[Sound.Logic]

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 !

[JohnR]

Hi, the best way to answer most of your questions would be for you to download The Edge and position the various drivers on your chosen baffle shape. You will clearly see how the response changes according to baffle size/shape and the position of the driver. It's worth doing, as just plonking a driver anywhere onto a baffle can have quite unexpected effects... I learnt that the hard way.

You can get the theory at LinkwitzLab.com but in practice The Edge seems to give pretty good predictions for non-round baffles. I think Rudolf likes another program, BoxSim, which I had intended to try but haven't... SoundEasy also will do this I believe.

The basic theory is that an open baffle will have a peak in the response at a certain frequency, and roll off at 6 dB/octave below that. Digital EQ is how I compensate for baffle effects but others don't do it that way, so there's lots of room to try it your own way.

This article may be helpful for some of the basic concepts (roughly the first half):

http://www.hifizine.com/2010/12/prototyping-4-way-open-baffle-speaker-with-the-minidsp-2x4/

[matevana]

While there are many considerations in OB design, perhaps the most difficult is addressing how low frequencies will be reproduced due to the affect of dipole cancellation. In your example, having separate subs in enclosures addresses what would have been partial cancellation from the rear wave in a true OB system, but also introduces some challenges along the way. One challenge that hybrid systems present is how to properly integrate a separate low frequency enclosure. Some people feel that boxed subs are too "slow" and cant keep up with the rest of the system. Another possible challenge relates to the different dispersion patterns you will now likely encounter, and how this might affect off-axis response. (Dipoles typically have a figure eight pattern while most boxed subs will be more cardoid-like in their behavior). Crossing low can minimize this effect but at 200 Hz you are likely to experience this to some degree.

Generally speaking I prefer the sound of mids and (usually) HF drivers in dipole orientation, meaning no enclosure what so ever, and not even any side walls. I like the sound of these drivers mounted on a flat baffle. Bass drivers usually integrate well when they are placed in an H-Frame, or some other type of "partially open" enclosure to allow for the many benefits of dipole bass.   

[scorpion]

Like JohnR is saying The Edge program will give you baffle influence on frequency response, supposing an ideal flat responding loudspeaker.
However coming to midbass this will not be so sensitive but to baffle width, 50 cm and a bit up on the baffle will be good for midbass/midrange. What you heard from the Beyma will be there. And active some small EQ in the low end is OK. Then if you would like to go with the constant dispersion school from midrange and upward, you could study the construction of John Kreskovsky's Nao Note to see how baffle is narrowed for the higher range units: http://www.musicanddesign.com/NaO_Notediscus.html . The main argument for constant or symmetrical dispersion is to achieve just this symmetry for reflections from walls in room.

/Erling