Anyone used Beyma or B&C coax 8" drivers?

[GordonW]

Anyone used Beyma or B&C coax 8" drivers?

Hey, I just did a huge speaker build using a pair of Tannoy 8" dual-concentric drivers (we sell Tannoy where I work, so I ordered them as spare parts)... and I'm thinking of building MORE of the same design. But, I am looking for a driver I can get easier (Tannoy is hard to get parts from on the high end stuff, they don't stock very many of them at the time... I got 2 out ONLY THREE TOTAL of the spare parts drivers of this type they had in the ENTIRE US/Canada headquarters!)... so, I was looking at the Beyma and/or B&C coax drivers as a substitute.

Now, I'm willing to tinker... changing out high-frequency driver units is NOT out of the question. I'm just wondering, if anyone has found either of these manufacturers offerings to give reasonably linear response, both in frequency and time domains... in other words, tested them to see how they compare to factory specs?

BTW: I'm also looking, naturally, at the Eminence Beta 8 CX and such. But, the Beyma and B&C look to have much better woofer baskets (more open in the back, less reflection of sound back at the cone, as well as stonger, being cast instead of stamped metal)... if there's a performance advantage to be had with the cast basket alternatives above, I'm certainly game. In addition, I'm not against modifying the drivers... changing the throat shape and such, if necessary...

Of course, if anyone has any OTHER pet 8" coax drivers, I'd be grateful to hear about them. BTW: I'm probably going to be using the 8" down to about 100-120 Hz or so (small sealed "doghouse" subenclosure), with an active-powered bass section in the same cabinet, and a "supertweeter" to extend the range and dispersion of the top octave (specifically about about 14KHz or so)... so drivers that "roll off" above 15KHz, could be fine, as long as they're well behaved...

Thanks for any info...

Regards,
Gordon.


Thanks!

Regards,
Gordon.

 

[cdnsoundguy]

8" Coaxial

Hi Gordon,

I have a ton of data on 8" coaxial loudspeakers. I can post measurements I have made between the Tannoy and the Beyma. The differences are few but do sound quite different.

1) The Tannoy uses a dome (see attached pic) with a "wave guide" while the beyma uses a ring-radiator.

2) The Beyma diaphram is Aluminium and the Tannoy is Aluminium-Magnesium

3) The Tannoy has a very low compression ratio wave-guide, and the beyma uses a higher compression ratio phase plug.

4) Both cones are polypropylene but the Tannoy is a Curvilinear profile compared to a Straight on the Beyma.

5) Both HF use Ferro-fluid but are different viscosity, the tannoy is using it to dampen the HF more

6) The time offset of the HF to LF is greater in the Beyma.

Now switching to the B&C, Eminence etc, these designs use a back-plate mounted compression driver. These designs require delay compensation for the HF to LF, for sure! Even though I have seen and heard them used in a passive situation the crossover region always suffers.

The design problem is getting a HF acoustic center into the LF acoustic center. In the case of the Tannoy they used a dome and wave guide. The disadvange is the low efficiency and low power handling. If they increase the voil-coil size the tweeter no longer fits inside the pole piece. Beyma decided a ring-radiator was a good alternative because of increase power handling and higher frequency break-up mode, but it moved the acoustic center a little farther back.

Hope this helps -

Rob

 

[GM]

Greets!

FYI: http://www.audioroundtable.com/HighEfficiencySpeakers/messages/1834.html

GM

 

[miguel2]

Hi,

Which Beyma are you referring to?

Miguel

 

[cdnsoundguy]

Beyma 8BX

The driver I was referring to was the 8BX.

Rob

 

[ScottG]

Re: 8" Coaxial

The design problem is getting a HF acoustic center into the LF acoustic center. In the case of the Tannoy they used a dome and wave guide. The disadvange is the low efficiency and low power handling. If they increase the voil-coil size the tweeter no longer fits inside the pole piece. Beyma decided a ring-radiator was a good alternative because of increase power handling and higher frequency break-up mode, but it moved the acoustic center a little farther back.

I suspect that this is the problem mentioned on the AudioRoundTable link that GM provided. I believe the BMS co-axials use a 3rd order on the tweeter and nothing on the mid..(could be wrong though).

Gordon I don't think you need (specifically) a co-axial driver to overcome this problem if you are using 3 drivers. Consider a "quasi" phase plug (i.e. just a pole) with a tweeter at the end (i.e. aproximating a normal "flush-mount" baffle condition). The crossover of choice would the the B&O filler driver concept (2nd order on the tweet and the woof parralleled with 1st order bandpass on the mid) - provided that the mid extends fairly high in freq. and that the tweeter extends low enough. This should also reduce problems with dispersion and diffraction of the tweet in conjunction with mid..

JohnK's site:

http://www.geocities.com/kreskovs/GenFiller.html

 

[cdnsoundguy]

note to GM link

Hi GM,

I looked at your link you post Dr. Geddes was reffering to a dual coaxial compression driver from BMS which does suffer from the same need for time alignment at the crossover freq. The comment about the quality of B&C, I agree very very much about their compression drivers and cone transducers. I have had a lot of success using them and I think they have done alot of work on the phase-plug's used in the design. I believe that Dr. Geddes has been investigating phase plug design for quite some time, in persuit of his research on a driver that incorporates a removable phase plug assembly, so that it could be optimised for the horn-driver combination.
I too have been researching this, and learned alot from B&C's drivers. I am looking forward to seeing what Dr. Geddes research discovers. I have found his AES papers to be some of my favorite reads.

Thanks,
Rob Robbins

 

[GM]

Greets!

Understood, I was merely pointing out that he had concluded that overall, he considered B&C to currently have the best coaxes. Glad to hear your experience mirrors his conclusion since I have no first hand knowledge of them.

GM

 

[Pallas]

My understanding of Dr. Geddes' position is that coaxes are kind of dumb, anyway. (I disagree with him, though I've never heard his OS horns.) His comments about B&C were about their compression drivers not their coaxes, and the BMS coaxial driver to which we was comparing them was this one: http://www.bmspro.com/products/4590.html. It has a 1" ring radiator coincident with a 2" ring radiator.

Maybe I need to hear more horns, and especially oblate spheroid ones, but I was astounded by the performance of the 4590 on 90x40cd horns. I also think onethis 15" cone + compression driver BMS coax looks on paper really neat for a big home monitor for use with a subwoofer, though I haven't heard it.

 

[GM]

Greets!

Maybe I totally misunderstood, but I took his remarks to encompass all of BMS's coax systems. Regardless, since he said "In the coaxial drivers the response at the crossover is a mess........", which contradicts their marketing hype of "excellent phase coherence" and "perfect time alignment without the problems of multi-source interference", I would be prone not to consider/recommend any of their coax systems as it seems reasonable to assume that the rest of them may be misrepresented as well, and I seriously doubt he would make such a statement unless he could back it up with measurements.

As always though, YMMV.

GM

 

[dwk123]

My understanding of Dr. Geddes' position is that coaxes are kind of dumb, anyway. (I disagree with him, though I've never heard his OS horns.) His comments about B&C were about their compression drivers not their coaxes, and the BMS coaxial driver to which we was comparing them was this one: http://www.bmspro.com/products/4590.html. It has a 1" ring radiator coincident with a 2" ring radiator.

My take on the discussion matches the above - they were discussing compression drivers, not conventional coax drivers.

Pallas, when considering Geddes position on coax drivers, you have to understand that his whole focus is on pattern control, and achieving constant directivity as far as possible. Typical coax driver arrangements make this pretty much impossible as they impose geometrical constraints that are at odds with pattern control. It'll also probably generate more high-order-modes, but I'm still a bit fuzzy on the math on that one.

For the 'best' approach to conventional coax drivers, I suspect this can't be beat:
http://www.eaw.com/products/AX/CSA_technology.html

Maybe I need to hear more horns, and especially oblate spheroid ones, but I was astounded by the performance of the 4590 on 90x40cd horns.

The 4590 is pretty well regarded, so this doens't surprise me. However, once again keep in mind that Earl is after constant directivity, and the BMS coax arrangement doesn't lend itself to this, since the radiation pattern at the throat will be non-uniform, and hence won't mate too well with his OS waveguides.

So, just because Earl doesn't like/use it doesn't mean it won't perform well. His OS waveguides do look theoretically attractive, but to a degree they're a 'finishing touch' - about getting that last 5-10% of the performance out of the system.

I also think onethis 15" cone + compression driver BMS coax looks on paper really neat for a big home monitor for use with a subwoofer, though I haven't heard it.

Yeah, I think those are interesting as well. I'm not sure how well they'll work in the nearfield, though. I suspect the cone relies to a degree on diffraction around the horn to fill-in the center of the soundfield, and it might need a bit of space to develop. Still, with a 60degree conical horn, it should mate reasonably well with the directivity of the 15 at the 1-1.2k xover, so in a larger-ish room it might work very well.


My personal track has put my Unity-concept experiments aside for the moment (I think the Unity is truly the ultimate coax-arrangement, but it's tough to get right) and am looking at something like this EAW setup: http://www.eaw.com/technology/proprietary/va4.html I'm looking at using a waveguide-loaded conventional tweeter though, to minimize the cross-section impact on the midhorn. Preliminary experiments are very promising - with my DCX2496, I got something that sounded way better than my Unity experiments after a couple hours of fiddling.

 

[Pallas]

I always thought that pattern control was one of the reasons why I almost always prefer coaxes over separate drivers. (I've not heard many GOOD systems with low-cutoff horns crossed over low to high-quality pro-audio woofers, as Dr. Geddes sells; the only one I really liked was the aforementioned 90x40 BMS 4590 with a 15" Beyma woofer and a huge Cabasse subwoofer. I would love to hear the Gedlee speakers....)

I like big coaxes much more than small ones, but even when I was shopping for inexpensive bookshelf speakers in the late 1990s the only one that sounded like it had decent pattern control (no horns in the power response) was the KEF Q15, and accordingly that's what I bought. I've also heard the same from Gradients, which use a Seas variant of the KEF Uni-Q.

Also, the Tannoys that Gordon W. introduced me to at Audio Atlanta and the DIY2003 gathering (the Tannoy D500's were the star there for me) sound like they have very good pattern control, with a consistent soundfield as you move around the room that differs in treble extension but not basic character.

Those EAW speakers look very interesting. Check your e-mail

 

[dwk123]

I think a coax can be engineered to have fairly uniform/smooth power response, but remember that Geddes is an academic/theoritician, and so 'good' isn't necessarily good enough.

Done properly, it should be possible to put the xover in a coax at the point where the directivity of the woofer matches the tweeter. If we follow the Geddes model, this would be ~1k for a 15" and maybe 1.5k for a 12".

I think the practical problem with this is that in many coax drivers the tweeter horn isn't large enough to provide true directivity down to the low end of the spectrum. On my Tannoy System 600s for example the tweeter 'waveguide' is only a couple inches long at most, and only maybe an inch in diameter. Contrast this to Earls waveguide in his speaker, which is 15" in diameter. This is one reason why those BMS drivers look interesting - the horn might actually be big enough to actually work.

 

[GordonW]

WRT: the Tannoy tweeter being relatively low power handling- I can attest to that. I actually managed to kill one element, during "abuse testing" of the speaker I did. Mind you, it was at levels that pretty much NO ONE would find tolerable for more than a few minutes in a home setting (unless you, like some people around here... *cough... GM... cough* happen to have had Danley servo-subs in your home ).

For the application I've been designing for, the Tannoys seem to be pretty much up to the task, power-handling wise. I work for a dealer that has carried Tannoy DC-drivered speakers for the last 15 years, and I can count on one hand, the total number of replacement tweeter elements we've ever had to order for the Saturn, Definition, Dimension, Sensys, Eyris, Churchill, Prestige or any other dual-concentric. I'm talking hundreds of pairs that we've sold, in use, with only 3-4 total tweeter failures... and some of these speakers are owned by DRUMMERS, for heaven's sake! (insert obilgatory apologies to any drummers reading this who don't fit into the normal musician's "drummer jokes" category, of course... )

Hence, my thoughts are to possibly take something like a Beyma or B&C driver, and modify it to use a directly-coupled dome element into the back of the bass driver magnet assembly (possibly including maching out the waveguide, to reduce the compression ratio, and such). There are some pretty efficient dome drivers out there, that behave reasonably well (Fostex, Vifa, etc)... and with the fact that the waveguide/horn load provided by the flare inside the coax, INCREASES their low-frequency output substantially (this is what allows Tannoy to use a 1.4KHz acoustic crossover point... it's about a 4kHz electrical network)...

Specific questions come to mind, about the B&C coax- is it a curvilinear cone, or straight-sided like the Beyma? I'd much prefer to stick to a curvalinear... as it seems to GREATLY improve the tweeter on-vs-off-axis energy dispersion, and makes the frequency response of the tweeter more "linear" (in the sense, that a crossover can be designed with minimal components, to achieve flat response... not many "fixes" are required to compensate for response discontinuities and drop-offs).

I'm also tempted, to see if Eminence might be able to build the Beta 8 CX into some sort of cast basket, one with low-profile basket legs. This would greatly help me decide the Eminence is a good idea- currently, with the stamped steel basket, there is far too much flat metal in the basket legs, reflecting sound back at the woofer cone...

I'm also not ruling out the possibility, of building/reconing/modifying a driver to do what I want. I've reconed enough speakers to be confident in my ability to assemble driver moving assemblies, pretty reliably... and I will only get better, with more practice, I would expect. So, even if I wound up with just the frame of something like a Beyma or B&C driver, and made up a custom cone assembly for it, that could possibly be a viable alternative...

Basically, my point here, is to try to find a driver where I'm not at the mercy of "spare parts" from a manufacturer NOT in the business of building raw drivers (but instead, finished systems). In actuality, I got TWO of the THREE spare-part Tannoy TDC 8" dual-concentric drivers in the western hemisphere, apparently... that kind of makes any sort of repeat build, rather difficult...

Regards,
Gordon.

 

[GordonW]

I think the practical problem with this is that in many coax drivers the tweeter horn isn't large enough to provide true directivity down to the low end of the spectrum. On my Tannoy System 600s for example the tweeter 'waveguide' is only a couple inches long at most, and only maybe an inch in diameter. Contrast this to Earls waveguide in his speaker, which is 15" in diameter. This is one reason why those BMS drivers look interesting - the horn might actually be big enough to actually work.

Well, if you really get into the Tannoy DC drivers, the tweeter does hold up pretty well, right down to 1.4KHz, where it "hits the wall", at least on the bigger (8" and larger) DC models. This is, as far as I can tell, because the tweeter actually uses the neck and body of the cone, beyond the waveguide (the cone flare does almost exactly match and continue the flare of the outside of the waveguide), and bolsters the bottom end response and pattern control. In actuality, the 8" DC driver acts as if it's got an 8" round conical horn attached to the tweeter... which easily can hold pattern control down to a pretty decently low frequency. It's still got a "mouth radius" of a half-wavelength, at 1.4KHz. Especially if, like in the Tannoy Prestige models, the driver itself is mounted into a "flare exension" which further extends the "flare"... this further extends the lowest point where pattern control can be upheld...

 

[Pallas]

Hence, my thoughts are to possibly take something like a Beyma or B&C driver, and modify it to use a directly-coupled dome element into the back of the bass driver magnet assembly (possibly including maching out the waveguide, to reduce the compression ratio, and such). There are some pretty efficient dome drivers out there, that behave reasonably well (Fostex, Vifa, etc)... and with the fact that the waveguide/horn load provided by the flare inside the coax, INCREASES their low-frequency output substantially

FYI, RCF Acoustics has an 8" coax designed to accept a conventional dome tweeter. It's called the PAC 81 CX, and has a cast frame with a carbon fiber cone that looks to have some curvature to it. IIRC, Solen carries RCF, though I don't know if they carry this specific driver or how much it costs.

As you mentioned, the waveguide substantially increases the low end of the dome. According to the Hobby HiFi review of the RCF piece (2/2002, pg. 62-3), the included dome tweeter (of which they weren't complimentary) rises ~12dB from 5kHz to ~1.4kHz, below which it falls off like a rock.

As for your other question, I'm 99% sure that the B&C 8CX21/Hi-Level RX200 has a straight-sided cone.

 

[GordonW]

Huh, that RCF sounds interesting. Especially since RCF now is, for all intents and purposes, an "independent" speaker manufacturer again (once they split away from Mackie). Probably much more likely to keep something like this in the line for a while...

I'll have to check it out... maybe my pro-audio contacts here in Smyrna GA might know where to find RCF more near-to-home.

Regards,
Gordon.