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RE: "If we replaced the wire with foil that had 1/4th the mass and 4X the resistance per inch" ...

Josh,

I guess it depends on what you mean by "the Magnepan foil". All I have is a pair of MG-12/QRs. But in case it might be helpful, I measured their tweeter foil:

On my MG12s, the tweeter foil is exactly 0.1-inch wide (2.54 mm).

There are eight vertical runs of foil, with the outer two going to the connectors. There is a space of exactly 0.125 inch (1/8 inch) between each pair of vertical foil segments (so, 0.225" between their centers), except that the last vertical foil segment on the right is 0.250 inch from the one to its left, instead of 0.125 inch like the others.

I did measure the total foil length (8.146 meters), and the resistance (3.60 to 3.66 Ohms), as precisely as I could. The calculated thickness, with measurement variations, came out to be in the range of .0009 inch to .0011 inch. So I'd have to assume that it is actually a .0010-inch thickness.

I measured the length of the MG12 tweeter foil from connector to connector, as 8.146 meters (320.7075 inches). [No, my measurement precision wasn't .0001 inch. I just kept the four digits from the conversions of 16ths etc to decimal fractions, until after I added everything up.] (The lead-ins from the connectors totaled 16.8125 inches. The length of each of the eight vertical sections is 37.6875 inches. There are six horizontal runs of 0.325 inch each and one of 0.450 inch.)

I unplugged the tweeter wires and measured the resistance, with my Tektronix DMM916 multimeter. Of course I also measured the resistance of the meter's probes and wires by shorting the probe tips, and subtracted that from the tweeter resistance measurement, each time. I repeated the entire process until the measurements stopped changing (five or six times). I got 3.6 Ohms, for the one tweeter that I measured (only one sock is off).

I also tried measuring the resistance of the tweeter foil using my TH2821A LCR meter, and got 3.66 to 3.65 Ohms, using frequencies from 100 Hz to 10 kHz.

To calculate the cross-sectional area, I used the equation A (in sq mm) = [0.026 (for Aluminum) x Length (in meters)]/Resistance (in Ohms). I then divided the Area by 2.54 mm to get the thickness in mm. Then I divided that by 25.4 mm/inch to get the thickness in inches.

Cheers,

Tom


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