Aperture question

[salvarsan]

If I recall, FEMM allows one to define one's own magnetic materials.

If they screwed up something so trivial as permeability, then they screwed up other non-trivial things that were uncorrectable by the rest of us. Credibility is important in software engineering, but that's only my opinion.

The biggest problem with FEMM is that it's 2D, but even so it can do a pretty good job on the center of a blade pickup.

FEMM was entertaining for a while and was much better than nothing at all, particularly since commercial magnetic modeling software is very expensive. Ultimately, we needed practical (= cheap) magnet testing to make any kind of modeling worth the effort, but that was not to be.


-drh

[Dave Kerr]

People got pretty pictures and a slightly better understanding of magnetic circuits, but nothing that improved their pickup-building craft.
-drh

Well, that "slightly better understanding of magnetic circuits" part could be beefed up to say "much better understanding of magnetic circuits" when your base level of understanding is just about zero. Many people (my hand is up in the air) struggle with written descriptions of what's going on here, so to them the pictures are not only pretty, but informative.

[Mike Sulzer]

Andy of Wizard Pickups, Possum, Me ... Steve Kersting of SK Guitars is the only one who left the results up. *You are quite free to research the Ampage archive for more names.


Magnetic fields in pickups
-drh

Thanks for the info, drh. One can use the first plot from that link to get a good idea of the aperture. For those not familiar, the lines indicate the direction of the field, the spacing indicates the strength. The color also indicates the strength. It is the vertical component that counts because you need the component of the flux normal to the surface, most simply thought of as the plane of the loop. So the magnitude plot below is a lot wider than the actual aperture. For example, when the angle is 45 degres, the field strength is sin(45) = .707 and the power is down by the square of this number, one half. This occurs close to the edge of the pole piece. This would apply in the direction along the strings; in the other direction multiple pole pieces affect the field.

FEMM uses the "dynamic" state of the magnetic material; the effective permeability is the slope of the B-H curve. So, for example, it can compute the correct result when the current in the coil affects the magnetization in the core, such as a loud speaker. In a pickup the correct permeability would be the slope of the curve in the material's normal magnetized state with no applied current. I doubt that FEMM has this wrong, and if it does, you can easiliy correct any property yourself if you know the correct value.