Open baffle and acoustic rolloff

Bauhausler

Rational Subjectivist
I'm thinking I'd like to exploit acoustic rolloff of an open baffle speaker to sharpen the LF cutoff of the response of a midwoof driver. As I understand the theory, below the frequency where the radius of the baffle equals 1/4 wavelength, the front and back waves destructively interfere and acoustic output drops way off. Is this correct? For a large driver (say, a 7" Lowther), where is the radiation point measured from? Driver center? Also, the speed of sound in air is, like 750 FPS @ STP or something close to that, right?
Under those assumptions a circular baffle with a 2' diameter would begin to roll off at 187 hz. To confuse the issue, if the driver area is a substantial fraction of the total area, then the sound does not have a point source and a single path length to the edge. How does this work in practice? I don't want to build a box if a baffle will work fine. Anybody have experience they want to share?
Cheers
 
If you go to madisound.com, and go to their messaga board there, there's quite a number of open-baffle discussions going on there right now. Not precisely about what you're asking, at the moment... but there's enough people there that know open-baffle dipole/cardioid speakers well enough (including Siegfried Linkwitz, of the crossover that bears his name!) that it's pretty sure this sort of question should be answerable...

Regards,
Gordon.
 
Thanks for the info. That gets me started. The plan is to use my Lowthers on top of La Scala bass bins. I only need the Lowthers to go down to 200Hz or so, but I want a fairly sharp cutoff at that point. In order to achieve this with a simple CR filter at the amp input I need some acoustic rolloff. The chart & the math tell me that a baffle 24" wide, the width of the La Scala bin, will only get me down to 285hz. 200hz rolloff would require 35", although practically, that should be about 10% below the electrical rolloff to give an appropriate shape to the response, so figure 24" is good for 315Hz and 36" is good for 220Hz. That's a board nine square feet on top of an 8 cu ft cube. Not a pretty prospect, since the speakers are in front of a window and I like the view. I have considered using heavy plexi, maybe double thickness, for the baffle board as that would impede the view the least. No idea what the resonant characteristics would be of that large a sheet of plastic, minimally supported. There would also have to be some damping to attenuate the high and mid frequencies off the back of the cone so they don't bounce off the window and into the room. That could be managed with a 'muff' of Dacron fiber or farbenglas on the back of the driver, covered in jersey cloth. Get the impression I'm free associating and thinking out loud? I am. I thinkI need to get a sheet of scrap OSB, cut a hole in it, bolt my $400 driver in there and see what happens. You'll be the first to know. Now where's that Lowther cutting template....
 
Ross,

You should look over the Bert Doppenberg site and in particular all the info on Oris Horns. There's quite a bit here.

http://www.bd-design.com/

The originator of the Oris uses a Lowther, but smaller than yours in a circular tractrix horn and sets it on top of a LaScala in one version of his speaker which is very dynamic.

I have a mold in progress which will produce a circular horn out of a casting process. It is not a copy of the Oris so I do not have to worry about design infringement. Unfortunately my design is for a 4" driver but you might consider building your own for your 7" drivers. One way is to cut circular holes in many sheets of MDF that represent the internal horn contour and then glue them as a stack. You then fill in the steps or sand them away.

The Single Driver Website from which I linked the baffle chart is also chock full of more than an evening's reading!

http://melhuish.org/audio/index.html

If you still want to try the open baffle idea consider that it is OK to fold the sides of the baffle backwards to reduce the frontal width!
 
Rob;
Thanks for the suggestion for loading the Lowthers. As cool as the Oris horns are, I think I need to stay with direct radiator types for the present use. The type 400 front horn on the La Scalas is what is causing the dispersion/directionality problems that I have in this room. I sit at a desk most of the time that's 8' behind and 6' to the right of the LaScalas' sweet spot. The timbre changes wildly as you move about the room, although it's about perfect in the listening chair. The Lowthers in the Fidelio cabinets give a good radiation pattern through the mids and don't start beaming until much higher than the Klipsch 400 horns. Mind you this is subjective and not measured, but correlates well to others' experiences. I expect an open baffle to have a god radiation pattern as well, or at least better than the 400 horn. The Oris is known for high sensitivity, but at the cost of being somewhat beamy. I just don't think that front horns are going to work for me. It's a shame. I've spent manymany hours looking at hobbyists' homebuilt wood horns, on tractrix, CD, expo and other patterns, and the awesome woodwork just makes me drool. I'd love to put a driver in a horn loaded box veneered in bookmatched walnut burl. So cool. The La Scala bin and baffle won't look nearly as good.
Now I had thought of a 'open-backed' box 2' square on top of the bin, with a Lowther on the front and minimal damping at the back. That's effectively wrapping the baffle back 90 degrees to get a better ratio of path length to front width, except then you also get a response step due to the sharp edge of the front panel to deal with. Additionally, I think the driver would have to be located assymetrically to avoid exciting internal box resonances/reflections. It's another thing to consider, but I'm not sure it would buy me anything. maybe a nice wood baffle with plexi extensions wrapped back 45 degrees to increase the path length and reduce front width while still not completely blocking vision would work. That's speculation, of course.
I'll take another look at the Doppenberg stuff. I think I first saw the Hedlund horns there. My friend Woody has those with these same DX3 drivers and they sound stupendous. I need a bottom octave, or near to it, though.
I still have no objective data to work with, so I think I'll get out the tripod, sine gen and RatShak level meter and cut a simple flat baffle to experiment with. Chances are I'll decide there's a good reason why nobody has Lowthers on flat baffles, but it will help to know why.
Thanks again for the invaluable references & encouragement.
 
Ross,

Sounds like a mid-hi driver that has a 360 degree azimuthal radiation pattern might be just the ticket for your room. Have you thought of creating something like an Ohm with a Walsh driver by pointing the Lowther upwards from a small sealed enclosure towads a conical dispersing reflector?! You could get some acoustical HP filtering from the size of the reflector and the size/shape of the cabinet containing the Lowther driver!

I don't think the Lowther with it's low Qts will work adequately on an open baffle, although a HP filter in the feedwires will help protect the driver from working without an adequate airload.
 
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Bob;
Sounds like you're describing an Audiovector. Upward firing Lowther and curved wood diffusor. Gorgeous things! That sort of thing might work really well in here, with 180 degree radiation pattern, as it's against a wall. It's a bigger woodworking project, but maybe not all that big.
I plan to roll off the power to the driver at a frequency higher than the acoustic baffle cutoff. It will get negligible power below 150hz so there shouldn't be a problem of excessive excursion like there certainly would be if I tried to drive it full range on an open baffle.
Lots of options present themselves at this point and I'm not sure what I'll end up with, but some experimentation is in order to see if I even like what I hear on an open baffle. It's an intriguing idea, but there must be some reason you almost never see them. Well, I aim to find out.

Cheers
 
Ross,

I love a challenge too. It can be fun trying new ideas. I have several speaker designs awaiting my time to build prototypes and right now I'm working on a prototype hi-end tube amp offering in an 805 SET.
 
Rob;
Speaking of ideas, I kind of blew past your statement about molding a horn. That sounds like a serious project, given the amount of work involved. I read with interest the recent horn construction articles in AX, including one using concrete in a paper form! What are your materials of choice?
I like the idea of front horns a lot, but the directionality issue bothers me. According to your experience, are there horn shapes that produce a wider dispersion?
I went back to the Oris DIY pages at Ron Wellborn's (excellent) site, and it looks like prices have gone up. For unfinished horn/stand assemblies with no drivers, but the required phase plugs it's near $1500 plus ship. Yow! It's a well engineered system and offers undeniable benefits, but that's quite a commitment for a hobbyist experimenter. Especially me, whose ideas start with a sheet of MDF and a saw, and end with a can of spray paint.
I do meticulous, deliberate and methodical tube amp work, but my speaker activities are strictly "lets turn these upside down and see what happens" quality. I should certainly have more focus on the speaker issues, as that's where most of my investment is.
 
Ross,

I haven't poured a horn yet but when I do the first pair they may be out of Plaster of Paris with diamond expanded metal plasterer's wall mesh imbedded for strength. In subsequent pourings my goal is likely to make them from see-ment. ;)

R&D like this is so time consuming and the materials can be expensive. I have $100 in fiberglass resin in my mold already. I am going to have to construct a shake-table machine on which to 'modulate' the mold so that it fills without voids. All sorts of issues to deal with.

The horn gets its gain from directivity. You can push the pattern shape around like narrowing the elevation zone and increase the azimuth zone (like Altec 811/511B horns), but there is no free lunch. I'm having a lot of fun, and success with dipole arrays at present. They can do some of the things that horns do really nicely (throw) without the coloration. Trouble is an array trades speaker cabinet size in horn length into width x height. Good window blockers!

I hear you about spending a lot of time on details in a tube amp. That is where I'm at right now. I tend to do this to all the projects I tackle though. I'm a real fussy perfectionist.
 
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