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RE: Foil Project for IIIa's

Posted by tom_gootee on February 21, 2012 at 03:52:03:
If the current going through the foil is the same amplitude as the current that previously went through the wires, then the force exerted on the current by the magnetic field ought to be pretty close to what it used to be, except maybe for variations in the field in the regions where the foil now is (since it's flat and spread out, unlike the wire) that the wire never occupied before.

But even assuming that the induced force is the same, the mass of the foil might be different than the mass of the wire, which would mean that the acceleration due to the same force could be different. If the foil has less mass, sensitivity could be improved, or vice versa.

It seems almost inappropriate, timing-wise, to mention the following. But, for future reference, there is a different and probably better foil layout which is being referred to as "the Epsilon layout", which can be seen at:

http://www.diyaudio.com/forums/planars-exotics/200038-analysis-epsilon-10.html#post2813499

It uses THREE foil runs for each gap between the columns of magnets, and the "Epsilon" layout pattern is such that the current is flowing in the same direction in the three foil runs in each gap. People who have used the Epsilon layout have reported that sensitivity is about tripled, compared to one foil run, even if the outer two foil runs overlap the magnets somewhat (which apparently causes no ill effects).

I haven't yet calculated-out any possibilities for the total resistance vs foil width, for an Epsilon layout, to see if a Maggie-sized speaker would need electrically-paralleled sections in order to present about four Ohms to the amplifier, or not, which could affect the overall sensitivity.

And here's a link to a different thread, where a guy is making a pair of DESKTOP planar magnetic speakers, with Neodymium magnets (which give a magnetic field that's about TEN TIMES stronger, at the Mylar surface in the gap, than my MG-12/QR speakers have, which was determined by comparing actual measurements of my MG-12s, made with my directional DC Gaussmeter, to FEMM simulation results for Neodymium magnets, produced by the OP of the thread below), and is using two runs of foil in each gap instead of three:

http://www.diyaudio.com/forums/planars-exotics/204348-mini-planar-magnetic-using-neos-7.html#post2903737

Cheers,

Tom