Detriments of a coaxial woofer?

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ReDress

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Joined 2014
#1
Hey everyone,

I've recently fallen in love with the idea of using a coaxial woofer in one of my plans, having read of their benefits of providing a point source - which is quite attractive to me for whatever reason. However, I'd like to know - what are the biggest downsides to using a coaxial configuration vs a component set-up (I'm aware of the intermodulation distortion element of it, but if one were to highpass the coaxial woofer out the bass range and solely as a mid woofer, couldn't that help alleviate some of the damage?)

But I digress, like I said, I'm mostly interested in the negatives of a coaxial configuration vs a traditional component set up.

cheers.
 
#2
As far as IM might be problematic, I suppose high-passing the cone would be a fair way of dealing with this.

I imagine a lot of the coax drivers out there might have rather poor HF waveguide/horn, including driver-throat and horn-cone transitions, leading to reflections. Eminence drivers seem to be a fair value, with a rather wide range of opinions on them. Radian and B&C is where I'd put my money if the cost wasn't above what I can justify spending on mere playthings, seeing how I'm not short on speakers or raw drivers.
 
#7
POOH said:
In a 6" I've been eyeballing these

P.Audio System Co., LTD

The best 6" coax I've personally used is the Tannoy coax. They are often found used on ebay including the crossover. Don't let the ceiling speaker part fool you. They are very hifi.

Tannoy CMS 6 TDC C Ceiling Mount Dual Concentric Driver 6 5" Speaker | eBay
Click to expand...

I have been working with a pair of the Tannoy 8 dual concentrics. Did you modify the standard factory crossovers and if you did what changes did you make?

DonM
 
#9
I have used them with the factory crossovers as mid/treble in a secondary system and think the factory crossover is great. I imagine it could be better - the crossover assumes the speaker will be mounted in the wall so they do not filter for baffle loss

I use them now triamped with subs where the coax is mounted above me firing into a 2' convex horn reflector for above ambient channels. 🙂
 
#10
Hmmm...

From what I can tell, you've either got a compromised waveguide, or you mount a horn in front of the woofer cone, which will give an interesting off-axis response due to diffraction.

While the point source imaging is nice, the high-frequency oddities aren't to be ignored.

Chris
 
#11
POOH said:
I have used them with the factory crossovers as mid/treble in a secondary system and think the factory crossover is great. I imagine it could be better - the crossover assumes the speaker will be mounted in the wall so they do not filter for baffle loss

I use them now triamped with subs where the coax is mounted above me firing into a 2' convex horn reflector for above ambient channels. 🙂
Click to expand...

I cross my tannoy at 100hz to a sub and find that works.

Thanks for that feedback!

DonM
 
#13
chris661 said:
Hmmm...

From what I can tell, you've either got a compromised waveguide, or you mount a horn in front of the woofer cone, which will give an interesting off-axis response due to diffraction.

While the point source imaging is nice, the high-frequency oddities aren't to be ignored.

Chris
Click to expand...

That was my concern as well before I tried it. The Ciare 12" I use is definitely not as smooth as a seos 12 above 3k or so but it still manages to sound more natural and lifelike. The shop I use it in is 40x60x16. When I open the large doors and listen to 100% reflected sound outside the difference is still noticeable.

Maybe it's just the particular models I'm using, but diffraction is proving not to be the problem I was expecting. Will need to investigate this further.
 
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#14
Hi Ernie,

My experience has been with some 12" and 8" coaxials for PA use. They both look nicely done, but the 12" exhibits a 9dB bump at 8-9kHz, likely due to the very short horn in use. The 8" driver has a cone profile more continuous with the mini-horn (the bit in the middle), so its not such an issue.
If I can, I'll dig out my measurements of the two. Of course, they include the manufacturer's passive XO, which I haven't messed with yet.

Chris
 
#16
I did some measurements on the IM issue when KEF came out with UniQ. The issue is response variation with shifting woofer cone position. You measure the tweeter response and then DC shift the woofer in and out. With the KEF units the response was pretty consistent, so no IM.

At PSB I made some coax units with a waveguide on the tweeter mounted on a pivot in front of the woofer cone. Response was way better than any post mounted tweeter. Although the waveguide was fairly large it had almost no effect on woofer response.

The BBC did some interesting designs with tweeters mounted on a perforated plate in front of the woofer. JBL sells some pro ceiling speakers with the tweeter on a slotted waveguide.

Lots of fertile ground here.

David S
 
#18
#20
ErnieM said:
BMS looks like they're following Tannoy's lead. Smooth transition profile.
Overview
Click to expand...

Mehhh...
IMG_20150128_094711.jpg

BMS 5C150
 
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