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Silent Screamer 11th July 2012 07:20 PM

First time OB builder
After reading through numerous pages of the thread about box colouration, it got me to thinking that perhaps OB is not a half-finished speaker after all.

To be perfectly honest I really don't know the first thing about them, and I would like to experiment with them before I go to the trouble of building BR boxes.

The speaker I have are ScanSpeak 12MU-4371T00 (4 of) and 12MU-8731T00 (6 of) midrange, and some RAAL 140-15D AM tweeters.

I read in the other thread that a low Qts is not a good thing for OB and being that both the 4 and 8 ohm speakers are low (.24 & .29 respectively) I was wonder if they could still be used.

StigErik 11th July 2012 07:30 PM

I used to dismiss OB's as novelties.... until I tried it out myself, and since then I've never looked back. :)

Your 12MU's are usable as midranges, from 500 to 1500/2000 Hz is my recommendation.

You will need some serious woofers in addition, lets say at least a 15".

I also recommend using a dipole tweeter. Your RAAL is not.

And before you begin: get some more amps and a decent digital active crossover. OB's are not trivial when it comes to crossover and EQ, and going active you can do things that are impossible with a passive filter.

Silent Screamer 11th July 2012 07:37 PM

Already have a Ground Sound DCN28 so well and truely sorted on the digital X-Over front, amp I have semi covered with more to come.

Will the monopole tweeter work or is it a deal breaker?

StigErik 11th July 2012 07:46 PM

Good, you have the right tools there.

Monopole tweeter will work (you will get sound...), but the speaker will be half dipole and half monopole, with different polar patterns on each side of the XO frequency. In my opinion not the best solution.

Good dipole tweeters are few. The only ones are either planars (B&G Neo3) or AMT's (Mundorf, Beyma, and a few more). RAAL do have a dipole ribbon tweeter if I'm not mistaking.

You should also read Rudolf's paper on dipoles. Too bad its only in German, but use Google Translate to help you. It was an eye-opener for me, especially the part on Constant Directivity (Dipole mit konstanter Richtwirkung).

Offene Schallwand (OB)
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Silent Screamer 11th July 2012 08:04 PM

Thanks I will look at that when I'm not half asleep (5am here and been up all night) ok so it look like it might be worth giving it a go.

One thing the RAAL is 95dB and the 12MU8731 is 87.2dB I was planning on running them in parrallel making it a 4 ohm load.

The 4ohm speaker has a sensitivity of 90dB I was figuring running two in parallel should give me ~96dB?

Being that the dipole drives both sides what would that work out to?

_henry_ 12th July 2012 03:43 AM


for OB you will need big woofers, either 15" pro woofer, or 10-12" subwoofer with lots of xmax, as u will need them when EQ or do the Linkwitz transform.


Silent Screamer 12th July 2012 08:02 AM

Thanks Henry from what I am seeing the basic design kills the bass to a certain degree, it seem logical it might need some reinforcing. I have a few different long throw drivers kicking around.

Can anyone advise if I still need the two mid drivers to lift to the same SPL of the tweeter?

I have both 4ohm and 8ohm drivers available.

StigErik 12th July 2012 08:09 AM


Originally Posted by Silent Screamer (
Can anyone advise if I still need the two mid drivers to lift to the same SPL of the tweeter?.

Since you have active XO, that really does not matter. Simply adjust the levels in your XO.

_henry_ 12th July 2012 08:12 AM

well silent, the good thing about OB, you dont need to spend much $$$ for experimental baffles.

you can always try to parallel the 8ohm and do MTM, see if you like it. generally it will increase SPL and lower distortion, but it maybe abit too far for the CTC on RAAL 140 as it may be less desirable dispersion.

as you are doing multich, i dont see any reason not to try few arrangements, and see what the tradeoffs are.

ive used 2x15 (eminence beta & alpha15 mix) + Beyma 12cx coax. they are good for movies and music, it may not have the refined response like hifi drivers, but they are truly dynamic.
i will use raal 70-10 for MTM, but havent found the CNC guy yet, as i got no tools here.

the succesful OB design reference is the ORION, u can see that SL didnt use high Q woofer, as he eq'ed it with his Linkwitz Transform. he loads the woofer in H-frame, and the latest one with stacked upwards H-frame style, which i think to lower the distortion due to the cancellation.


_henry_ 12th July 2012 08:15 AM

i agree with stigerik, although there will be alot of attenuation for the tweeters.

btw, stigerik got one of the best OB (prob the best) on the forum :D


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