B-H Curve Chart?

[JGundry]

Does anyone have an explanation of how to read a B-H curve chart as it relates to guitar pickup magnets? Thanks!

[PremiumPlus]

I may be way off base here, but since I haven't seen or heard of "B-H curves" since I was an EE student, I'll be surprised (but delighted) to see a discussion of magnetic properties in terms of equations and direct applications, as they refer to pickups. I say this because pickups have already been designed empirically, that is, by trial and error. We know what sounds good, and what the ramifications of changing magnet strength/coil turns/wire gauge/shielding material and so forth are. We know that more turns of wire equals more output in voltage, but that the treble response and chime suffers with more windings because the inductance of the coil rises with more turns. We know that more magnet strength will increase the output but that in turn effects the string's physical response in the magnetic field... It's kind of like making a barbeque sauce for grilled chicken...we know what more lemon juice is going to do, but we never consult any recipes...we just throw in some more sugar or lemon juice or vinegar or oil...know what I mean?
It would be really cool if we had a reference to work from in regards to mathematically modeling pickup response to various changes in design parameters. We could write software to virtually design pups. But until that is done pickup design will remain kind of a black art.

[Mike Sulzer]

Originally Posted by JGundry

Does anyone have an explanation of how to read a B-H curve chart as it relates to guitar pickup magnets? Thanks!

Go here: Permanent Magnet Design Guidelines

Scroll down to section 4.2, The B-H Curve, and look at the first figure. Start at the origin where we have a non-magnetized, but magnetizable material. Apply a magnetic field to the material, either with a magnet or a current. This corresponds to increasing H along the horizontal axis. The magnetic material material responds: little domains of current line up, making the B field. As the H field is increased, the curve starts to flatten out as the material saturates. Now decrease the H; B remains high because this material can be permanently magnetized. Take H to zero and note where the B value. This is the permanent magnetic field.

1. If you have a pickup with alnico magnets as the pole pieces, this magnetic field magnetizes the string since the field extends out of the magnet, decreasing in value some, of course.

2. The curve intersects the axis with H = 0 at some point. Draw a line through this point parallel to the curve (the tangent line). This line defines the permeability of the material. The steeper the slope, the higher the permeability. This tells us how much B field is produced when a small additional H field is applied. For example, the vibrating magnetized string applies such a field that is changing in time.

Thus for a given string magnetization, materials with high permeability give a higher B field and thus more signal from the pickup coil. Alnico materials generally have a permeability less than steel, but much higher than a vacuum. Neodymium has a permeability only slightly larger than a vacuum. When it is used in a pickup, the highest output is obtained by using it with steel or ferrite pole pieces.