I know a lot have been said about absorbing back wave but I can hardly find any info for the following specific case: Do NOT block the wave at the back but strongly dampen the baffle enhancement. Then, I would expect some reduction of the rear radiation around lower mids. On the other hand, higher mids, once they start to beam, would not be much effected - hence the ambiance could be there. Any experience or reports??
You see half of a sonotube in the drawing for the curved baffle. Here the idea is to tame the loading inside the half cylinder as well.
You see half of a sonotube in the drawing for the curved baffle. Here the idea is to tame the loading inside the half cylinder as well.
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What kind of OB do you want? Ideally a dipole should have the same radiation front and back. A cardioid however has it's rear output attenuated
The first famous comercial OB in the 50s' had a velvet felt like ! I assume it's giving a little load while giving a little db attenuation (so more cardioid pattern) !
It's also maybe a question of rooms and placement (the nearer to the front wall, the more usefull such attenuation...)
I always wanted to try an OB with a rear big inflatable ballon (a blue one!) as a near sealed load ! I'm asking myself what is the frequency where the energy of the rear wave is making unproof such flatable sealed load !
Did some of you remember the flatable tweeter Audax made? ... it was very good while not very proof with time !
It's also maybe a question of rooms and placement (the nearer to the front wall, the more usefull such attenuation...)
I always wanted to try an OB with a rear big inflatable ballon (a blue one!) as a near sealed load ! I'm asking myself what is the frequency where the energy of the rear wave is making unproof such flatable sealed load !
Did some of you remember the flatable tweeter Audax made? ... it was very good while not very proof with time !
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yep, I was not born but I remember having read it has a felt behind... Most of the still exosting ones were moth-eated ! Maybe Troels Gravsen site talked about that around his OB designs around it ?!
Could be funny to shape a felt like a hat with vapor to make a not proof load just resistive ! But it will mostly attenuate the highs !
For instance variovent which is just a little very resistive overture gives a -18 db slope... The OB is near -12 dB iirc above the Fs...
If the op want to make sort of bass trap behind the front bafle : one should measure the response behind with mic then maybe calculate a Schroeder trap ! The shematic above is more like S. Linkwitz made with the LX mini !
For instance variovent which is just a little very resistive overture gives a -18 db slope... The OB is near -12 dB iirc above the Fs...
If the op want to make sort of bass trap behind the front bafle : one should measure the response behind with mic then maybe calculate a Schroeder trap ! The shematic above is more like S. Linkwitz made with the LX mini !
OB to me is dipole, no compromise, zero tolerance dictatorship. Start doing something else and you're back to a box within a box. In the beginning OB (open back box of some kind) was the norm. Then WAF raised it's ugly head and the trendmongers spied an opportunity to make money out of downtrodden men the world over. 
An externally hosted image should be here but it was not working when we last tested it.
The beamwidth of a dipole depends on the pathlength between the front and the back. Therefore, if you're putting the rockwool there to absorb that radiation, things are complicated by the change in pathlength imposed by the curved baffle.
Subjectively, I've tried spherical baffles for dipoles and I preferred the sound of no baffle at all. The spherical baffle tended to sound like a plain ol' closed box speaker, none of that dipole magic. YMMV
OB to me is dipole, no compromise, zero tolerance dictatorship. Start doing something else and you're back to a box within a box. In the beginning OB (open back box of some kind) was the norm. Then WAF raised it's ugly head and the trendmongers spied an opportunity to make money out of downtrodden men the world over.
you have a tolerance as you have a room with a front wall : true OB is on free space... in space (but with air 😱 ) (is there floor in space ? 🙄)
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There's no space on my floor, speakers are in the middle. Subs double as handy landing space within the box that is my room 
Hi,
I recall reading Troels Gravesen's OB11 project where he had use 10mm felt on the front side finally removed because a slight reduction of the mids. I suppose the same to be expected applying felt to the rear. As for the semi cylindrical baffle to my eyes seems like a U frame. M.J. King's articles show how to calculate the depth so that to avoid resonances without any damping needed. But this pretty much make it useful only for bass drivers. For higher frequencies it may take treatment that cancels the dipole function. And finally, IIUC Dr Linkwitz, the walls behind the open baffles should be reflective rather than absorbing so that by carefully placing the speakers the reflected waves will be masked by the front radiated waves.
I hope I helped!
I recall reading Troels Gravesen's OB11 project where he had use 10mm felt on the front side finally removed because a slight reduction of the mids. I suppose the same to be expected applying felt to the rear. As for the semi cylindrical baffle to my eyes seems like a U frame. M.J. King's articles show how to calculate the depth so that to avoid resonances without any damping needed. But this pretty much make it useful only for bass drivers. For higher frequencies it may take treatment that cancels the dipole function. And finally, IIUC Dr Linkwitz, the walls behind the open baffles should be reflective rather than absorbing so that by carefully placing the speakers the reflected waves will be masked by the front radiated waves.
I hope I helped!
Thanks for all responses...
Patrick, can you tell a bit about the spherical baffles you tried? Were they hemispheres and what was the size of them?
MagicBus, the semi cylindrical baffle is actually the one in my mind. The simple idea is about getting a *narrow* baffle (for the sake of imaging) and a *wide one* at the same time LOL... The latter one, of course, is for a lower reach. In OB11 project, I couldn't find info about the felt. But, if 10 mm felt cut some mids, 50 mm thick rockwool in my cylinder may really help with the horn-like loading of that U-frame.
This is the subtlety I was trying to explain. I am not willing to absorb lots of back radiation. I would like to cut just enough to kill that probable horn loading at the back. Not even by absorbing the radiated sound, rather by controlling the baffle enhancement a bit. Perhaps, some cardioid pattern at low mids only wouldn't harm much...
Cheers...
Patrick, can you tell a bit about the spherical baffles you tried? Were they hemispheres and what was the size of them?
MagicBus, the semi cylindrical baffle is actually the one in my mind. The simple idea is about getting a *narrow* baffle (for the sake of imaging) and a *wide one* at the same time LOL... The latter one, of course, is for a lower reach. In OB11 project, I couldn't find info about the felt. But, if 10 mm felt cut some mids, 50 mm thick rockwool in my cylinder may really help with the horn-like loading of that U-frame.
This is the subtlety I was trying to explain. I am not willing to absorb lots of back radiation. I would like to cut just enough to kill that probable horn loading at the back. Not even by absorbing the radiated sound, rather by controlling the baffle enhancement a bit. Perhaps, some cardioid pattern at low mids only wouldn't harm much...
Cheers...
What speaker(s) are you using? Is space a limiting factor, is that why you want more of a cardioid? You would still have to have a minimum distance to front wall of 3ft if you are only absorbing the lower frequencies
I went back and forth between some sealed speakers and tons of OB experiments. Current brainstorming is not about a system I currently have but rather about gathering ideas to try something like shown in the semi-cylinder drawing.
About the distance from wall: I can go upto 5-6 ft but even that I find insufficient for OB. 4-5 years ago I had a looong living room, which gave me something like 10+ ft from the wall behind speakers. That was WOW...
About the distance from wall: I can go upto 5-6 ft but even that I find insufficient for OB. 4-5 years ago I had a looong living room, which gave me something like 10+ ft from the wall behind speakers. That was WOW...
Oh, even if you skip the cardioid part, I don't want a horn (semi-cylinder), which is directed toward the rear wall especially at low mids...
with no diffusion is no good 
, toeing in can help here. 
OBL11-baffle
I'm not sure if a folded baffle would act as a horn load. I would rather expect a resonance peak at the frequency of wavelength four times the depth of the semi cylindrical/U frame followed by another one of the double frequency half the magnitude and so on. The so called comp effect, something very difficult to tame with XO or damping.
I have something similar by lucky unplanned happenstance. 😉
Built a Jordan VTL http://www.ejjordan.co.uk/PDFs/Eikona_2_VTL.pdf with a removable back panel.
With back removed it starts to
off around 200Hz, over an octave lower than it should in a 1ft wide baffle 😀
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