“Loading” of tweeters in large drivers Coaxes - diyAudio
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Old 27th October 2008, 01:09 PM   #1
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Default “Loading” of tweeters in large drivers Coaxes

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 driver (for OB or other)

panomaniac’s experience:
http://www.audiocircle.com/circles/i...1181#msg541181

While I havent seen it in specs for the driver, http://www.audiocircle.com/circles/i...action=search2 mentions it having a dual stage radial phase plug, and an acoustic lens. I could be misunderstanding those terms but in the photo http://www.p-audio.co.uk/products/db..._bm-18cx38.htm (or the smaller BM15 CX38 at http://www.loudspeakersplus.com/html/paudiocoax.html), I can't identify an acoustic lens. Might it be sitting 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 difficulty getting a really smooth crossover, especially at the lower end of the tweeter’s range.

Might P.Audio's CXHA range (shown at http://www.loudspeakersplus.com/html/paudiocoax.html which clearly have waveguides), *if its "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 it's loaded by the woofer's cone.

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

Thanks
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Old 27th October 2008, 06:12 PM   #2
Pano is offline Pano  United States
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Hey Otto,
You can see my response (FWIW) over on the Audio Circle OB Forum.

I'd like to hear what others have to say, too.
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Old 28th October 2008, 01:05 AM   #3
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Hi Pano

If the cone (18”) is the waveguide, its a decent size waveguide, might be good down to about say 600 Hz.

A woofer cone is also relatively “shallow” for a waveguide, which I believe is often better than a high rate of expansion. Maybe that’s why you find the off-axis response is good.

OTOH, quite likely the cone profile is a compromise between what’s good for the woofer and what loads the horn. That would cause the tweeter’s response to be less even.
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Old 28th October 2008, 03:35 AM   #4
Pano is offline Pano  United States
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Yeah, 600Hz sounds about right. Maybe the baffle takes it down a little further.

I would assume that the woofer cone isn't the perfect waveguide - but it does seem to do pretty well. Much better than I would ever have thought.

Crossovers can be tricky no matter what. But I think this horn can be tamed. =) It already sounded much better than most, I just wanted to hear "perfection." The insane amount of fast flutter echo in the hotel room didn't help matters any. That flutter may account for a portion of the slight midrange trouble we were having. Only time and hard work will tell!
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Old 28th October 2008, 03:22 PM   #5
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I've been very interested in the dual concentric type coaxial loudspeakers like these for quite some time now, but still haven't taken the plunge, for a number of reasons.. mainly just leery/skeptical about several things regarding the design of these drivers, and the impact these things probably have on the sound. Some of these issues have already been mentioned, such as the "snout" or tube referred to by Duke on the OB forum at AC, which panomaniac linked to above..

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."

For what must have been the last couple of years now, I've often pondered all the Tannoy style coaxial drivers from several pro loudspeaker manufacturers, including: these P.Audio's, Tannoy, BMS, B&C, Beyma, Radian, Hemp Acoustics, JBL, Eminence, Hawthorne Audio, SammiSound, Atlas Sound.. and probably tons more that I can't remember right now off the top of my head.

One thing I think pretty much all these types of drivers have in common is that the frequency response of the high frequency driver is very ragged. Think along the lines of a large fullrange driver with a whizzer.. sometimes even worse, and probably less predictable. From what little I've been able to observe during all this time, I think this is most likely due to a combination of things, all of which are detrimental to the resulting output from the high frequency driver.. Like the long "snout" mentioned earlier that any sound from the high freq driver must pass through. Then, there's a far from optimal cone shaped "thing" attached to the end of that "snout" that sits just behind the coaxial driver's dust cap (if it has one, and most of these pro coaxial units do, unfortunately). From there, there's a nasty transition from that little round metal cone shaped thing to the woofer's cone, and often over the woofer's voice coil former in between (yuck!). Then of course, you have a suboptimal shape for a large waveguide, which just so happens to be whatever woofer cone is used.. and then there's the bumpy surround, rubber/foam/cork gasket thing, screws etc, and then any small gaps between the woofer's frame and the baffle that it's mounted to. From what I can tell, I've always assumed that most (if not all) of these loudspeaker manufacturers haven't put much (if any) effort into custom designing woofer cones to be used as waveguides for these drivers. I might be wrong here, but it seems to me as though the vast majority just use whatever woofer cone for T/S parameters etc (or whatever), and whatever the high frequency response ends up being is basically what you get, without much effort put into changing it. I might be wrong about that though, but that's just what it looks like to me. Tannoy, on the other hand, is probably a different story, although I still question some of their pro models and in-ceiling products.

Some other things that might be worth mentioning.. Correct me if I'm wrong, but I always just assumed that any conical waveguide that was anywhere close to being constant directivity would have the typical falling frequency response, following the increased output from loading of the horn. Strangely, this isn't seen on spec sheets for most of the coaxial drivers from the manufacturers listed above, except for a couple of the smaller ones where it can be seen a bit.. like the 8" P.Audio BM series coaxial, as well as a nice looking 8" coax without a dustcap from BMS, if I remember correctly. As for pretty much all the others, I don't think this is the case. The response just keeps going everywhere, really ragged. With those smaller coaxials where the HF response droops down, it's ragged too.. but maybe not quite as bad. This is mostly all observed just from looking at manufacturer spec sheets BTW, as I haven't seen very many measurements from 3rd parties, maybe a couple, don't remember.

Also, I've seen a couple models from B&C and Beyma where they somehow integrated the HF driver into the same motor assembly as the woofer. Seems like this might eliminate some problems caused by traveling through that long "snout", but from the looks of the spec sheets, the response still looks pretty awful. I wonder what's going on here?

What's that funky looking metal thing that Tannoy uses in the "snout" of so many of their drivers? I came across something about it a while back, but I can't seem to find it now. I'm sure it was something to improve the HF response, and I'm guessing it must have worked, for the most part.

Anyway, it's really coincidental to me that this thread was posted.. not only because of the topic of coaxial drivers, but also because the P.Audio BM-18CX38 has been the main driver of discussion. For the last several days, I've been looking at that particular driver (again) very closely. If I remember correctly, I was modeling T/S params for it again real quick in WinISD Pro a few nights back, and realized I could probably get a pretty good amount of output from it in a sealed enclosure for use with subwoofers.. without having to use a highpass filter on it. Of course, it'd only go down to 80Hz 2nd order though. But at "normal" listening levels, some EQ could probably be used to get it a little lower if I wanted to cross to subs at say 60 or maybe even 50 Hz instead of 80. Without the highpass filter, group delay would be much lower, and I figure it'd just sound a lot better overall in that range.. as long as there's a good way for me to have full control over the phase alignment of the subwoofers. I dunno.. was just a thought.

I'm sure I can think of more to post, but I gotta get back to some other stuff for now. I might have some PDF's and/or images to add that might be of interest too, I'll have to look.

Anyhoo, if there's any way someone could get ahold of some good measurements of that 18" P.Audio coax (or any of them for that matter) and post them, it'd really make my day.
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Old 28th October 2008, 03:31 PM   #6
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how about converting this beast into coaxial
http://www.pyleaudio.com/itemdetail.asp?model=PDW21250
Remove dustcap and attach compresion driver with horn to pole piece. I plan to do it after finishing other 4 projects
P.Audios look very nice but they are quite expensive.
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Old 28th October 2008, 03:42 PM   #7
BHTX is offline BHTX  United States
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Quote:
Originally posted by MisterTwister
how about converting this beast into coaxial
http://www.pyleaudio.com/itemdetail.asp?model=PDW21250
Remove dustcap and attach compresion driver with horn to pole piece.
..haha, I've often thought about that too. I'm just not sure what I'd use for the horn tho. I can hardly make enclosures, much less fabricate anything like that, so that's probably out of the question for me. Any ideas? What are you planning to use for the horn? Whatever it is, a 2" exit HF driver would have to be able to go pretty low, probably crossed at 600-650 Hz at the highest, and I like at least an octave of flat response below that. That might be asking too much though, I dunno.

And yes, I agree.. the big P.Audio coaxes are expensive. It's tough to take such a risky plunge into the unknown, especially after you've done it far too many times in the past.

edit: For some reason I was thinking that 21" Pyle had a 6" voice coil, but it's only 4". In that case, I wonder how you'd find a small enough driver that'd go low enough to cross to it. Probably impossible without mounting it some other way, too far from the 21" driver, and then you'd have a difficult time trying to use the 21" cone as a waveguide too. Hrmmmm..
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Old 28th October 2008, 04:40 PM   #8
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BHTX, I haven't thought about details yet.
many compression drivers have overall diameter a little less than 4in. maybe it is possible to mount compression tweeter to the back of woofer, firing through vent hole into attached horn.
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Old 28th October 2008, 05:08 PM   #9
Pallas is offline Pallas  Pakistan
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Quote:
Originally posted by BHTX
[B]One thing I think pretty much all these types of drivers have in common is that the frequency response of the high frequency driver is very ragged. Think along the lines of a large fullrange driver with a whizzer.. sometimes even worse, and probably less predictable.
While I'll grant you that the top octave isn't generally as clean as one finds from better domes flush-mounted, in the better designed ones it's not that bad. Here's Stereophile's 30deg averaged response of the Tannoy TD12, for instance.
Click the image to open in full size.

IMO, the benefits in midband power response are worth the less-than-stellar top octave.

Quote:
Originally posted by BHTX
[B]What's that funky looking metal thing that Tannoy uses in the "snout" of so many of their drivers? I came across something about it a while back, but I can't seem to find it now. I'm sure it was something to improve the HF response, and I'm guessing it must have worked, for the most part.
There's some propaganda about it in this paper.

Here's a color cutaway:
Click the image to open in full size.

I can't say anything about other coaxes, because except for a brief interlude with the 8" B&C I really only have experience with the Tannoys.
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Old 28th October 2008, 07:09 PM   #10
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Hello All....

Interesting conversation on the P. Audio BM18CX38.


OK... this will likely be a long thread... much to talk about. Alot of what I will say is pretty well known... so I hope I do not bore too many with this. Plz forgive spelling-syntax errors!!

After fooling with... or should I say experimenting with many coaxials over the past 40 years... mainly the large 12-18" pro versions... including EV, Altec, Selenium, Tannoy (Very long ago) and now P.Audio, I would like to share the following.

One thing I have learned is that any coax that has a decent design can be tamed... it comes down to mostly crossover and of coarse tuning that crossover to suit the users intended application.

Most manufactures that supply or advise a recommended crossover assume the main use of these drivers is high output pro... commonly for PA and or monitor work. So the main crossover design compromise is balancing power handling (durability) with sonic signature. This usually means taking the LF section up as high as possible. Of coarse this is not desirable in a small room application... at low to modest play back volumes in small room near field listening conditions....

And ... in the past... nearly all manufacture supplied crossovers did not fully correct for the horizontal offset of the voice coils... so the outcome was not phase correct through the cossover region... The last of the Altec crossovers got close to getting it... Jeff Markwart does a great job of explaining this and his phase correct crossovers work VERY well.

My interest in the BM18X38 is in OB use... so my crossover solution is geared toward the special considerations OB requires. I do not want to get into all the design aspects of this kind of crossover at this time. But one of the main considerations is the performance of the HF section.... what it naturally does and does not do well. And key to that is the loading effects of the woofer cone and baffle combination. The BM18's I have worked with have HF FS in the low 500 hz area. And from crude measurements they really get up to over 18Khz on axis before a quick death. And with out any crossover they can do well up to about 12 Khz or so at 30 degrees off axis. This is better than a lot of dedicated dome and cone midranges mounted on a typical baffle. So... it was obvious to me that with a proper crossover combined with the wave guide effects of the large woofer cone and the output ability of the HF driver were conducive to... and could deliver excellent power response in a near field envionment.

Add the nearly 16" diameter of the cone and some baffle area ... say 800-1000 sq. inches... in a home hifi situation the HF will work from a practicle sense down to under 600 HZ. And is fully loaded from about 780 HZ up. This makes getting the crossover right much easier. Maybe more on this in another post.

As for the HF section... it... by modern considerations is pretty straight forward. Most if not all large format compression drivers use a radial (or similar) phase plug... with notched sides and exits. It appears the BM18 also has some sort of (fiberglass??) mesh at or near the exit of the HF that acts.... along with the dust cap as an acoustic lense of sorts. Once you establish a phase correct crossover.... this seems to happent around 950 Hz acoustic...Give or take... all of this come together to make for a speaker that can image with the best of them... regardless of design or type.

As for the drooping repsonse of the HF up above 10K... this is typical for most compression HF drivers and I think is more indicative of a mid broad mid band peak than a high end roll off. This also can be cured with crossover tricks... JM uses a voltage divider... I try to solve it with a staggard third order BW. Or a combo of the two. The important thing is to preserve the phase correctness of the crossover while taming the mid band HF rise. Maybe more on that another time to. This is already WAY too long.

Best...

JBSpeakerman.
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