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-   -   Alternative material for stators? (http://www.diyaudio.com/forums/planars-exotics/64642-alternative-material-stators.html)

furly 20th September 2005 08:19 PM

Alternative material for stators?
 
Looking for suggestions for a inexpensive alternative to perforated sheet metal stators.

Haven't been able to find any perforated sheet metal in my area without having to make a special order which is quite $$.

I was thinking about steel fly screen for windows (if they even make that any more).

Any thoughts?

SY 20th September 2005 08:24 PM

I think McMaster-Carr delivers to Canada. And the perf metal from there is pretty moderately priced.

dstockwell 20th September 2005 08:48 PM

Takes more time, but 28 - 30 guage wire on egg crate.

I_Forgot 21st September 2005 02:09 AM

This is not what you're looking for (too expensive), but I have been playing with the idea of using a perforated carbon fiber/resin (epoxy or polyester) composite panels for stators. I just finished building a bicycle using the stuff so I've learned how to work with the materials, and I have some scraps to play with. Hmmmm. Maybe this winter...

The perforations are always the tricky part. CF/resin is not particularly hard on tools, so some sort of CNC step and repeat set up for drilling might be possible. Another way might be to make a "mold" with pegs (nails?) all over the place. Just lay the cloth in with the pegs poking through it then pour on some epoxy and spread it around. The trick would be making sure you can take it off the pegs when the epoxy has set.

Another way might be to mount the cloth on a frame, pull every other "thread" out of the cloth, and then apply resin. This would leave lots of holes. CF cloth loves to come apart when you're working with it, so this technique might work well.

The CF cloth is conductive. I measure 30-50 Ohms between my meter probes placed a few inches apart on a small piece of cloth. I think the resistance is unlikely to affect the sound.

5.7 oz./sqyd CF cloth typically sells for $35 per running yard in small quantities. I estimate that a single layer, 1 sqyd piece of CF/resin composite would end up weighing about 8-9 oz if it is vacuum bagged, and a few more ounces if not.

Another way might be to buy CF "roving" and use it the same way the wire stator guys use wire, but then apply resin after the "weaving" is done. This might have some advantages over wire stators in that it would weigh less (maybe NOT and advantage) and has very nearly zero coefficient of thermal expansion (not much advantage if the speakers sit around at room temp all the time anyway- maybe good for car audio?).

I_F

Brian Beck 21st September 2005 07:07 PM

This will be an interesting project to follow. Please keep us posted. This is reminiscent of Beveridge's work, but, to my knowledge, he didn't have carbon fiber to play with.

Lucius 23rd September 2005 11:41 AM

"The CF cloth is conductive. I measure 30-50 Ohms between my meter probes placed a few inches apart on a small piece of cloth. I think the resistance is unlikely to affect the sound." ...I_forgot

Would the cf/epoxy stator need to be painted with a conductive coating?

Brian Beck 23rd September 2005 01:39 PM

Keeping in mind that the secondary circuit of an ESL (from step-up transformer to stators) is an extremely high impedance environment, small resistances are probably not of any consequence. Think of it this way: If the ESL’s step-up ratio is 1:75, and let’s say each stator has as much as 500 ohms of resistance from secondary to stator surface (1000 ohms total in the secondary loop), that would equate to 1000/(75^2) or 0.18 ohms in the primary circuit. That would be less than most tube amp’s output impedance, and probably typical of a solid state amp plus cable. The DCRs of both primary and secondary will compete with these values too. I think a few tens of ohms in a carbon conductor stator can generally be forgiven, although some means of distributing voltage so that all points on the plane of the stator have close to the same resistance presented to the diaphragm surface might be useful.

reverber 23rd September 2005 02:08 PM

What about using CF rods to build the stator?
An array of small carbon fiber rods arranged horizontally would make for an interesting look. And couldn't one bow the rods to make a curvilinear panel?
What would the effect of pushing sound around these cylindrical surfaces be?


Cody

Brian Beck 23rd September 2005 02:14 PM

I_Forgot, a caution that you may have already considered:

As you know, charges concentrate on points of high curvature (charge density is proportional to the radius of curvature), such as in lightning rods. The tiny cut tips of carbon fibers would certainly have extremely high curvature. If these points, even one or two, were allowed to face the diaphragm, I believe they would encourage corona formation and possible arcing. Even if the fibers facing the diaphragm were continuous, they still have high curvature in one dimension. If continuous fibers are woven or placed very close to one another relative to the stator-to-diaphragm distance, you may be OK. The electrical properties of the binder or resin that you use to hold them together will certainly effect the behavior.

I was reminded of this concern by a device I have. It is an air-ionizer/cleaner. It uses a weak high voltage supply connected to a small exposed rope of interleaved carbon fibers that is wrapped around a plastic insulator. Here the designers were using the high curvature properties of the carbon fiber to create coronas. And it works. You can hold your hand nearby the rope and feel the electric “wind”.

I’m not trying to discourage you at all. In fact, I think if you’re careful to address these corona concerns, you may have an excellent approach. Of course, this brings us back to Beveridge. Suppose you used a more homogenous conductive additive to epoxy? Could powdered graphite be mixed in sufficient quantities to give low enough resistance without making the mechanical properties go to hell? Inquiring minds want to know…

I_Forgot 23rd September 2005 02:49 PM

Quote:

Originally posted by Lucius
Would the cf/epoxy stator need to be painted with a conductive coating?
I would embed an electrical contact to the CF in the composite, so there would be no need to add a conductive surface over the CF/resin composite.

Quote:

Originally posted by reverber
What about using CF rods to build the stator?
An array of small carbon fiber rods arranged horizontally would make for an interesting look. And couldn't one bow the rods to make a curvilinear panel?
What would the effect of pushing sound around these cylindrical surfaces be? Cody

There are a few commercial speakers that use metal rods or tubes for stators, so CF rods should work fine. I don't think they would make a very efficient speaker because you need to allow some space between them which means the area they present to the diaphragm will be relatively small, like using a large mesh screen for a stator.

There are other problems- where do you get them at a reasonable cost? The retail price is quite high. On the other hand, fishing rods can be pretty cheap, and if you collected enough old ones from local garage sales you might eventually have enough of them to make a speaker. You might be able to get rejects from rod manufacturers in Taiwan pretty cheaply. Fishing rods are tapered, so if you kept the spacing between them even you'd have a speaker that was wider at one end and narrower at the other, which might look nicer than a plain rectangle.

As for sound squirting between the cylindrical surfaces of the rods- should be no problem. In fact, with perforated sheets, those little holes actually form cylinders, thousands of them all the same size, that have some specific resonance frequency. A series of rods should have no such resonances. But then again, what about the resonance of the spaces INSIDE the hollow rods? Hmmmmm.

Curved panels are another set of problems, the biggest being curving the diaphragm while keeping it under tension. Martin Logan does it, but not for the reason that most people think. Curving the panels doesn't broaden HF dispersion much. The HF beaming is primarily a function of the width of the driver- it always was and always will be. The real reason to curve the panels is to make them stiff. A flat steel panel would be very wobbly (I know because I have built some that way). Curving the steel makes it stiffer. Try it with a piece of paper.

If you're making stators out of CF rods or tubes, you can make them flat or curved easily and without having to bend the rods/tubes, unless you want the panels curved in two dimensions. The real problem is making the tensioned diaphragm do that. If you figure out a way to do it (maybe using air pressure?) what have you gained besides a nifty bit of marketing copy?

I_F


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