 PM   #8
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I ran through what you gave me for information and checked it against what is up on Godzillas site. It seems to all calculate out well.

I (think I) have one last question for you. Would you agree with this statement: If Vas is around 50 litres or smaller Sm=4.25Sd should suffice where as if the Vas starts getting much higher than 50 litres Sm=8Sd would do better.

So in other words the FE166E would sound good in a box that has a calculated Sm=4.25Sd because its Vas is 45 where as the 6.5 inch hempster has a Vas of 94 so it would sound restricted in the same box compared to if Sm were increased to be 8Sd.

Thank you.
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Old 3rd March 2007, 05:04 PM   #9
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Mostly.

Providing Qt is 0.4 or lower and Vas is below about 50 litres then 4.25Sd should work reasonably well. If Vas is 50 litres or above, and Qt is ~0.4 or over, then the 8Sd fudged from the Altec alignments is the better bet. This doesn't take relative motor-power into consideration though (hence the alignment tables I'm working on) as things are more complicated. These aren't refined alignments -they're just old-fashioned ROTs that people would use and then adjust from there. But they'll probably do for about 75% of the drivers likely to be stuffed into them, which isn't bad going.

You might just about get away with the 6.5in hemp unit in a box where Sl=8Sd, providing they were very firmly stuffed into corners, but frankly I doubt it. This driver is one of the 25% where the above basic ROTs don't work, or not very well at any rate. It doesn't have the motor-strength which would help offset the high Vas a little. Your summary in general terms is quite accurate however -a high Vas driver will tend to demand more Vb (unless Qt is low enough to offset it) than a lower Vas unit, and will sound somewhat constricted in a smaller cabinet, which can't provide sufficient LF gain.

Cheers
Scott
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Old 3rd March 2007, 07:27 PM   #10
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Awesome, thank you so much for all your help.
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