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Crazy PA Coaxials for Hifi

voodooless

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... So, were going to talk coaxial again... Well, deal with it :cool:

We all know that KEF and Genelec (and there are a few others out there as well) make the top end of HiFi coaxials. They can be best used in 3-way systems, but obviously, KEF also has 2-way's. Obviously, 3-way systems add complexity and also need the extra woofer. But the other thing is: they are small. The best ones are 5.25" at best. That's not going to give you bass extension and SPL for a larger room.

So in come the PA coaxials. You can get them in all kinds of sizes, up to 15". For a 2-way, this has some advantages, the main one is that it drastically lowers the need for excursion, leading to less modulation of the tweeter signal, and thus less IMD. To further help with that, I would recommend putting the 2-way PA Coaxial in a reflex enclosure. This will significantly lower excursion in the bass compared to a closed box. As an example a beast from FaitalPro, the brand new 15HX500. Let's have a look at the excursion at 40Hz at the same 106 dBSPL. For this, the closed box (both same volume of 100L) needs significantly more power, about 100W more, so 130W vs 30W for the reflex version:

1656924538345.png

That is a big difference. The BR advantage will be there all the way up to 200 Hz, so It is quite significant. Obviously, you'll need to control the excursion below 30 Hz, but that should be done anyway. So, that 2 mm movement will get you > 106dB from 40 Hz on. Pretty good! It also means this thing will go loud! > 112 dB > 30 Hz o_O .

So these things seem to have some compelling arguments going for them. Obviously, the main question is: what about the dreaded coaxial frequency response and directivity issues? Well, here things get obscured quickly. Sadly one cannot find much information about this just about anywhere. You can find lots of people claiming they used a PA coaxial and that they sound great, but rarely does one find a frequency response, and even more rare is an off-axis frequency response :facepalm:. The manufacturers aren't helping either with their (lack of data)-sheets...

So time to try to consolidate all the information we can find on these things so we can make some more sense out of it all!

There seem to be two notable groups of coaxial available:
  1. Coaxials without a horn. These usually have a smooth transition of compression driver throat to the woofer voice coil. The profile of the woofer is usually shaped as a waveguide to provide a better off-axis response.
  2. Coaxials with a horn. These use a horn for the HF part. The horn then covers most of the woofer cone. This leads to some difficulties regarding crossover design because of the profound effect on the woofer response this configuration has. Obviously, the horn will give a much better HF response and will yield lower IMD because there is far less chance of modulation. Some horns are symmetric, others are not. Dispertion is generally less wide than the coaxials without horn.
Then there is a special category of devices from BMS, which are actually tri-axial drivers. These sport a coaxial compression driver, and a woofer in one, with also a horn added in for good measure. Only a glance at the frequency response makes one want to gouge your eyes out:
1657022982352.png

But possibly this can be very well tamed? Once again, data is lacking... I just hear they seem to sound fantastic... Oh, did I mention it is really expensive?

The FaitalPro 12HX500 (the smaller brother of the earlier referenced 15" version), seems to sport quite a smooth directivity:

1657023144828.png

Obviously, this is highly smoothed.

This 14" B&C also:
1657023249392.png

I especially like the between 12" and 15" size.

Arguably better than a horn-loaded version of the same brand:
1657023349336.png


This horn-loaded 15" 18 Sound also doesn't look too bad:
1657023481507.png


This Beyma seems to be a popular one as well:
1657023643144.png


So, this is just a fairly random collection. I did find some DIY projects with some of these. But I'll need to compile some more links. And many are for PA wedge speakers or tops. Essentially that should not matter as long as the data is decent. I'll add some follow-up posts with a few links coming days. For now though, this might already be some food for thought and discussion.
 
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LearningToSmile

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There's at least one non-PA commercial speaker I know of that uses a similar coaxial, the Monkey Banana Silverback "studio monitor":
180_144344335.jpg

But I was never able to found measurements(or even that many reviews) back when it originally caught my eye. 15 inch woofer, 3 inch compression driver.
 
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voodooless

voodooless

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1800 euros a pop :eek: No off-axis response data either. That horn looks to be a bit small-ish for a 1.5 kHz crossover point as well.
Thats a crazy looking speaker.

But how and if this can work?
You basically have just a very narrow window for the crossover:
23_15_05_360_6eGnGfep.jpg

Steep filters will help a lot here. The recommended crossover is 1.2 Khz, so right next to a major response dip. I would actually argue to stay below 900 Hz. For HiFi that is probably still fine.
 

tomtoo

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1800 euros a pop :eek: No off-axis response data either. That horn looks to be a bit small-ish for a 1.5 kHz crossover point as well.

You basically have just a very narrow window for the crossover:
23_15_05_360_6eGnGfep.jpg

Steep filters will help a lot here. The recommended crossover is 1.2 Khz, so right next to a major response dip. I would actually argue to stay below 900 Hz. For HiFi that is probably still fine.

Sure we talk about active xover and eq.
I just like the look, spray the horn golden. ;)
 

Plcamp

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I have often considered coax to replace 8” fullrange in my OB speakers. With two supporting 15” woofers, a 12”- 15” coax could operate with tiny excursion and match well. I got turned off of the idea by the messy response in the specs, and an apparently better approach to use an ~ 600 hz horn and compression driver.

I’m also not sure a bigger cone does much of anything to lower tweeter modulation, because regardless of the woofer cone diameter, you’re still moving same amount of air at a given spl.

Basically I am unconvinced coaxials can deliver bass and mid/highs at the same time. That’s the same problem I oerceive with my fullrange, although I expect coaxials are better.

And I am quite likely to be proven wrong on some of the above
 
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voodooless

voodooless

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Looks like Radian does provide some off-axis data:
1657026825012.png

Seems to be quite uneven around 1.2 kHz
 
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voodooless

voodooless

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I’m also not sure a bigger cone does much of anything to lower tweeter modulation, because regardless of the woofer cone diameter, you’re still moving same amount of air at a given spl.
Effectively, less movement is less modulation. So yes, it does work.
 

AwesomeSauce2015

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This is something I am interested in as well.
I mean, pro coaxials have incredible sensitivity and have very good SPL capability compared to traditional coaxial drivers. This would make them useful in home theaters and large spaces.
Edit: Also, CD-based coaxials can actually hit the 105db mark for reference level at distance without compressing the HF. So far only KEF's Q950 and Reference (X) Meta seem to be able to do that, and even then, a compression driver still gets way louder.
The only concern is whether we can smooth the "jagged" frequency responses. In theory, since directivity is good we could use EQ to fix any response issues. However, I would venture to say that most response issues may not be audible due to the very narrow nature of the peaks and dips.

Then we also need to consider that these graphs may not be representative. From what I've seen, it seems as though measuring HF horns can be tricky to get right, and if done wrong, you end up with horrendous predicted responses like we see here.

What I think we need to do is figure out how to interpret / represent the frequency response of compression driver / horn-based-coaxial speakers. Once we figure that out, we can move on to a potential design for hifi use.
 
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voodooless

voodooless

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Then we also need to consider that these graphs may not be representative. From what I've seen, it seems as though measuring HF horns can be tricky to get right, and if done wrong, you end up with horrendous predicted responses like we see here.
The same horn on a flat baffle would yield a much smoother response. Seems like the abrupt edge and gap to the woofer edge have a big influence?

One of the BMS coaxes has a nice soft edge on the horn, possibly to get a smoother transition?
1657137383487.jpeg
 

AwesomeSauce2015

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The same horn on a flat baffle would yield a much smoother response. Seems like the abrupt edge and gap to the woofer edge have a big influence?

One of the BMS coaxes has a nice soft edge on the horn, possibly to get a smoother transition?

You could solve the issue for woofers with a horn structure by smoothing the horn transition, or you could use a coax driver where the woofer is the horn waveguide...
Looking at a driver which seems to use the woofer as part of the waveguide: (However, I think there is still a horn of sorts hidden under the dust cap)
1657137801404.png

The B&C 15CXN88 seems to have a fairly smooth frequency response... But still not amazing.
Also, this driver seems to narrow the HF a bit which isn't great, but still, the FR is good.
And lastly, this coax driver suffers from the issue of being insanely expensive at $800 each... lol.

One other thing to note is that there is decent agreement in the 1-2khz region with both directivity and frequency response which should allow an easy crossover.
So all that being said; Pro (compression driver based) coaxial drivers certainly can work, you just need to find one that does work...


Edit: I wonder if someone who has pro coax drivers as part of their system could measure the in-room response? I wonder if the response might potentially even out in an actual room? That would explain why people like them.
 

gnarly

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Hi, I gave a prosound coax a try... the B&C 15cxn88. https://www.bcspeakers.com/en/products/coaxial/15-0/8/15cxn88-8

Goal was to mimic the Danley SM-80, but use a larger driver ported to get down lower than the SM-80..
I tried a foamboard wave guide like in the pict, but it didn't really work.
Got rid of the waveguide, slapped duratex and a grill on the box, and it now sits between two single 18fh500 ported subs up in my lake dock rafters .
Pretty good sound for a dock !

edit: haha AwesomeSauce....i see you posted about the 15cxn88 as i was typing !

coax horn try.jpg
 

TimVG

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Here's a build of mine around the RCF CX15N351




Pros: Powerful and relatively compact, given enough space they image incredibly well and sound huge. Relatively large vertical listening window (compared to non-coaxials)
Cons: Some more ripple in the response than high quality home loudspeakers. Dispersion is narrow and you don't want to move too far off axis horizontally or imaging suffers so don't expect to use these in the nearfield to good effect.
 

Katji

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This topic excites my pro audio modes. (Which includes aesthetics.)

... [subscribed]

...but I want DAC+amps in the box and network connection to control from computer. I guess that does exist, at the super-expensive real pro level.
 

AwesomeSauce2015

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What is plotted here exactly? Probably something very common, but I struggle to recognise it.
Excursion. I believe it's a ported box (green) vs a sealed box of a similar driver.
Basically, it illustrates that prosound coaxials can have massive output especially in the bass department... Especially compared to more traditional coaxials like the KEF Uni-Q
 

AwesomeSauce2015

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Hi, I gave a prosound coax a try... the B&C 15cxn88. https://www.bcspeakers.com/en/products/coaxial/15-0/8/15cxn88-8
Do you have in space measurements you could share by any chance? A sort of comparison between the datasheet and actual real-world performance?

Also, can you see some kind of horn structure under the dustcap? If so, does it seem to protrude out beyond the woofer cone or does it transition to the woofer cone?
(obviously not asking you to tear apart your speakers lol, just any visual observations would be appreciated as I try to identify the source of potentially jagged response - the sort which seems to be lacking in that B&C driver...)

Thanks
-Me
 
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