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Old 24th April 2007, 11:11 PM   #511
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Quote:
Originally posted by Lynn Olson


I'd be the first to admit that hemp cones really are the thing - so much better than those awful carbon-fiber, Kevlar, and metal cones, and they bring a whole new dimension to paper, getting rid of the "cone cry" and compression of ordinary cones.
Are there any woven kelvar cone speakers that get good reviews? I purchased a B&W midrange on eBay that has a curve-linear profile, woven with variable thickness kevlar cone... very good sounding and better behaved than metal cones. The technology to weave one continuously variable profile cone seems critical. The Auto woofers use cut Kevlar and carbon fiber sheets with straight profiles, and the glassy thermal resins seem to create fast moving high frequency noise.

Hot pressing paper or hemp fibers in a form seems like lower tech that more shops can afford.


http://www.bwspeakers.com/index.cfm/...7F00D0B7473B37
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Old 24th April 2007, 11:22 PM   #512
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Quote:
Originally posted by LineSource


Are there any woven kelvar cone speakers that get good reviews? I purchased a B&W midrange on eBay that has a curve-linear profile, woven with variable thickness kevlar cone... very good sounding and better behaved than metal cones. The technology to weave one continuously variable profile cone seems critical. The Auto woofers use cut Kevlar and carbon fiber sheets with straight profiles, and the glassy thermal resins seem to create fast moving high frequency noise.

Hot pressing paper or hemp fibers in a form seems like lower tech that more shops can afford.


http://www.bwspeakers.com/index.cfm/...7F00D0B7473B37
Well, if the opinion of $tereophile and Absolute Sound are important to you, Kevlar is the way to go. When I think of Kevlar, I think of the Focal Kevlar driver shown below, which was used in Wilson Audio and other extremely expensive speakers of the day.

It is significant that Kevlar and carbon-fiber cones are typically used with 24 dB/octave crossovers, which tells you something about the out-of-band performance. Reviewers who have never designed a speaker confuse the characteristic upper-mid harshness with "detail" - when it is nothing of sort, it's just a series of resonances in the upper-mid region.

After following the B&W link above, I find it a significant that after writing at considerable length about the superiority of their proprietary Kevlar cone, there are no impulse response and no frequency response curves of the driver by itself. If it is as good as they say, where's the raw, unsmoothed data of the raw driver? B&W certainly owns MLSSA and even more powerful measurement systems- where's the data to support the claims?
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Old 25th April 2007, 08:03 AM   #513
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Default Good vs Bad FR Curves

I was a little too quick to dismiss Kevlar and carbon-fiber cones - the ones I measured were some ten years ago, and were pretty bad examples of the breed. Surely by now, they've improved - I would hope so, after all the money that's been thrown at them by big-name vendors like B&W, Celestion, Focal, Audax, and others.

I admit when I see a low-efficiency 5" to 7" driver with severe resonances, I get annoyed - little drivers are easier to get right than big ones, and with efficiency in the very modest 86 to 89 dB/metre range, there's room for a couple of dB of efficiency to throw away on cone-damping techniques. There are plenty of smooth-measuring drivers in this size and efficiency category - although there are plenty of dogs too.

The reason for developing little drivers with rigid cones is to satisfy the market and the magazine reviewers, who are endlessly thrilled with each and every tiny speaker with "bass slam". While this might seem laughable to anyone that loves 12 and 15-inch drivers and horns, the reviewers really get off on the whole "bass slam" thing.

There's only one way to stuff a big-bass quart into a pint pot - linearize the gap geometry and magnet structure as much as possible (which can cost efficiency), select a 200-watt high-damping amplifier to overcome the low efficiency with brute electro-motive force, and use the most rigid cone possible (to minimize cone flexing in the midbass region). There's your slam for you - which is all about midbass performance. Car-audio vendors in particular have gone down this road to an absurd extent, with kilowatt amplifiers being nothing special anymore.

But nothing is for free. Rigid cones, although good for midbass, are more prone to standing-wave resonances, which exacts a cost at higher frequencies. This tradeoff goes unmentioned by the rigid-cone advocates, just as the converse tradeoff occurs with floppy poly cones in any size beyond 6 inches. Paper has the charm of being a good all-around compromise, and of course there are a zillion ways to add this or that composite to the mix, which is nothing more than a slurry of paper fibers. Paper itself can be made from ground-up fibers of any sort - wool, cotton, wood pulp, and the historic favorite, hemp. I'm surprised nobody's tried papyrus yet.

Getting big, efficient drivers to behave - now, that's more interesting. What's been sad has been discovering just how bad coaxial drivers are - B&C is a solid, well-respected vendor making top-flight pro drivers, but the curves for coaxes they make are just horrendous, way worse than the 5" Focal 5K013L I was banging on in the last post. And B&C has plenty of company in the pro world - even the 18Sound coaxials are only just OK in the measurement department. Other vendors like to conceal the true state of affairs by only publishing a smoothed composite curve, never a good sign.

When you see 15 dB peak-to-valley ratios, that points to geometric problems (resonant structures) that I'm pretty sure are beyond the range of cone treatments to remedy. The small area where the coaxial horn joins the bass cone just seems to be a very difficult design problem, both for the horn termination and the VC former/center-of-cone area of the bass driver.
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Old 25th April 2007, 08:42 AM   #514
SunRa is offline SunRa  Romania
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Hello Mr. Olson,




Here are some people that worked with TAD drivers (the 15" one mainly). I guess contacting them and the pioneer staff would be possible to aquire some more info on them.
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Old 25th April 2007, 08:53 AM   #515
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Hi Lynn,

Much of the upper range colouration (UK) and increase in output from a driver arises from the centre region.

I know you mention using an additional driver to lift efficiency at the lower frequencies, but this still won't give us clean HF.

Using a coaxial gives point source. Using mid, tweeter etc. gives us a reproducing panel where the sound changes as you move about, and in stereo there are two independent changes arising about what ought to be the original image. To me the 'field' errors due to flat panel driver arrangement far exceed those due to coaxial anomalies.

I have not yet seen any discussuion in this thread about bra-strap or pole piece mounting a mid/high cone driver in front of say a 12" running full range, with the back of the central driver being covered in sound absorbent material to both absorb centre of main cone HF disturbances and protect the mid/high from cone excitation.

The placement of damping material in front of a main driver will improve its entirel natural roll-off/crossover, and allow simple C filtering to the mid/high (with possible suitable R+C matching/damping) and produce a main assembly which needs only one amplifier.

Also the alignment (needs to adjusted empirically with C and damping material) can augment the crossover characteristics.

This allows most use of the main driver (with LF augmentation as/if necessary) with a much smaller one - short wavelength spaced in front....... any thoughts ?

Cheers ........... Graham.
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Old 25th April 2007, 09:28 AM   #516
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Hmmm - some interesting thoughts here.

The drivers of interest have efficiencies in the 95.5 to 99 dB/metre range - this is the baseline T/S efficiency in the piston-band, so any peaks are elevated above that.

These figures rule out dome tweeters, which don't exist in this efficiency range. No old-school cone tweeters, either. We're left with with prosound ribbons and horns, each with their own set of problems.

Several pages back, we had an interesting proposal for a custom driver with a 4" VC assembly and a coaxially mounted ribbon where a pole-piece would normally sit. The ribbon could sit fairly deep in the woofer, almost touching the interior of the cone, or it could extend further out on a cylindrical pole-piece. The extended version would be compatible with your suggestion for local felt damping near the center of the cone, where it could do the most good.

As for horns, hmmm - most of them are kind of big, in order to have a successful 1.5 ~ 2 kHz crossover, which we're going to need. This means using the woofer cone as a horn extension, a la Tannoy, with all the problems we've seen so far, or a more Altec Duplex approach where the horn actually shades part of the cone.

The most extreme shading is on the BMS 15CN680, where the horn is certainly big enough, but the bass driver is looking distinctly unhappy around 1.2 kHz, which is obviously caused by the cavity between the horn and the woofer cone. But maybe a little bit of strategically placed felt would make a difference - at least de-Q the notch at 1.2 kHz.

The UREI-modified Altec Duplex took the approach of covering the surface of the small horn with foam, which had a modest degree in success in taming the raucous midrange of the 15" Duplex. But UREI Duplex was still a long way from flat in the midrange.

I guess the real way to test the theory is start with a known-good 12 or 15-inch driver, and suspend a damping pad in the center and seeing what it does. I'm not sure many drivers will tolerate a several cubic-inch object suspended in the center of the driver - although streamlining could make a quite a difference to the overall result, with the object ending up looking like a sort of an egg-shaped phase-plug as seen from the woofer cone.

In essence, a phase-plug, and a good one, has to be designed, and then a very compact horn fitted in the middle. This is do-able, I guess, but we're talking about a serious research effort just to optimize the phase-plug, so it improves the response instead of degrading it. The phase-plugs I've seen so far optimize the response off-axis, but have negative effects on the impulse response. Again, do-able, but with a different emphasis than we've seen so far.
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Old 25th April 2007, 10:15 AM   #517
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Phase plug - like a felted Lowther one with a tweeter cone in the end.

Are there any high sensitivity mid/high cone drivers as made for professional horns ?

That small Tannoy above with its own conical felted rear damping layer and running for hi-mids upwards in front of a 12" or 15" ?
Depth likely a problem though.

Cheers ........ Graham.
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Old 25th April 2007, 02:15 PM   #518
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Default Raal and Hemp

Great thread, and right in line with my current interests, too.

I have read only to page 29 and expect it to take a couple of days to get to the last post and find out if this has been raised:

In looking at the Raal site, I see on the "news" page that they have apparently severed business relations with Hemp Acoustics, as of Nov 06.

I wonder what that's about.

http://www.raalribbon.com/news.htm

Russell
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Old 25th April 2007, 06:10 PM   #519
aerius is offline aerius  Canada
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Default Re: 12" Beyma... Liberty 8

Quote:
Originally posted by chrismercurio
I was looking for a 12" guitar speaker with smooth response yesterday and stubled upon this speaker.

I'm wondering if it is a candidate for your project? It has smoother (published) response than a great deal of hifi and pro product. It also doesn't have any of the ragged topend or peaky breakup.

Just a thought,
C

It's funny since I was also thinking about Beyma drivers, as a local speaker company uses them in their high efficiency designs.
I was just doing a little browsing around on the Beyma site looking at the curves when I came across the curves for the 12G40, they look quite nice through the midband region with a peak at 5k, after which they drop off. The info can be found on the Beyma product page by selecting G40 series in the Low Frequency box under "Pro". The graphs are in the PDF downloads. There's probably several other useable drivers there, I haven't had the chance to go through them yet.
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Old 25th April 2007, 06:48 PM   #520
dmason is offline dmason  United States
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Beyma make some very interesting stuff. Their smaller DIA Nd midband units are very impressive, however looking at that 12G40 curve, impressive though it is, makes the FR of the TT Alnico 12 on Lynn's page look even better. All that and HempTone as well. My vote goes for the Tubbies...
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