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Old 5th December 2011, 08:01 AM   #1
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Default To open baffle or not

Hi People ,

A little background first , I am trying to design a 4 way active loudspeaker , I will be using the following drivers per speaker for the stereo pair
Dayton RS315HF-- Go into 65 L Sealed Enclosures employing some good old brute force construction
Beyma 10BR60 -- 10 inch Mid Bass
Seas Mca 15 RCY-- dedicated Mid range
And the LCY 130 ribbon tweeter for the highs

The subs have already been constructed and sound good , however im facing a bit of a conundrum as far as the other enclosures go . I spent many a sleepless night trying to come up with enclosures that best handle the back wave , trying to figure out the bracing so that I get the least amount of reflection , trying to figure out the stuffing and damping .Then one day while I was tooling around I decided to hook up the Beyma and place it above the Sub supported by a few books to see what it sounds like along with the sub , I high passed it at 177Hz ,and I MUST say, I was BLOWN away . I loved the sound , open , airy , no colouration , fast and absolutely effortless , then I had the epiphany of removing the boxes completely .

Which is what brings me here .

I am no stranger to making traditional coffin box audio speakers . However im slightly out of my element with OB . I plan to use my existing sub as the base for this project and build an ob above the box .

I cant quite wrap my head around the calculations and the various aspects I need to look at , primarily what are the considerations I should be making when designing an ob speaker . I figure the baffle is going to be about 18 inches in width at the base and taper towards the top .

How does a loudspeaker react in an ob , I figure diffraction must be treated the same way as a sealed enclosure , by using corner rounding , flush mounting , felt etc .

But what is the OB dip and peak that people refer to ?

How do I figure out the bass response of the lower mid , if i cross it over at 177 or 200 hz , what am I looking at ?

How do midranges typically behave in OB , are there any differences from traditional sealed boxes aside from the Dipole radiation?

What about transmission lines , since the midbass will not be used under 100 hz can I mount it in a sort of aperiodic transmission line style of enclosure like Troels Cyclops treatment for the Mid? or is it better to just let it be free of boxes ?


Too many questions , but I am starting to think you OB guys might have a convert !
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Old 5th December 2011, 08:11 AM   #2
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In short, a driver put into OB would not have a flat response.

Woofer region: Sloping down 6db/oct

Midrange region: sloping down 6db/oct then peaking, then dip. Depending on baffle width and midrange diameter

Tweeter region: Flat on axis, but most likely horrible off axis. Need special approach (read: Nao Note, waveguide,etc.)

Of course we want to have a total response of FLAT. Hence you can easily see how an active implementation is the best approach here to correc these 'deviations'.

You almost need to build one and measure yourself in reasonably anaechoic environment (outdoor, raised). Try different baffle width. Forget the simulations.
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Old 5th December 2011, 08:11 AM   #3
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I suggest you start reading Linkwitz Lab - Loudspeaker Design and Music and Design. These are great sites which will give you insight in open baffle design.
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Old 5th December 2011, 08:31 AM   #4
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At what point would the woofer response start to fade . Lets assume 18 inch width for the baffle and probably 16 inch high . Im trying to figure out if I can set the xo points in an intelligent enough manner to avoid using EQ . I plan to go completely active and will be using a DBX 234xl for the xo duty .



As far as the midrange goes , is it possible to give it its own baffle , (minimal ) .The mid is a 5.5 incher ? I can design the baffles around the system and xo points if I have to . If I run the lower mid upto about 600 hz and then transfer to the midrange what is the likely scenario ? The tweeter I plan to have in a separate turned sphere . I plan to make each speaker its own individual baffle and stack then one ontop another like how people who make separate boxes do it . only here it will be a U shaped box with open sides , only having a bottom and a bare minimum top , just enough to support the other piece of baffle on top and so on.
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Old 5th December 2011, 08:33 AM   #5
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I agree with the Idea that experimentation is the best way forward , but a little help in narrowing down the options and some directions would be awesome .
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Old 5th December 2011, 09:02 AM   #6
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You can download musicanddesign's free "ABC Dipole" to figure out the rolloff behaviour. As the baffle is square it's not immediately clear where it would start, but I think a normal crossover point of 100-200hz would safely put it in the dipole rolloff region.

For midrange, a general rule of thumb is Baffle width <= 2.2x midrange diameter. It would safely put the undesirable dipole dip above typical tweeter freq. of 1khz-2khz.
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Old 5th December 2011, 09:22 AM   #7
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So for the 5.5 incher it would be around 12 inches maximum baffle size . Forgive me for my ignorance but what exactly happens if I increase this baffle size or lower it , from what I know is that the larger the baffle the more low frequency gets reinforced . How would reducing/expanding the midrange baffle , assume its rectangular or square , move the dipole dip? I would love to cross to the tweeter higher than 2000 , I feel it sounds the best when it is around 2500 cycles. Thanks for the abc dipole tip ! much appreciated , are there any simple "explain like im five" guides out there?
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Old 5th December 2011, 09:35 AM   #8
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It's basically a relation between wavelength and distance between back & front. At certain point they will add (peak) and cancel (dip). The larger the baffle, the "later" it's wavelength is acoustically smaller.

Electro-acoustic models

Note also that diameter = cone diameter, not with the frame.

Yes, of course the benefit of larger baffle is the downslope is intruduced lower in the frequency ("fequal" becomes lower in that linkwitzlab graph). It's a compromise between SPL and polar response. Hence 2.2x seems to be the "magic number"
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Old 5th December 2011, 09:44 AM   #9
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I see ! Another question , what are the differences between a H frame and a U frame , H frames seem to defy logic , it is standard practice to refrain from placing any driver in a kind of cave , but here it seems to be done quite a lot , what is the deal with that? U frame I can understand , it adds wings so that the baffle looks larger to the driver , again , trade offs ? are there any?
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Old 5th December 2011, 10:01 AM   #10
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I experienced also the no-baffle , which is just...a frame .
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