Mark Audo CHR-70 BiB noob help please - diyAudio
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Old 1st September 2009, 05:31 AM   #1
geepee is offline geepee  Australia
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Default Mark Audo CHR-70 BiB noob help please

Hi all, I am thinking of doing some projects with the chr-70. I want to try a BiB first but i have a couple of questions that I am sure have been answered in the main BiB thread but there is no way I am trawling through that thing again..!

1. The BiB calaculator that I am using from the zillaspeak site tells me i need a folded line length of 48 inches for the 70Hz Fo of the chr-70. This is a fraction over 1244mm. This is a bit over the standard sheet size for mdf in these parts..(well, the mdf I have access to..) I know i can shorten the line length to 1200mm (47.24 inches) with a hit in low frequency performance but how do i recalculate ZDriver for the new line length? Also, is shortening the line by this much going to affect the performance greatly?

2. I seem to remember someone (lots of people..?) saying that the spreadsheets (the version 1 and the version 1.66..) don't take into account the thickness of the slanting baffle when the depth of the cabinet is calculated. As a definitive answer, if i use 18mm mdf, do i need to add 18mm to the internal depth of the cabinet that the spreadsheet gives me?

thanks

GP
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Old 1st September 2009, 07:09 AM   #2
Ian444 is offline Ian444  Australia
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Hi GP,

1. Not sure if I read you right. 48" = 1219mm. So if you propose to make it 1200mm you're only losing ~2% so it should be OK. Z driver = 0.217 times line length, according to the GM 166 calculator. Another option is keep bumping up the resonant freq until you get a folded line length of ~47.24" (try 71.3Hz) then Z driver will be correct for that line length. You may find the real resonant freq is a bit higher than advertised anyway, which would shorten the line length.

2. The GM v1.66 spreadsheet has an entry for baffle thickness. You need to "Enter the Preferred Dimension" before it will calculate external dimensions. If you use the V1 spreadsheet then add the baffle thickness, by the looks of it.

Thanks for bringing this up as I am about to cut some wood very soon BTW proly best to ask future BIB questions in the BIB thread to help others who come along late, I'm sure the guys won't mind, in fact I think I saw that suggestion inside that huge thread.

Last edited by Ian444; 1st September 2009 at 07:12 AM.
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Old 1st September 2009, 01:08 PM   #3
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Unless you have very low ceilings I would suggest (as has been suggested many times) you flip the cab over and have it floor loaded. Zdriver changes to .417 line length IIRC and you can use spikes, bolts or adjustable wooden legs to adjust the spacing to the floor. You can also have it exit out the bottom but at the back of the cabinet and just pull it out from the wall to tune.

And I don't think a cm here or there will have an audible difference so don't overcomplicate things. Just make sure everything is sealed real tight!
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Old 1st September 2009, 04:13 PM   #4
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Quote:
Originally Posted by giantstairs View Post
a cm here or there will have an audible difference so don't overcomplicate things. Just make sure everything is sealed real tight!
What he said.

dave
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Old 1st September 2009, 08:15 PM   #5
GM is offline GM  United States
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Quote:
Originally Posted by geepee View Post
.........there is no way I am trawling through that thing again..!

As a definitive answer, if i use 18mm mdf, do i need to add 18mm to the internal depth of the cabinet that the spreadsheet gives me?
Greets!

There's no need to trawl through any long thread since we've always had a thread search option (below the thread on the old forum, now at the top of this one and there's always Google, which historically has been the most beneficial for me).

Hmm, I have V2a, but it's probably a couple of years old, so not sure if it's a bone-fide revision number or something I made up for whatever reason. Anyway, it all nicely done up in multiple colors and a BIB drawing out to the right of the calculated columns.

Regardless, it displays the formulas in the formula bar up top, so they can be rearranged to find zdriver based on a fixed line length or sneak up on it like Ian suggested.

One more time, theoretically, how you lay out a BIB determines whether or not you should account for the divider's thickness, i.e. if laid out like TC's original you don't add it (look at its layout at the terminus), but I intentionally made the formula calculate a ~10% larger Vb on average than theoretically required to allow for driver specs variances, bracing, etc., so as a practical matter, adding the baffle thickness will potentially only have a positive effect if a driver's spec is far enough off and/or there's some unaccounted for output resistance that raises the effective Qts, whatever, to 'eat up' this extra cab Vb.

GM
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