Beyond the Ariel

[planet10]

Originally posted by berm
He goes on to write that he was surprised to find that Museatex has a patent on a similar device, but he sees his as a “soft piston” and the Museatex as a “bending wave” transducer. I believe the Museatex design is the Melior One, a single-driver point souce which I think moves a mylar film without the foam sandwich feature.

I've seen one of these. My buddy Bill Perkins designed the motor on it, and i believe that our own Moray James was also involved (hopefully he will drop in and comment).

dave

[moray james]

was the parent company started by Paul Burton and myself. Paul had the idea for a traveling wave driver. I had been working on ESL's at the time. The two of us worked solid for a year and developed a line source version with a magnetic drive. Shortly after doing this work we realized that we had a potential product and started the company with a lot of quality help and good fortune.
I would not say that Bill designed the driver however his assistance and design input along with his machine work skill was what made it all possible to come together. The basic team at that time was Paul myself, John Wright with Bill making the motor parts. Paul's Dad designs Circlitrons and when we took our motor design to him for an assesment the said there was nothing left to do. That felt pretty great comming fro him. I will also say that the point source design was the undoing of our little company. All the original designs had been line source designs and I can say that to this day I have not heard the equal in terms of image quality. Only the the big German MBL's do the kind image we were getting from the line sourcees. We feared that magnet structure cost would kill us and time was pressing as we had borrowed a lage sum of money to proceed. We had been playing with very inexpensive point source design motors at the time and felt that we could solve the problems they had by designing our own. With the commitment made we shelved the line source and proceeded with the point source version. The cost in time and development expenses struck a near killing blow. At the end of the day we had a low mass single layer 4 ohm coil point source with good enough bandwidth. The motor cost alone was about $360.00 each (this in 1989). The driver used ferro fluid as a liquid bearing (a first as far as I know) to replace the spider to stabilize the coil as spider talk through a 150 gage mylar diaphragm was not acceptable. While the point source was very good we never achieved the quality of sound from it that we had from the line source design. The point source design was a traveling wave design and could be discribed as a mechanical version of the Quad 63 approach. I liked to think of it as a hornless horn loaded design.
If you consider the old pebble in a pond visualization the VC pokes the film diaphragm and launches a forward traveling wave into the air the size if the coils cone (in this case 1.25 inche). At the same time the diaphragm comes back to centre and a secondary wave spreads outward along the diaphragm toward the frame. The leading edge of this wave in the diaphragm then spreads out the outside edge of the forward in air traveling wave . So the traveling wave acts like a wave guide forming the resulting wave front. This then generates a half hemisphere wave front (we are talking now of just half of what is going on as this is a dipole design). You need to do some fancy work at the frame to eat up the bulk of the reflection which would normally occur. Additional diphragm damping to controll tympanic resonances is also required. We did all of the initial work by ear and by the seat of our pants. Later on Bill Perkins built us a world class measurement system. This was both a good and a bad thing as it is easy to follow the lead given by the measured data and not trust your ears. We got over that issue and developed a fine speaker, not perfect but time and money were pushing hard. A very simple idea but very difficult to implement.
Considering the billions that must have been invested and spent in the following years by NXT think that what we did stands as a superior overal acomplishment. The time and money that we spent to jump into the point source (given that a line source had been just about ready to go to market) combined with issues that our distribution partner (Sumo) had were too much to overcome and things unwound. Paul cut loose from the project and was given a golden handshake to go away. We ended up having our investors essentally float (buy) Meitner and merged the two companies and things went worse from there on. Lots of reasons but not worth going into. That's the story in a nut shell. Regards Moray James.

[jzagaja]

What is so special with line sources imaging? The wavefront is less prone to interfere with a room? Does it apply to the linear array based on small wide band moving coil drivers? Needed minimal distance?

[Salas]

I don't like it. Sounds like a clean, forced & detailed vertical 16:9 version of a spherical event.

[berm]

Hi, moray james. I remember from another forum your mentioning you had been invoved with Sumo, which had been mentioned in the article also. I had thought to ask about this from you, but never got around to it.
I'm wondering if this different take on the concept, and it does seem different - a "soft piston" using a foam sandwich - has any merit. This would seem to combine your original idea with a larger soft piston area. It would limit the bandwidth to the bass region, and should deal with many of the issues that the EnABL and Mamboni processes deal with, though maybe not so well nor so elegantly. Instead of a large woofer mounted in a larger panel, the woofer membrane itself would form most of the front baffle - or the side, or sides, of a fuller arrangement.

Wish I could have heard your line sources. Ever think of trying to bring them to market again? Or is the meat grinder world of audio business not something you want to face again?

[moray james]

Our original line source design used a very thin tight diaphragm in conjunction with periferal wave traps which loaded each side of the diaphragm. The diaphragm was fairly thin prior to stretching and I believe that this resulted in a far faster wave transmission than with the thicker far lower tensioned diaphragm of the point source version. Yet with the point source version we were able to measure with a B&K capsule mic facing the outside edge of the speaker frame (same plane as the diaphragm) 10 KHz which would never happen with a conventional planar such as a maggie or an ESL.

berm: Soft won't get you there. If you want to have the waveform shape preserved you have to aim to transmit in the diaphragm at as close to the speed of sound in air as possible. Remember the sucess that guys were getting gilding paper cones with aluminum foil to speed the surface transmission of energy? Same thing here, once you launch the wave into the air you have to attempt to match the speed of transmission in the diaphragm to that in air or you will get waveform deformation. We used 1.5 mil mylar on the Sumo Aria diaphragm and heavier would have been nice for better bass reproduction.
You borrow a lot of money to make a business go and things change. Reasons for doing things change and you have investors looking over your shoulder. Partner relations change. Enough stuff went down you could have choked a horse on it so yes that took its toll. There was a bad taste associated with the whole thing for a very long time.
I am toying with an old idea from a long time ago that may or may not ever see the light of day. If it does I will let you know. Regards Moray James.

[Lynn Olson]

Originally posted by ScottG
While the 12NDA520 is a fantastic driver.. I'm not so sure this is the driver that should be run fullrange.. Dispersion becomes poor about 1.4 kHz, and it looks like at 1.8 kHz you start into time domain problems.

Now of course you *might* be able to modify the driver to an extent to clear things up (in time).. but that still won't do anything for the dispersion pattern.

It could be that the Tone Tubby is FAR better with time at the top of its passband.. don't know. I also don't know about its horizontal dispersion (which, including price, is why I didn't mention it to Paul in his thread).

One possibility to improve in-room power response while even further limiting dispersion, (creating a more linear off-axis response), and near time room reflections would be a hypercardoid based on a free air bipole with a pair of 12" drivers (mounted back to back). It would however need a very similar dispersion pattern at higher freq.s - which could be accommodated via those 18 sound horns. (in a paired bipole config.). Note that such a config. for the free air bipole would significantly reduce frame energy build up, (provided the two drivers were connected physically via a high transmission material), and mildly reduce 2nd order distortion. More importantly though, it should reduce time problems at the top of the driver's passband (..when compared to virtually any other mounting scheme). Additionally, you could always vary the rear in-phase output for most of the passband with a simple acoustic trap.

(Note that such a bipole will NOT have the side combing problems of traditional bipoles).

Well, it's an idea.

Many many thanks for all the good stuff in the Large Midrange for OB thread. Tons of information, fabulous photos and test data, along with a wealth of subjective information what this stuff actually sounds like.

As for the whole idea of "pushing" a large diaphragm to the top of its range, good question about the 12" Alnico Tone Tubby vs the 18Sound 12NDA520 . The TT is plenty directional, as you'd expect from a straight-sided cone, but the hemp cone does have good damping qualities at HF, as shown by my MLSSA measurements on a flat baffle.

A more "classical" approach would be the 18Sound 8NMB420, with near-flawless FR and quite respectable T/S normalized efficiency of 95.6 dB/metre. Considering that I'm looking at de facto bi-amplification of the midbass and bass drivers anyway, that could remove the unpleasant necessity of a passive highpass filter at 200 ~ 250 Hz for the 8NMB420. Good-quality caps in the 100 uF range pretty much don't exist - and the trick of a smaller film cap to bypass a yucky back-to-back electrolytic doesn't do much to get rid of the electrolytic coloration. The better the driver, the more poor-quality parts in the crossover become audible, unfortunately.

So there are several options for the widerange driver, depending on how much I want to take them out of their recommended ranges - or what I ask of the tweeter. The 18Sound ND1060 plastic-film diaphragm CD combined with the XT1086 80 x 60 elliptical horn look interesting, particularly since I'm strongly considering a pair of XD125's on the rear of the speaker - one pointing straight back to illuminate the back wall, and the other pointing out at the side wall. Both the ND1060 and XD125 use the same plastic-film diaphragms and similar horn profiles, so there shouldn't be a conflict in the dissimilar-driver coloration for the front and rear sound.

As for the bipole - or hypercardiod - using a pair of 12" drivers, one in front of the other, I'm very concerned about degradation of impulse response. Diaphragms are pretty much transparent acoustically, and the impulse from the rear driver would travel right through the front one - but with a 0.4 to 0.5 mSec delay, depending how close you can get the drivers to each other. That's going to create large ripples in the time response, comb filtering in the frequency response, and some narrow lobes at high frequencies. It would certainly tax the crossover design, since there would have to be a very sharp cut (probably 24 dB/oct) before the comb filtering starts disrupting things. There's nothing the crossover could do to fix the time domain, though.

But - even though I am wary of doing this in the midrange region - the deep bass is quite another thing. In fact, I was thinking along similar lines this morning before I turned on the computer and read your message.

I was considering a version of the speaker with a pair of 12" drivers side by side, when it dawned on me that they don't need to have the same type of enclosure. In fact, one could be in a closed-box (a simple monopole), and the other could be in a short resistive-loaded tube (a dipole). Just as in a microphone, this creates a cardioid speaker.

But then I started thinking about the inter-driver phase relationships and the effect on the cardioid pattern. If the rolloff of the monopole creates a 180 phase rotation (relative to the dipole), that would shift the cardioid pattern all the way from front to back! Not a small difference to the sound, and something that's not obvious from inspection of the FR curves for each type of loudspeaker enclosure.

Since closed-boxes, vented-boxes, resistive-loaded boxes, TQWT's, etc. are all minimum-phase highpass filters, the phase relationship is predicted by the frequency response. For Butterworth filters, the phase shift at the -3 dB frequency is half of the phase shift at the ultimate attenuation frequency. So for a 12 dB/oct highpass (a closed box), the phase shift is 90 degrees at -3 dB, and approaches 180 degrees at zero frequency (although it gets pretty close one decade away from the -3 dB frequency). Similarly, for a 24 dB/oct highpass (vented boxes, TQWT's, etc.), the phase shift is 180 degrees at the -3 dB frequency, and approaches 360 degrees a decade lower.

This has implications for combining a dipole with an adjacent box enclosure. The dipole's natural 1/f rolloff is creating a mild degree of phase shift (90 degrees or less), depending how far away from the 1/f transition frequency we are (the path-length of the baffle). This is fairly close to what the closed-box is doing, but we're going to get in trouble with the vented box (in all its forms, including back horns) thanks to its higher phase shift interfering with the dipole. This problem would be greatly worsened if the TQWT or horn have ripples in their response, which creates associated phase shifts. By themselves, these phase shifts are reasonably benign, but could create cancellation problems if the TQWT or horn output is summed with a dipole.

So I guess the moral of the story is: we have to be very careful about both phase and time relationships when we have two or more drivers close to each other. If the drivers have well-matched phase relationships and are on a common plane (at the same distance from the listener), things are fairly straightforward. This why combining dissimilar drivers at high frequencies gets us in trouble, because small differences in FR result in large differences in phase, rotating the polar pattern.

[ThomasW]

Originally posted by planet10

MDF -- not the ideal material...

That's just the prototype and no it's not MDF. It's multiple layers of 1/4" 'hardboard' laminated together with epoxy.

[berm]

I don't want to get this thread too far off topic, since I got interested because I wanted to read what Lynn Olsen's thinking was on speakers at this time. However, another question for moray james: With the "Spandex woofer", as it seems to operate somewhat more like a piston than a bending wave device, do you still think it would be bad as a bass driver? I was thinking that the things that inherently limit its use as a fullrange driver might make it a good bass driver. It would have a natural low roll-off, and be able to move a large mass of air. Since most of the pad is moving rather than starting as a point (mostly), would this cause a wave at the lower frequencies to deform too much? I suppose the foam core in the sandwich could be replaced by a more rigid panel, but then we are getting into a different area with its own problems.
I'm mostly looking for potential options for a bass dipole to augment whatever full/widerange driver I would mount topside. And I'm learning a lot about what I should look for and how to treat it in Lynn's and others' posts on this thread.

p.s. I saw your post on another forum about the CyberHome DVD & picked one up. Nice, and thanks, though it wasn't written to me.

[Lynn Olson]

Originally posted by moray james
If you want to have the waveform shape preserved you have to aim to transmit in the diaphragm at as close to the speed of sound in air as possible. Remember the sucess that guys were getting gilding paper cones with aluminum foil to speed the surface transmission of energy? Same thing here, once you launch the wave into the air you have to attempt to match the speed of transmission in the diaphragm to that in air or you will get waveform deformation. We used 1.5 mil mylar on the Sumo Aria diaphragm and heavier would have been nice for better bass reproduction.

Regards Moray James.

Hello there Moray, maybe you could answer something I've always been curious about: when a material with a high speed of sound (like aluminum) is bonded to something with a slow speed of sound (like paper), what happens to the speed of sound in the new composite material?

Is it as fast as aluminum? Are there two wavefronts travelling at different speeds through the new composite, with the first arrival followed by an echo? Or does the faster speed win, with altered decay characteristics compared to either material on its own?