Thanks

Yes this is old stuff. John's paper is also misleading or wrong as proved by JBL's own recent product moves.

Its not just the gap geometry that lowers the distortion in an ALNICO structure. ALNICO is a good electrical conductor, unlike ceramic, an so it is its own a shorting ring! So his conclusion that the geometry accounts for the changes is incorrect. JBL recently started to use ALNICO again for variuos reasons - lower flux modulation and near zero flux changes with themerature, both of which are very bad in ceramic.

Then there is the fact that second and third harmonics are not really the important ones.

John's paper was the prevailing philosophy at the time, but now we know more than we did then.

Its hard to beat ALNICO for performance, but its price and weight are real drawbacks. The better drivers today are using laminar motor structures with each lamina being copper plated.