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RE: Apogees and ethics - comments

Posted by josh358 (P) on February 10, 2012 at 12:09:43

In Reply to: RE: Apogees and ethics - comments posted by Satie on February 10, 2012 at 11:05:28:

Some other advantages of top/bottom ribbon configurations I think of include the fact that the fundamental resonance is so low, that the field is more linear as a function of excursion, that the front wave can be unimpeded (though it seems that a magnetic circuit across the back is needed to avoid an efficiency loss of c. 40%), that it has more effective displacement since it dishes only vertically and not laterally, and that aluminum can be used without a plastic diaphgram -- though this presents its own set of problems such as impedance and I've heard varying opinions, some say they're comparable overall, the Transmission Audio web site says it causes stored energy and other problems (but then, they make metal ribbons). I gather they segmented their bass because it's impossible to make a full-width ribbon driver that goes below maybe 250 Hz, because of the impracticality of producing a field across a gap greater than 3". I don't know how this speaker or the all-ribbon designs from Transmission Audio sound since I haven't heard any descriptions, but I suspect the main drawback is cost -- both the cost of multiple large neodynium magnets, and of direct-drive amplifiers necessary because of the limitations of transformers at low frequencies. This has to be a very expensive speaker.

Magnepan uses edge damping as well, not sure what material they use but I've wondered if it deteriorates with time.

I don't think you could get away with putting a ribbon tweeter in the middle of bass panels. I'm not sure about midrange, though. Apogee put ribbons in front of and near the sides of its 2" midrange ribbon in the Scintilla and it seems to work -- I've seen one person say he thinks he hears intermodulation, but no other complaints. And there are the side-by-side single-magnet 2"+1" Transmission audio ribbon assemblies that I mentioned in my email. Would the center be any worse? I'm not sure why Apogee put the Scintilla's tweeter ribbons on the sides, or ran the rear ribbons out of phase. Presumably they used a laterally symetrical arrangement for the same reasons one uses any symmetrical arrangement, to reduce crossover lobing and improve imaging specificity. But wide separation will give you lateral lobing, and running the rear ribbon out of phase will give you a rear wave suckout at the crossover point, as well as giving you a different, omnidirectional polar pattern (though separation > lambda sources tend to be directional, with lots of lobes, so maybe that isn't an issue). One though I had is that they put them out of phase to keep the backwave from stimulating resonances in the midrange ribbon -- since one tweeter moves forward as the other moves backwards, the net force on the midrange will be zero. Another is that with the primitive magnets of the time, they weren't able to achieve a high enough field strength in the center of the magnet assembly.

So I still don't know how well a center coaxial tweeter ribbon would work. In light of what Apogee did, I do think you'd probably want to run it in a separate section, rather than in front of/behind the midrange ribbon as I'd speculated the other day -- at least, I don't like the idea of that phase flip on the rear tweeter.

Getting back to the TR audio, I don't think anyone knows how to make a coaxial true ribbon that covers all three frequency bands, bass, mid, and treble. Even the coaxial quasi ribbon designs don't try to do that, the Wisdom only goes down to 80 Hz, ditto the central tweeter OEM version of the RD-75. I assume this is because the IM would be too high. But since localization below 300 Hz is less critical, I'd think you'd want to coax the mid and tweeter and not coax the bass and mid, since whatever you do you want the mid as close to the tweeter as possible, both for imaging and to reduce crossover lobes.

Regarding neodynium, you know far more about commodities than I do. I gather though that the runup in prices has to do with the Chinese monopoly and their restriction of exports to force electronics manufacturers to make their products in China, and that the first competing rare earth mine is about to come on line in IIRC the Phllippines, capable of supplying 15% of world production, with other mines, including the mothballed one in the United States, slated to join it in the years ahead. I'm hoping supply will move ahead of demand and push the price down, as happened with oil when conservation measures and new exploration broke the back of the OPEC cartel at the end of the energy crisis. But I don't have the impression it will happen overnight, and as I said, I don't know much about commodities and haven't looked into the issue, so have no sense of the supply/demand curve.

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