[djn]

Hi All, a friend of mine and I decided to do a down and dirty OB line array just for fun as neither of us has ever messed with OBs.

I have 16 4" drivers and will use 8 on each board.  In the middle of each board between the 8 drivers we will use a 6" x 6" JBL wave guide with a Selenium D210 compression driver to cover from 2khz on up.

So, not knowing how to dimention an OB, I thought I'd ask here how wide the baffle board should be and if the drivers should be off center.

Here are pics of the drivers. 

These drivers will cover from 100hz to 2khz.  They will sit on subs that will cover from 100hz on down.  Thanks in advance.

The T/S for the drivers are below the pix.







Fs =  108 ( 7)       
 Qts =  .620 (.040)       
Qes =  .703 (.046)       
Qms =  5.260 (.385)       
Re =  7.830 (.120)
Le =  .300 (.036)   
Vas(L) =  3.02 (.60)     
Mm(g) =  3.12 (.32)     
cms(mm/N) =  .705 (.150)
Rm(mechanical ohms) =  .40 (.05)         
Efficiency(db/watt/m) =  89.0 (.29)       
Measured area(sq. m) = .0054
Xmax (measured by cutting a driver open!) =  1.1 mm

[mightym]

I believe your question about the baffle dimensions can be answered here:www.quarter-wave.com/OBs/OB_Design.pdf

As to the XO from mid to sub I'd refer you to the same article.

Martin's pretty sharp...

John

[mcgsxr]

Interesting project, should be fun.

I would place the drivers offset, but would not go overboard in doing so - 2 inches off center should do fine.

[djn]

Thanks John.  I agree about Martin.  I've read his stuff and built TQW but did not look at his OB stuff.

These drivers roll off very steap under 100hz and my subs can be adjusted up to 150hz so I will play with that to begin with and go from there.  Cheers.

I believe your question about the baffle dimensions can be answered here:www.quarter-wave.com/OBs/OB_Design.pdf

As to the XO from mid to sub I'd refer you to the same article.

Martin's pretty sharp...

John

[djn]

Interesting project, should be fun.

I would place the drivers offset, but would not go overboard in doing so - 2 inches off center should do fine.

I was thinking the same thing but did not know why it needs off set.  Maybe that will covered in Martin's material.  Cheers.

[mcgsxr]

You are trying to ensure that the distance from driver to edge of baffle is different in all directions - that way the frequencies affected by that distance are spread around, rather than concentrated.

[djn]

Thanks Mc.  That makes sense.  I was also wondering if putting a small wave guide in the middle of the array will hurt the sound....any thoughts on that?

The board will be 50" tall and 10" wide.  So if I have a 2" offset, one edge of the drivers will be 1" from the edge and the other side of the drivers will be 5" from the other edge.  Do you see any issues there?  Cheers.

[mcgsxr]

I would play around with the MJK site - not sure that 10 inches wide is going to be wide enough to get to 100Hz though.

I have not tried anything with wave guides, but have used wings with some success - not suggesting that you build a box with no back, but even 3 inch extensions off the rear of the baffle will make a big difference, compared to just a flat baffle - but, it will also alter your dispersion pattern - as always, complications, considerations and compromises in design!

[djn]

I am a much better building than engineer.  I messed with the Edge program for a while but just can't seem to make it work.  I will keep working on it though.  3 issues I want to focus on:

1. Just how wide does the baffle board need to be

2. will the width determine the need for comb filter EQ, then how do I design the EQ (that is another thing in Edge I can't figure out.

3.  Here is my layout, and below that are pix of the 6" x 6" horn and CD is will use for 2khz and higher.  The plan now is to put the horn in the middle of the array.

[mightym]

I spent a little time searching out the formula, to no avail.  I was hoping to be able to give you the cut-off freq. formula.

I can't remember it.

I do know that you will need to measure from the center of the front of the driver to the center of the back of the driver, including the bits that stick off the back of the baffle ( like a "U" frame ) and that there's a formula related to the wavelength of the cut-off frequency, and the distance from front to back.

IIRC I was looking at a similar project last year.  I "think" I worked out that I would need the baffle to be 16" wide, made of 3/4" material, with a shallow "U" of 4" on the baffle edge to move the roll-off down to 100 Hz.  I can't seem to find my notes. 

I'll keep looking, in the mean time here's a link that may be of some help:www.geocities.com/dmitrynizh/labaffles.htm
Here's another thread on an OB linearray:www.diyaudio.com/forums/full-range/140338-open-baffle-line-array.html
Another place to do some research would be the old linearray files here at AC.  Look here:www.audiocircle.com/index.php?board=127.0
Hope this helps.
John

[djn]

Yep, that helps a lot.  I have been doing some research as well, but my understanding of this stuff is limited.  In the EDGE program it has an OB EQ circuit planner that allows the lower freqs to come up with a simple eq circuit

[djn]

I am using 8 4" mid drivers and the JBL 6x6 wave guide in the middle of them.  By using the bass comp I am flat to about 150hz and 3db down at 100hz.  My subs will go up to 150hz so that is fine.  However, I see the top roll off starting at 1khz and I know my drivers will go higher.  So will the open baffle cancel anything over 1khz?  The wave guide will cross over at 1.6khz.  Will I have a sonic gap between the two?

Also in the compensation box, it shows the scheem for the comp but I can't figure it out.  What is meant by the triangle with a + and - on one side and + coming out the other?  Where does the + on the bottom of the triangle go to? 

Also, Also, the baffle board is 10" wide with the drivers offset about 2".  Can I use an 8" board with the drivers centered then attach a 1" x 2" board off the back on one side and have it act like a 10" board?

We are building these on Sunday and I will post pix.  Cheers, Dennis.