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In Reply to: RE: Apogees and ethics - comments posted by JLindborg on February 15, 2012 at 08:20:46
You are absolutely right, the mitigating factor is that the membrane becomes more flexible over short time periods as stretching increases temperature in the membrane and reduces resistance to elongation. It is noticeable on the Neo8s as the detail retrieval and dynamic swings after a very loud passage improve and then deteriorate slightly if no further loud passages follow. I should point out that when I use the modified Nuforce class D amps, the dynamics are better and the effect is even more pronounced but dynamics are better overall. I think this better dynamic presentation has something to do with the driver being within the feedback loop since the little amps get way hotter than the energy they are supposed to deliver would imply. Which indicates some sort of electric damping, probably of the return tension on the membranes.
The membrane material in the Neo8 is rather stiff and has little elongation and is responsible for the sheer drop of the bass response as stretch limits on the membrane are approached. It is alot more like kevlar than it is like rubber.
There is definitely energy storage and compression going on with the BGs. But it is not that great so long as you keep bass freq out of it.
The big deal is that the bigger the magnets the smaller the effect of air damping on dynamics. Remember that you have the ribbon sittin in a narrow slot and pushing air into and out of the slot with turbulent air flow and at the kinds of displacements that Valvetude reports that is going to play into damping - and the more magnet you have the better you are able to overcome the air resistance. While you can get "cheap power" to do it instead of expensive magnets, the quality of the "cheap power" in something as revealing as a ribbon is far more significant a cost than it would be on a bass driver.
Torsion is also magnetically/electrically damped, though weakly, which is why that is a limiting factor in ribbons. The torsioning is caused by greater air resistance to motion by the edges of the slot. Once an asymetry initiates the torsion it will not go away till the ribbon's vibration is much reduced in amplitude.
Re Atkinson - it is what he is implying in comparing to the Quads.
http://www.stereophile.com/content/quad-esl-63-loudspeaker-measurements
http://www.stereophile.com/content/quad-esl-989-electrostatic-loudspeaker-john-atkinson-may-2003
Also Iverson's review of the SF Guarneri stand mount speaker actually compares to the Quad ESLs.
Follow Ups:
While it's true that larger magnets increase electrical damping, air damping so predominates at the higher frequencies at which ribbons operate that I'm not sure how much of an effect electrical damping has, even with strong magnets. More on torsion and edge-gap distortion perhaps, since when that occurs the air is turbulent. But as I think about it, I'm not even sure if it has much of an effect there. Amplifier damping can be compromised by the crossover network, which would leave only self-damping from eddy currents. Since ribbon resonances are below the crossover point, the diaphragm could literally be left to twist in the wind. Another consideration is that the amplifier can only damp the ribbon as a whole. Since torsion is symmetrical, it may not be much affected by amplifier damping at all. Again, you're back to eddy currents. Which would however be stronger in a stronger B field.
The idea is that larger magnets and thicker tweeters end up with larger relative magnetic forces vs. air resistance and damping. Thus less compression.
In the torsioning case, I know from listening and from the basic physics of it that it can't be damped effectively, only prevented - as magnepan do with their little beads - which allows them to have no torsioning and no significant lengthwise resonance.
One of my tweeters is loose, while the other is nice and tight and beaded. The loose one torsions when playing signficant output below 5 khz, but performs just fine with the 10khz XO. The higher frequencies don't seem to be affected by the torsioning when playing at lower XO, but the 2-3 khz range becomes rough and screechy like the Apogees can sound.
Once it starts oscillating the damping would already have proved insufficient, so it makes sense that it wouldn't stop while still driven. But I assume that if you could achieve critical damping, you wouldn't get the torsional oscillation in the first place.
I gather the Apogee ribbons had foam supports to achieve the same thing as Magenpan's adhesive. But of course they still had the torsion problem, I assume due to the width of the ribbon.
Not any measurements done by myself but from the book "Ribbon Loudspeakers Theory and Construction" ISBN: 1-882580-24-X.
Corrugated ribbon have larger/more THD then an uncorrugated ribbon in 3kHz and 10kHz except for 1kHz where the uncorrugated ribbon has slightly higher THD.
IMD is higher for corrugated in 2&5kHz and slightly higher for an uncorrugated ribbon in 5&7kHz.
Interesting I would think.
The one who succeeded was the one who didn't know it was impossible.
I was looking for that book a few days ago, the only copy I could find was going for more than $300. But I saw the web page that had that info. I wonder whether an uncorrugated ribbon would be more susceptible to torsion. Also, whether it would allow sufficient displacement to play at high amplitudes at the lower end of its frequency range.
There are sadly no comparisons in those areas in the book.
But it mentions and shows a research of how a corrugated ribbon does behave as a uniformly moving piston, but has many modes imposed and torsional movement in, what they call 8th resonance mode, that would be 244Hz.
The one who succeeded was the one who didn't know it was impossible.
Edits: 02/17/12
Now that's really ineresting. Wish I could get hold of that book. I'm going to see if I can get it on interlibrary loan.Meanwhile, I found the paper mentioned in the caption:
Edits: 02/19/12
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