# New Coaxial from Celestion



## Patrick Bateman

Celestion is getting into the coaxial compression driver game.

When I first read the press release, I got the impression it might be a BMR-type driver. But after looking at the cutaway, it appears to be very very similar to the BMS coaxial compression drivers.

It's interesting that there are now *three* companies all using a similar design:
BMS
JBL
and now Celestion

One of the interesting elements is that they claim that there's no crossover required. Most have assumed, probably correctly, that they mean that there's no *additional* crossover required, IE, it's probably built into the compression driver. But back in the 90s I recall there was a crossover-less coaxial, so perhaps it's made a comeback?


----------



## dgage

Do you have a model # and rough cost?


----------



## thehatedguy

AxiPeriodic Drivers - Axi2050 - Celestion - Guitar, Bass & Pro Audio Speakers


----------



## SPLEclipse

140dB Sensitivity. I'll take 10 please.


----------



## Patrick Bateman

dgage said:


> Do you have a model # and rough cost?


It's a neat driver, but it looks like we'll have to wait for QSC to stick it in one of their models. Because it looks to be an 'OEM' item. Based on the talk on the forums, the price will be around $375ish. (If you can find it.)

It took me close to five years to scare up a set of Celestion midranges, I stumbled across them at Loudspeakers Plus and bought every one they had. (all four of them, lol)


----------



## oabeieo

Pretty cool, too bad there's not a small format co-ax 1" exit specific for 500-20k 
Minus the d2 but something along those lines would be intresting , 

No crossover , 50$ and my left nut says there's a really crappy electrolytic cap buried in there somewhere unless it's phenolic. Which in that case I could see it getting down to 300 but wouldn't have 140db sens. 

Curious as to its materials


----------



## dgage

I was partially wondering as I'm working on a HT speaker (OEM) but the top-end rolloff doesn't really make it a viable contender to the BMS coaxial CDs in my opinion. And I just have a hard time believing that CD can be 140dB sensitive and also play down anywhere close to 300 Hz. If it could then maybe it would be worth trying to get a super-tweeter or similar to assist on the top-end but I'd have to test it to believe it and with the difficulty of trying to source, not sure it is worth the effort. If anyone has experience with Celestion drivers, please let me know if I'm off base on questioning their specs.


----------



## thehatedguy

The 140dB was/is on a plane wave tube, which is always going to be the highest sensitivity possible for a driver. Put it on a horn, and the numbers will come down...the wider the dispersion, the lower the number.


----------



## Kevmoso

Neat driver but it doesn't look like a coax to me. 
Page 6-7 http://www.tubeampdoctor.com/pdf/Celestion/Celestion_Pro_Speaker_Catalogue16.pdf


----------



## thehatedguy

Yeah, I didn't think it was either


----------



## Victor_inox

two voice coils loaded to two independent diaphragms coaxially- that is coax driver to me unless I`m missing something.


----------



## subwoofery

Freq resp looks good. Would like to see the power response. 
Disp pattern 90° x 40°

Kelvin


----------



## Kevmoso

And in the same paragraph says "Axi2050 uses a single, patented, large
diameter annular diaphragm..." 
Then looking at the exploded view I only see one voice coil. 
Konfusing

Edit: I don't see two coils stated anywhere. Neither of the two links provided in this thread say coax.


----------



## Patrick Bateman

oabeieo said:


> Pretty cool, too bad there's not a small format co-ax 1" exit specific for 500-20k
> Minus the d2 but something along those lines would be intresting ,


In the original documentation for the D2, JBL indicated that it was going to be an entire lineup of coaxials.

I wonder if they've backed off that commitment? There hasn't been a second or third driver announced, and the D2 has been out for years now.

I'd speculate that the BMS / Celestion solution may work better. Here's why:

To maintain the same level of output and go down one octave, you have to increase displacement by four. The JBL D2 has a second diaphragm, but they're both playing the same bandwidth. *So the second diaphragm only 'buys' you about half an octave more bandwidth.*

In the Celestion and BMS designs, the second diaphragm is larger and covers a lower frequency range, it's basically two compression drivers in a single unit.

This might be the reason that TAD loves beryllium. Basically there's two ways to extend the bandwidth of a compression driver. You can add another diaphragm, like JBL, BMS and Celestion do. Or you can use *one* diaphragm that's larger, and make the diaphragm out of beryllium.

It's too bad beryllium is so darn expensive. 


oabeieo said:


> No crossover , 50$ and my left nut says there's a really crappy electrolytic cap buried in there somewhere unless it's phenolic. Which in that case I could see it getting down to 300 but wouldn't have 140db sens.
> 
> Curious as to its materials


Yeah I think that's likely. 
The diaphragm is titanium. Aluminum usually offers wider bandwidth than titanium, but titanium is way more durable. That's why Radian / Emilar use an aluminum diaphragm with a mylar surround iirc. (100% aluminum would fail eventually, mylar makes it more durable.)


----------



## Patrick Bateman

Kevmoso said:


> And in the same paragraph says "Axi2050 uses a single, patented, large
> diameter annular diaphragm..."
> Then looking at the exploded view I only see one voice coil.
> Konfusing


Go figure, it looks like this *isn't* a clone of the BMS.

I read the press release, and took a cursory look at the driver, and assumed that it was *two* rings.

But it's not - there's one ring and one voice coil. So this *is* something new. Neat.










I marked up their pic, to show what I mean. There are two acoustic pathways *but only one voice coil and one diaphragm.*









In some ways this is the *opposite* of the JBL. The JBL has two diaphragms, two phase plugs, and one acoustic pathway. The reason that the JBL works is because everything is equidistant. This is key, as a pathlength difference of even half an inch would create comb filtering in the top octave.

If the Celestion only has one diaphragm, but has two pathways, than that would seem to indicate that they've designed the diaphragm so that the highs are coming off one portion of the diaphragm, while the lows are coming off the other. Perhaps by using BEM they were able to optimize the diaphragm to do this. (Similar to how small full range drivers work, but it's not a cone it's a ring.)


----------



## Victor_inox

Very helpfull Patrick-Thank you!


----------



## Kevmoso

oabeieo said:


> No crossover , 50$ and my left nut says there's a really crappy electrolytic cap buried in there somewhere unless it's phenolic. Which in that case I could see it getting down to 300 but wouldn't have 140db sens.
> 
> Curious as to its materials


I think you owe us $50 and a nut. Lmao!


----------



## oabeieo

Kevmoso said:


> I think you owe us $50 and a nut. Lmao!


Lol, yeah but that blow up doesn't show leads , which still doesn't answer how it can't use a crossover? So it's safe to hook it to a 300w sub amp? Or full range amp? Unless it is surround less and the titanium is 1/8" thick and sub frequencies just get absorbed into heat?? 

There's something going on ( unless I missed it) 


@Pat , so if there's 2 pathways one pathway is longer which would be higher GD numbers no? And potentially some phaseing issues where the pathways line up?


----------



## Kevmoso

oabeieo said:


> Lol, yeah but that blow up doesn't show leads , which still doesn't answer how it can't use a crossover? So it's safe to hook it to a 300w sub amp? Or full range amp? Unless it is surround less and the titanium is 1/8" thick and sub frequencies just get absorbed into heat??
> 
> There's something going on ( unless I missed it)


Quote from the pdf:
Essentially performing the role of two speakers in one, Celestion’s Axi2050 is a high
power, high-output driver delivering a frequency range of 300Hz to 20,000Hz without
the need for a crossover.

I interpret that as still needing a high pass crossover at 300hz (or whatever low frequency cutoff the horn it gets coupled to will allow.) It is a tweeter after all. I wouldn't hook up a seas millenium without a crossover either.


----------



## oabeieo

Kevmoso said:


> Quote from the pdf:
> Essentially performing the role of two speakers in one, Celestion’s Axi2050 is a high
> power, high-output driver delivering a frequency range of 300Hz to 20,000Hz without
> the need for a crossover.
> 
> I interpret that as still needing a high pass crossover at 300hz (or whatever low frequency cutoff the horn it gets coupled to will allow.) It is a tweeter after all. I wouldn't hook up a seas millenium without a crossover either.


Ooh I get it now . Okay , so the Hf driver can just handle the abuse probably, so it's two VCs on a single diaphragm?


----------



## thehatedguy

I thought the diaphragm was similar to the b&w dual dome tweeter.


----------



## Kevmoso

thehatedguy said:


> I thought the diaphragm was similar to the b&w dual dome tweeter.





oabeieo said:


> Ooh I get it now . Okay , so the Hf driver can just handle the abuse probably, so it's two VCs on a single diaphragm?


It looks like only one voice coil and one diaphragm to me. And it only shows one magnetic circuit.


----------



## thehatedguy

Maybe it was Tannoy...

But one used a dome and glued another dome that was like a egg with the top cut off behind the visible dome.


----------



## dgage

Kevmoso said:


> Quote from the pdf:
> Essentially performing the role of two speakers in one, Celestion’s Axi2050 is a high
> power, high-output driver delivering a frequency range of 300Hz to 20,000Hz without
> the need for a crossover.
> 
> I interpret that as still needing a high pass crossover at 300hz (or whatever low frequency cutoff the horn it gets coupled to will allow.) It is a tweeter after all. I wouldn't hook up a seas millenium without a crossover either.


I'm not sure how they can even say the CD goes from 300-20,000 Hz with the way it drops off on the top end starting at below 10,000 Hz. I guess since it is OEM, they expect someone to DSP the heck out of it to make it flat. Or I guess they could spend forever trying to get a horn to match up or have a huge passive crossover network to try and get that thing flat. If the price is right and it isn't too hard to get them, I'd be willing to try them in an OEM speaker I'm working on but the thought of developing a passive crossover for it doesn't entice me.


----------



## thehatedguy

It is a 2" exit driver...and that HF performance is really good for such a driver.


----------



## Eric Stevens

dgage said:


> I'm not sure how they can even say the CD goes from 300-20,000 Hz with the way it drops off on the top end starting at below 10,000 Hz. I guess since it is OEM, they expect someone to DSP the heck out of it to make it flat. Or I guess they could spend forever trying to get a horn to match up or have a huge passive crossover network to try and get that thing flat. If the price is right and it isn't too hard to get them, I'd be willing to try them in an OEM speaker I'm working on but the thought of developing a passive crossover for it doesn't entice me.



In the pro world driver bandwidth is defined as its -10 dB point. It appears it meets that criteria. I doubt you would want to use this much below 500 Hz in the real world.


----------



## Eric Stevens

Clearly not a Coaxial driver, it is descrbed by Celestion as "single annular titanium diaphragm, integrated suround"


----------



## dgage

thehatedguy said:


> It is a 2" exit driver...and that HF performance is really good for such a driver.





Eric Stevens said:


> In the pro world driver bandwidth is defined as its -10 dB point. It appears it meets that criteria. I doubt you would want to use this much below 500 Hz in the real world.


I looked at the BMS 4594 coaxial compression driver with which I'm familiar in my personal home theater speakers and I guess the frequency response does look good in comparison, at least on the top end. Thanks for the feedback and clarifications.


----------



## Rybaudio

This is based on fairly recent research into compression driver design:

AES E-Library » Wideband Compression Driver Design, Part 1: A Theoretical Approach to Designing Compression Drivers with Non-Rigid Diaphragms
AES E-Library » Wideband Compression Driver Design. Part 2, Application to a High Power Compression Driver with a Novel Diaphragm Geometry


The classic/typical compression driver phase plug design dates all the way back to 1953 and a guy named Bob Smith

An Investigation of the Air Chamber of Horn Type Loudspeakers

His idea was to place and size the annular slots exiting the compression chamber at locations that suppress the effects of acoustic resonances in the compression chamber. Some slots are placed at nulls of the resonance - sort of like placing a subwoofer at a null to avoid exciting a room resonance. When that can't be done, two slots' relative sizes are adjusted to cancel each other - just like placing 2 subs in a room at different locations and adjusting their outputs to cancel a room resonance.

That design/idea has been used for many years, and it works good!

But now we have powerful computers with FEA simulation capabilities to look closer. Jack Oclee-Brown's research takes it a step further. He computes the mechanical resonances in the diaphragm as well as acoustic resonances in the compression chamber. He then computes the coupling

(mechanical resonances in diaphragm) --> (acoustic resonances in chamber) --> (outside world through phase plug)

to see how much each resonance effects the end performance. Not all resonances in the diaphragm matter, because they may not couple to the chamber or outside world. Similarly, not all resonances in the chamber matter because they may not be excited by the diaphragm. His framework allows you to see if something is likely to be a problem (coupling is good between all) and just how much of a problem.

This driver is supposedly designed using that framework. They ran a lot of simulations and dialed in the geometries to minimize exciting bad resonances and their coupling to the outside world. Seems pretty cool to me - will have to see if some of those ideas also trickle down to cheaper/smaller products and make a significant impact.


----------



## Patrick Bateman

oabeieo said:


> Ooh I get it now . Okay , so the Hf driver can just handle the abuse probably, so it's two VCs on a single diaphragm?


One voice coil, one diaphragm.



















I think most of the innovations here are in software. In a nutshell, Celestion used software to build a model of a ring radiator compression driver with a novel phase plug. 

The 'trick' with loudspeakers is that the bigger you make them, the lower they play, but you lose output at the top end. For instance, the B&C DE250 (second picture) has a 1.7" voice coil and struggles to reach 16000hz. The Celestion has a *five* inch voice coil - about four times the area of most subwoofers - and makes it out to 10,000Hz+

The obvious thing to do here would be to scale it down by about half, and make a ring radiator with a 2.5" voice coil that can cover 500hz - 20,000Hz. By reducing the size by half, you reduce the cost by over 50%. Plus, it makes it a lot easier to work with, the AXI2050 is the size of a small subwoofer, at 7.8" x 4.4"


----------



## oabeieo

So a large annular , with the lost mass it would be as light as a smaller dome, but larger outer ring to play lower and lighter-ish and annular should lessen break up for that size. 

I bet it would be a good player in the 1.4"-2" world. 

I wonder how a single large annular would do as a dvc, one on outer and one on inner with a fold in diaphragm to separate the movements to some extent .


----------



## Patrick Bateman

I'm too cheap to pay for the AES paper, but there are some hints in the press release.

For instance, it mentions that the width of the annulus is more important than the diameter.

This might explain why the voice coil is so large. 

Here's some maths:

1) The area of a 1" dome tweeter is 5.07cm. 

2) The area of a ring with an outer diameter of 13.97cm (5.5") and an inner diameter of 11.43cm (4.5") is 50.7cm.

I'm not sure if it was intentional, but if they're using a 5" voice coil, and a ring with an outer diameter of 5.5" and an inner diameter of 4.5", that gives them the output of ten dome tweeters.


----------



## Rybaudio

The driver in the paper (presumably the one this thread is about, but they could have done some tweaks before production) has a diaphragm diameter of 125 mm and width of 36 mm. That gives an ID of 53 mm. So figure 5" OD and 2" ID.

It also has a max displacement of 1 mm which is pretty stout for a compression driver that's intended to be used all the way up. The paper shows a max SPL vs frequency graph of this driver compared to a coax driver and the new one does similar on the top end but has way more usable LF output - usable down to 200-300 Hz. Distortion comparisons are also shown which support this.

Overall, it looks pretty bad ass - if the roughness above 10k isn't an audible issue, you have a driver that can cover 200 Hz up with authority for home/hi-fi purposes. In pro sound, probably more like 300 or 400 up.


----------



## oabeieo

Rybaudio said:


> The driver in the paper (presumably the one this thread is about, but they could have done some tweaks before production) has a diaphragm diameter of 125 mm and width of 36 mm. That gives an ID of 53 mm. So figure 5" OD and 2" ID.
> 
> It also has a max displacement of 1 mm which is pretty stout for a compression driver that's intended to be used all the way up. The paper shows a max SPL vs frequency graph of this driver compared to a coax driver and the new one does similar on the top end but has way more usable LF output - usable down to 200-300 Hz. Distortion comparisons are also shown which support this.
> 
> Overall, it looks pretty bad ass - if the roughness above 10k isn't an audible issue, you have a driver that can cover 200 Hz up with authority for home/hi-fi purposes. In pro sound, probably more like 300 or 400 up.


So like what was mentioned earlier , we wouldn't use it past 500hz in most situations, if we built a 2" horn for a car horn would be pretty big to get to 500hz 

Maybe if they release them on the cheap at first I'll grab a set for the shop stereo


----------

