[rick57]

Hi
Is this is an issue for most or all coax drivers? I'm wondering how good is the "horn loading" shape of the tweeter in a driver like the P.Audio BM-18CX38, which otherwise seems a great OB option

panomaniac’s experience with your recent OBs:
http://www.audiocircle.com/index.php?topic=60599.msg541181#msg541181

JB you mentioned
http://www.audiocircle.com/index.php?action=search2
the 18CX38 having a dual stage radial phase plug, and an acoustic lens.
I could easily be misunderstanding those terms but in the photo http://www.p-audio.co.uk/products/db_product_1_6_bm-18cx38.htm (or the smaller BM15 CX38 at http://www.loudspeakersplus.com/html/paudiocoax.html), I can't identify an acoustic lens - is it behind a dustcap?

Or is the waveguide for the tweeter is the woofer's cone? And if it is, that wouldn't be an optimal shape, and would cause the difficulty getting a really smooth crossover, especially at the lower end of the tweeter's range.
Might the CXHA range (shown at http://www.loudspeakersplus.com/html/paudiocoax.html which clearly have  waveguides), *if it's well designed, have a smoother tweeter’s low end?

Hawthorne Audio's Sterling Silver Iris 15" coaxial uses a Radian high frequency driver 'rated' for an XO of 1000 Hz, but needs to be crossed at about double that as its loaded by the woofer’s cone.

Are any 15" coaxials that are known to use a *well designed (if not that large = higher XO, less gain) integral waveguide?
Or for it to be well designed, is a separate waveguide better?

Thanks

[pbrstreetgang]

I assume you know about the Altecs, Tannoys and Emminence

[panomaniac]

These are very good questions.  I'll answer what I can.

• Acoustic Lens
  If there is one, it's behind the dust cap, for sure. In fact, it may be the dust cap. JB will know more. The horn seems to have very good off-axis response, so there must be something going on.
• Wave guide
  On a driver like this, the woofer cone is the waveguide.  In the case of this driver, that's a pretty big waveguide! Sure, using just the woofer cone would seem less than ideal - but here it does work
• Crossover
  Not to worry. The HP filter to the horn is well within its range - at least an octave higher.

I do understand your concerns with the horn/waveguide thing.  Because I had exactly the same worries!  I was working on a design using one of the P.Audio 12" coax, sort of a "little boy" with helper woofer.  I wanted to get one that had a "real" horn like a Urei or Altec.  So I was alarmed when JB bought the 18s because they just use the cone.

Well, I have not compared the P.Audio drivers that use a short Altec style horn vs those with no horn, but I have to say that the 18CX38 works very well. Someone did a great job on that driver.  It may simply be that the big compression driver and the big cone waveguide work well together.

I don't know if that helps much, but that's about all I know at the moment.

[rick57]

I know of Altec, Tannoy and Eminence coaxs. All coaxs would have similar issues.
The fact that the cone profile is a compromise between what’s good for the woofer and what loads the horn, would cause the tweeter’s response to be less even, which would I think account for the challenge JB is having getting the XO right.

Hi Pano

Thanks for your response.
As I think there’s more knowledge of waveguides at diyaudio, I think it’s better to respond there
http://www.diyaudio.com/forums/showthread.php?s=&postid=1642716#post1642716

Cheers

[Duke]

I would lean towards a coax that does not have a separate internal horn or waveguide.  For one thing, while not perfect the edge of the woofer cone and adjoining frame/baffle is probably a better "mouth" than is the sharp edge of an unbaffled horn or waveguide.  Next, using the woofer cone should give good pattern control down to the crossover frequency, which I think matters.  Finally, the horn or waveguide itself is a diffractive obstruction in the path of the woofer's output, and may well be a source of coloration in the upper end of the woofer's range.

I'm a bit leery about the long "snout" that, in most cases, the compression driver's output must pass through to reach the woofer cone.  Big Tannoys and several B&C drivers use the same magnet for both woofer and compression driver, which significantly reduces the snoutlength - but that format may be more prone to flux modulation.  Which is the more audibly significant issue, I do not know.

As for whether a separate horn or waveguide is better than a coaxial, that depends on the specifics and on what the top priorities are for a given application.  One of these days I hope to spend some time working with a good coaxial, as there is a certain undeniable elegance to that format.

Duke