Anyone used Beyma or B&C coax 8" drivers? - diyAudio
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Old 3rd March 2005, 01:56 AM   #1
GordonW is offline GordonW  United States
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Default 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.
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Old 4th March 2005, 12:38 PM   #2
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Default 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
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Old 4th March 2005, 01:02 PM   #3
GM is online now GM  United States
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Greets!

FYI: http://www.audioroundtable.com/HighE...ages/1834.html

GM
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Old 4th March 2005, 01:09 PM   #4
miguel2 is offline miguel2  Portugal
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Hi,

Which Beyma are you referring to?

Miguel
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Old 4th March 2005, 09:00 PM   #5
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Default Beyma 8BX

The driver I was referring to was the 8BX.

Rob
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Old 4th March 2005, 10:44 PM   #6
ScottG is offline ScottG  United States
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Default Re: 8" Coaxial

Quote:
Originally posted by cdnsoundguy


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
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Old 5th March 2005, 07:57 AM   #7
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Default 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
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Old 5th March 2005, 03:38 PM   #8
GM is online now GM  United States
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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
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Old 6th March 2005, 09:09 AM   #9
Pallas is offline Pallas  Pakistan
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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.
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Old 6th March 2005, 02:10 PM   #10
GM is online now GM  United States
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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
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