[JohninCR]

I was reading about Dr. Geddes' Summa speaker system and he makes good points about the benefits of constant directivity in his white paper here: http://www.gedlee.com/downloads/Summa.pdf

It made me think that dipole cancellation at the sides gives dipoles constant directivity through a portion of the frequency range, which is likely to be a big part of why OB's easily sound more natural.  I'm interested in building a set of OB's that has true constant directivity throughout the range.  Multi-way is a given with baffles as narrow as possible.  Then at the very top end either dual front/rear firing tweeters mounted on a small sphere or a tweeter with a waveguide to match the directivity of the rest of the system.  Of course, some type of filter will be required to offset dipole roll-off in order to create a flat response for each section, but otherwise as long as my drivers aren't beaming and have a nice balanced dispersion, the system should have good constant directivity.  A thin baffle material should help eliminate edge diffraction effects, as long as I keep each set of drivers functioning only below their Fpeak depending upon the baffle width of their section of the baffle.

Am I missing something with this approach?

[Rudolf]

Congrats John, you are catching up on Siegfried Linkwitz real fast.

Honestly, you´re missing absolutely nothing with your considerations. SL himself has just published his result of your dual front/rear firing tweeters proposal:
http://www.linkwitzlab.com/orion++.htm
You might also look at the related thread in the Orion User Group forum:
http://orion.quicksytes.com/viewtopic.php?t=382

I find this evolution of all-over dipole configuration most interesting.

[JohninCR]

Thanks Rudolph and for the links.  The more I agree with you tech types, the more comfortable I get.

Right now I'm discussing waveguides on another forum to put on an OB front and rear for a wider bandwidth of each segment's controlled dispersion.  I'm going to try it first on an OB array, then later you might see something similar to the diffraction ring that I did for the B200 on the front and rear, making it look like some kind of double horn.  The horn guys still aren't ready for me.  They keep saying "That won't work, the rear wave is going to mess everything up." and I just chuckle to myself.

[konut]

Forgive me for being pedantic, but it would be a good idea to have an articulated baffle to align the voice coils of woofer and tweeter/s in the same plane to maintain phase coherancy. You could do this electronically but that approach has it's own problems. Depending on xover frequency the distance between divers must be taken into consideration as well.

[JohninCR]

Konut,

Time alignment and spacing are issues to consider.  I've read of aligning voice coils before, but it doesn't seem logical to me, since the VC formers can be different lengths.  For a physical time alignment, I'd want to align the membranes where the sound originates at the XO point.  The software driven XO program I use can adjust for perfect time alignment, making the physical alignment more a matter of aesthetics for my speakers.  Also, I think that time alignment is much smaller issue than balanced polar response, and here I'm talking about addressing polar response to the nth degree.  Time alignment can only be exact for a specific listening height anyway.  Do speakers sound very different if you aren't sitting upright, get a new chair, or for someone who is taller or shorter?

Also, I believe an articulated baffle would do much more harm than good other than in the bass frequencies.  This is because the staggered baffle plane is going to cause all kinds of diffraction problems.

[Duke]

John, I have the good fortune of being friends with Earl Geddes and was peripherally involved in the development of the Summa (which is 100% his design - I merely had arrived at some similar conclusions before meeting him). 

I do indeed think that uniform directivity, or at least gradual change in directivity, is one of the oft-overlooked benefits of OB as well as single-driver speakers. 

The most truly uniform directivity speakers I know of are the big SoundLab fullrange electrosts, which are dipoles that use a faceted-curved array that radiates uniformly over a 90 degree arc.  So we have a figure-8 pattern in the bass, narrowing (marginally) to a 90 degree pattern front & back farther up the scale.  I personally believe that this remarkable - perhaps unprecedented - degree of pattern uniformity is one of the most desirable characteristics of the SoundLabs, and is why I bought my first pair unseen & unheard seven years ago, subsequently crossing over to the "dark side" and becoming a dealer for them. 

Anyway, I think you're barking up the right trees.

Duke

[JohninCR]

Thanks for the input Duke.  Yes, I think it's a good tree, but we'll see if I have the capacity to climb it...yet.  Also, thanks for mentioning the 90deg dispersion, because that's what I had in mind for the wave guides front and back as an educated guess, at best.  My problem is that keeping it dipole, the waveguides make too big a baffle, so I may have to cross in an identical manner as Summa and rely on the dispersion of 15's in the same way except with dipole polar response in the lower frequencies.

It will be an interesting looking contraption to say the least.

[JohninCR]

I just love it when experiments work as planned, and I'm even more excited than SL is about his rear tweeter.  The front and rear waveguided OB array that I just built did everything it was supposed to do with very even and highly directive response +/- 45deg on axis.  Then further off axis, response falls off quickly, but remains tonally balanced unlike normal dipoles.  I was even able to imitate Dr. Geddes' foam plug, which dampens away the HOM's (High Order Modes caused by reflections within a horn or waveguide giving them a distinct "horn" sound) leaving me with a very natural sound instead of the "horny" sound of the undamped WG.

The next step is to reduce the size.  This will entail 2 separate projects.  First I'll recycle the array waveguide pieces, that I sectionalized, to create a rectangular dipole front/rear waveguide housing 3 or 4 FE108EZ's.  One 108 lacks the potential in an open alignment that I need, and my little Decware Select amps love low impedance.  Plus as good as this array sounds, the el cheapo drivers used make it not worth building the other to make a pair. 

I also plan build a dipole waveguide for a single B200, which will end up looking kind of like a double set of the diffraction rings in my earlier thread, but obviously with a smooth expansion.  It should retain much of the point source sonic benefit I noted with the diffraction rings, but function in a very different manner.  The damping required to address HOM's will certainly make a tweeter required, for which I'd prefer to engineer a concentric mount, but the damping should help tame the B200's hot top end.

Both will use simple well damped H using a pro-type 15" for bass fill, with a sub required for those who like their bass fully fleshed out like I do.

I'll post some pics of the array before I cut it up.

Everytime I think I some light at the end of my speaker tunnel some thing promises improvement.  I wonder what is next.

[johnk...]

SL was actually a little late to the party with the rear tweeter. For years he held the opinion that it was totally unnecessary. In the design of the NaO it was immediately evident to me that, at least in my listening environment, a rear tweeter was a necessity and I incorporated one in the design as an integral part. When I first introduce the NaO several Orion fans were more than highly critical of the rear tweeter. Of course, now, 4 years later,  it's the Holy Grail.

So much for the rant. Yes the rear tweeter yields more constant directivity and also smooths the power response a little through the mid/tweeter crossover.

http://www.musicanddesign.com/Integrating_a_tweeter.html

[JohninCR]

John,

What do you think about waveguides down thru the midrange for constant directivity?  I've already done a couple of test baffles for dipole waveguides and the results are very promising.  Here are pics of an array and a work in progress for a B200.

[Rudolf]

So much for the rant. Yes the rear tweeter yields more constant directivity and also smooths the power response a little through the mid/tweeter crossover.

johnk,
while everybody is talking constant directivity and power response now - isn´t dipole directivity still prone to a nasty irregularity in the midrange? When I look at some measurements of front/back radiation of midrange speakers, I see a critical area from 1500 Hz upward.

First diagram is for the SS 21W/8554, second for the Ciare HX 132 (red for 180°):





SL seems to avoid this problematic area of irregular behaviour by crossing over to the Orion tweeter at 1440 Hz LR4. The NaO II is crossing at 2.5 kHz, where the Ciare measurement signals big trouble.

Can you comment on the way you are steering clear of different front/back radiation with the NaO II midrange? A 180° measurement added to your 90° diagram would be appreciated.

[johnk...]

(JPK) The crossover point and directionality issues of the mid are dictated by the size of the mid and the baffle configuration., and the rear structure of the driver.  The smaller mids coupled to the narrow baffle of the NaO allow for a higher crossover point. Also, the crossover in the NaO is closer to 2.2 k Hz since the revision at the beginning of 2006. Starting with a higher crossover point means that the 90 degree off axis response of the front tweeter alone won't be as great as at 1.4k Hz simply because the tweeter will be more directional. This isn't really as important as the rear tweeter since the difference in the tweeter response at 1.4 and 2.2 K Hz at 90 off axis isn't really very much, but I felt it important and therefore when with the smaller mids. The tradoff, if you consider it one, is the requirement of an MTM format to have sufficient cone area in the midrange. But I happen to prefer MTM's so.... Also, I still believe that more uniform group delay in the midrange is a good thing and a higher crossover point meas lower and more uniform GD through the midrange, and a smaller change in GD through the crossover region. The GD is controversial as to audibility at the mid/twe crossover range, but there is no doubt the less isn't a bad thing.


There is always a trade off between these parameters and there will always be some asymmetry between the front and rear response. However, if you care to believe that the response you have posted form SL's site of an SS 21W/8554 on a 16" diameter circular baffle behaves the same was as a different driver mounted on a more realistic baffle .....I won't get into that.

But here is a plot of the front and rear response of the NaO II measure on axis from the front and rear:




The front is the black curve and the red is the rear. (Sorry for changing color of the front response.)

It isn't perfect and it is more sensitive to variations in mic position than the front is (baffle supports and such), but I think you have the idea.

Wave guides with front and rear tweeters could potentially be an advantage. As I tried to explain in my web page, by the time you get up in the the tweeter range the front to back path length difference is into the frequency range where the tweeters act as directional, uncorrelated sources. So while there will be cancellation at 90 degrees, if too much power "leaks"  around the edge of the baffle the power response actually can be worse in the crossover region with a rear tweeter instead of better. As you can see from by previous post, without the rear tweeter on the front tweeter is down by 10dB relative to the on axis response. That could be improved with wave guides as the controlled directionality would make them act more like isolated sources.  The idea would be to try and maintain the power from both front and rear to the -4.8 dB level. Than means  narrowing of the radiation pattern of the tweeters below 10K Hz.

[Rudolf]

John K,
thank you very much for that prompt answer. The rear response looks impressive. My expection was a much less congruent distribution. It is kind of you to share some more of your considerations regarding the midrange/tweeter crossover point. Helps me to remind that there is a lot more to keep in mind than plain frequency response.

BTW: I didn´t expect the NaO response to be anything like my examples - those pictures only wanted to illustrate what kind of problem I was adressing.

[johnk...]

I want to make one thing clear. The rear measurements of the NaO I presented were the best I could obtain. You might expect that I would want to present the results in the best light possible.  But being an honest man I want it understood that there can be much more irregularity in the rear response at difference mic positions. The rear response does not behave as nicely as the front. There are a lot of reasons for this: Baffle support braces, the driver cut out depths  and treatment, reflections off the rear tweeter mount, blockage from the driver motor structure.... The only way around that is to use two driver mounted on oppsite sides of a box in a back to back format (like and isobaric without the rear box). That would cases other problems though.

The good news is that supper smooth response isn't as importand on the rear side since what is really desired is uniform power from the front and rear. As you know, the rear radiation is going to be reflected and diffused before it gets to thelisteners ears. The most important aspect of the reflected sound is that the spectral balance be in proportion to the front direct sound.

[JohninCR]

John,

Other than double the driver cost what would be problematic with drivers mounted front and rear?  I'm considering mounting mids and tweets front and back on a cylinder with plenty of damping and a largish Vb.

[AJinFLA]

Waveguides to control directivity and lower distortion in tweeters used with dipoles.
That sounds like a pretty good idea to me. Might have to try that someday 

cheers,

AJ

[JohninCR]

Waveguides to control directivity and lower distortion in tweeters used with dipoles.
That sounds like a pretty good idea to me. Might have to try that someday 
cheers,
AJ

AJ,
I'm talking about waveguides more to control directivity of mids all the way up thru the tweeter range.  Initial tests show real promise.

[Kevin Haskins]

Waveguides to control directivity and lower distortion in tweeters used with dipoles.
That sounds like a pretty good idea to me. Might have to try that someday 
cheers,
AJ

AJ,
I'm talking about waveguides more to control directivity of mids all the way up thru the tweeter range.  Initial tests show real promise.

They would have to be large. 

[JohninCR]

Are these big enough for you?  Note that the pentagon shaped ones for the B200 still have big roundovers to come for the mouth, and both are without damping to deal with High Order Modes caused primarily by reflections and diffraction.  The mini-line is for FE108's and the rear will have a similar guide.  I'm erring on the large side, until I get a good grip on behavior and can optimize the size.

[Kevin Haskins]

Are these big enough for you?  Note that the pentagon shaped ones for the B200 still have big roundovers to come for the mouth, and both are without damping to deal with High Order Modes caused primarily by reflections and diffraction.  The mini-line is for FE108's and the rear will have a similar guide.  I'm erring on the large side, until I get a good grip on behavior and can optimize the size.

Yep... that will do it.      How does the FR look on/off axis?