Calculating K-Slot for iBIBk & Zdriver considerations

[scaramoucheii]

Hi All

I'm a new member and I'm currently designing a pair of BIB's for use with Fostex's latest FE108ez. These will be used with my Triode Electronics Dynaco ST-70 and Meitner DAC. I'm going for maximum holographic imaging and that magical midrange with this setup.

I have the drivers and BB ply on standby and about to start this weekend.

I have followed the threads here on this forum.

As the cabinets will be relatively short I'm considering configuring them as iBIB's so that the opening is not displayed, and offered as a catch all for things like Nerf Basketballs and waste paper balls ......

As said I have read through the threads and, downloaded the BIB_Calculator and k-slot calculation spreadsheets. But, seem to be having some trouble entering the data correctly to get useful k-slot information. Using the BIB_Calculator the SM will be 35" sq. Any advise on using the k-slot spread sheets to calulate a reasonable k-slot. The cabinet depth will be 7" and I'm thinking of splitting the k-slot in half and placing them on the sides at the rear of the cabinet, effectlively removing the lower rear corners of the cabinet.

I'm planning on 3 feet raising the cabinet off of the floor for acoustic and astetic considerations, therefore I can adjust the floor loading gap in addition to the K-slot. The single foot at the rear center and this should make the rear of the cabinet appear to float if I cut the k_slots into the corners.

Zdriver Dimension

The Zdriver Dimension is 18.9" So this is measured from the Open end of the cabinet ? When I turn the cabinet upside down this will place the driver about 20" above the floor, perhaps a bit too low below ear level ! I have read in some BIB threads that the driver placement in the baffel can moved slightly, but generally in the direction of the opening. Some even suggest placing the driver as close as possible to the start of the horn. In a iBIB this would place it even closer to the floor.

Thoughts on Zdriver and K-slot spreadsheet use greatly appreciated as I plan on cutting the BB Ply this weekend and building the basic BIB box and start listining to them in normal upright and inverted modes.

Scaramoucheii

[rjbond3rd]

Hi Scaramoucheii, on my living room floor are a set of BiB cabs (half completed) and I'm debating whether to invert, or not.

1. For inverted BiB, you use a different Zdriver, apparently the rule of thumb is .416 * line length. This puts it on the "front" close to the top of the inverted cab (very roughly!) so a very good height for ears:

http://www.diyaudio.com/forums/showt...t=#post1558582

2. Among the threads you read, did you check out the iBiBk thread? I'm not sure it has your answer though:

iBIBk developement thread.

[rjbond3rd]

Oops, sorry about #2 above, I now see that "iBiBk" is in your thread title so obviously you read that thread. On my computer (Firefox on Linux) the thread titles are really, really tiny compared to the rest of the text.

[scaramoucheii]

Hmmmm ...

The height of the BIB cabinet for the FE108EZ , calculated by BIB_Calculator will be 43.7" (Total Line Length = 87.4") and Zdriver = 18.9"

So if I multiply 43.7 by 0.416 I get 18.17" Which is what i stated in my first post !

I think the 0.416 number is for BIB's in general. So nothing new there. If the cabinets were standard BIB's with the opening up, then the driver would be approximately 24" aboove the floor, Which would be a nice height for my ear when I slouched, but i would prefer to get the driver a bit higher.

I think I will start with the calculated box for this driver and listen to it for a week in the "Nerf Basketball Hoop" position and then experiment with inverting it ! Perhaps a innovative arrangement to create some wide open space below it when inverted ?

Simon

[rjbond3rd]

Howdy, just to clarify, I'm saying .416 * line length (not height). If that is correct, your Zdriver is .416 * 87.4" = 36.3" from the bottom of the cab, which would be a nice driver height.

I could be totally wrong on this though, being a newb.

[scaramoucheii]

I Agree,

I you could be totally wrong !

I believe that Zd is L / 2 * 0.416

As L is calculated according to the driver frequency specifications then

Zd = Quarter wave lenght ?????? Coincidance ???

If the line length of a FE108EZ BIB is 87.4" then the folded height is 43.7

therefore 43.7 * 0.416 is 18.17, which is approx 1/4 the length of L

Which is VERY close to what the BIB_Calculator calculates for this driver (18.9")


So My question remains

When building a BIB with such small drivers the driver height is close to ideal when the BIB is oriented with the opening pointing UP, and a stabalizing foot arrangement is built at the bottom raising the cabinet up a couple of inches ....

but , then the cabinet opening will be approximately 4 feet from my 8' cieling.... is there going to be sufficient bass (Yes I know it's a 108 mm driver) So what I was proposing was to invert the cabinet to increase the base response in the room, and building a iBIBk

I guess the thing to do is build a standard BIB this weekend and listen, then experiment with inverting and K-Slots at a later date.

Which still leaves the other question unresponded.

I'm having trouble calculating the k-slot using the spreadsheets expcal95 and tractrixchuck. Any advise on creating a 35" sq exponential bell curve with a 5" width base using these spreadsheets. I seem to be missing something there !

Simon

[rjbond3rd]

I believe that Zd is L / 2 * 0.416...

That is not correct.

Originally posted by Scottmoose
...Easy T/S based formula & stages GM created.
1) Set line length to 1/2 wavelength of Fs.
2) If a regular BIB, Zdriver = 0.217 line length, if inverted, 0.416 line length.
3) Vb=(20Vas)*(Qt^1.25)
4) Calculate the CSA of Sl by dividing the answer to stage 3 by the cabinet height,
5) Set WxD dimensions to the CSA of Sl using a ratio 1:1.4142

http://www.diyaudio.com/forums/showt...21#post1306021

[GM]

Originally posted by rjbond3rd

I could be totally wrong on this though, being a newb.

Nope, you got it right and assuming ~9 ft. ceilings I'd build them without a fold, aiming them at the ceiling with the driver at the ~36.3" point. Mounted on a base to stabilize/space the drivers up a little (you may want to experiment with different thicknesses to find the optimum speaker/ceiling spacing) and no one's the wiser if full width sides are used, though if positioned close to a wall or corner you'll need to damp this cavity.

GM

[GM]

Originally posted by scaramoucheii

Any advise on creating a 35" sq exponential bell curve with a 5" width base using these spreadsheets.

Well, you can't calc an exponential flare with a Tractrix calculator.

The K-slot SS is designed around a frequency input, so you'll either have to try different frequencies until you get Ax = 35 or rearrange the formula and work it out longhand as you currently have to do to lay out the curve since I never got around to finishing it. With such a narrow base it's going to be a very long cutout though and this will in theory raise the pipe's tuning considerably if it's not resistive enough, so be forewarned.

GM

[scaramoucheii]

OK, So it seems I understand things correctly from the begining .....

I greatly appreciate GM's suggestion and building a unfolded BIB did cross my mind, Also the comment on the difficulty of a K-Slot in such a narrow cabinet.

So the Zdriver .....If I build the cabinets without the fold, Then the cabinet will be approximately 7.25' hight As L = 87.4" , then Zdriver in this case = 87.4" * 0.416 = 36.35"

Yes a nice ear height !

NOW .... if the cabinet is folded (Regardless of Inverted or not) the Cabinet height will be , 43.716 " HOW do you then mount a Driver in the Baffle at 36.3" ????? It will be at the very top of the cabinet. In the case of a folded BIB then the 0.217 number as used in the BIB_Calculator spread sheet, and this works fine .......

So to summerize ..... it seems that with a driver as small as the FE108EZ then the iBIBk is a bit of a difficult build. (Adding in GM's comments about the K-Slot ) I'm leaning towards now just building a standard folded bib for them, placing Zdriver at 18.9", BUT, building an extension into the bottom cabinet(Closed end of the bib) to get the FE108EZ to ear height. I can then line this void cavity with duct seal and fill it with sand! This will also give the cabinet a very bottom heavy CofG, and if it dosent work out I can just roll the cabinet on the table saw and remove the void cavity and experiment with other configurations.

Thanks ALL ...

Simon