In Reply to: RE: "Here is a mod that Wendell suggested", REALLY posted by gregmacknass2@netzero.com on June 30, 2011 at 23:22:51:
I'm not sure why the efficiency of the tweeter would change, assuming the resistance per unit length of the foil was matched to that of the original wire. Did he unwittingly change the DCR of the tweeter? Also, the foil covers a large percentage of the tweeter surface so he may have reduced its compliance too much. Not sure how much of an effect that would have on the tweeter. In any case, it isn't an issue for the MMG since we're leaving the tweeter as is and using the lower mass tweeter foil for the woofer to improve transient response, wiring sections in series/parallel to maintain a nominal 4 ohm DCR. Any changes in output should be limited to the woofer.
Unless I'm missing something (which I do at least nine times a day), the reduction in woofer voice coil mass will increase efficiency, raise the resonant frequency Fs, and reduce Q. Planar woofers use an underdamped alignment to compensate for the 6 dB/octave dipole cancellation: measured up close, they have a big resonant peak -- 20 dB on some of the measurements I've seen. So if we reduce Q and raise Fs with respect to Fequal, the amp may have to do some of that lifting instead, as it does in equalized dynamic dipole woofers like the ones on Linkwitz's site. Which will put a heavy demand on amplifier power.
I'm guessing that there could also be some minor changes to compliance and suspension loss since the not-very-elastic foil will be "tying down" more of the Mylar, and compliance is inversly proportional to element length (to an approximation, the width of the Mylar that doesn't have aluminum glued to it). This could also have an effect on resonance frequency and Q, but per the earlier discussion in the thread where Mart corrected my mistake, I'm guessing it's relatively minor -- the woofer foil doesn't tie down much of the Mylar surface.
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Follow Ups
- RE: "Here is a mod that Wendell suggested", REALLY - josh358 07/1/1107:12:18 07/1/11 (0)