a couple magnet theory questions...

[FunkyKikuchiyo]

I've been messing with a couple different things, and I can't seem to answer my own questions...

Let's say you're dealing with steel slugs and a long magnet on the bottom of the pickup.

First question: Does the full length of the magnet affect the overall magnetic field or the sound? For example humbucker mags sometimes are of varying length, will that length make a difference?

Second question: A given magnet 2.5" long will have one effect. Will using two 1.25" magnets or four .625" magnets have a different effect? If you look at magnets placed together like that with viewing film, the magnetic field has a very different characteristic, but I'm not convinced it'll have a practical difference.

I'm struggling with both of these. Common pickup knowledge base might advise that the shape of the magnetic field is a bit part of the character of the pickup, but I don't quite buy that. Without a permeable baseplate or something, the signal is still going to be dependent on the magnetic field being altered via the pole pieces, and any extraneous gauss lines passing through the coil don't seem useful. I think many theories erroneously treat the copper wire as though it is magnetically permeable, like drawings I've seen showing P-90s as having a very wide aperture. The pole pieces are still what are doing the "sensing", the coil doesn't become an extension of the sensing area. I do see how it could affect the behavior of eddy currents and the inductance for the outer coils though. It seems to me like it might be a case of good observations but bad conclusions, which I run into a lot with theories of luthiery.

My other problem is that field lines would (in theory) work towards the permeable material in the same fashion based on the placement of the magnet and pole pieces rather than dependent on the shape of the magnet. The field lines will orient within the pole pieces in a fashion determined more by the properties of that material rather than the shape of the magnet if my thinking is correct. The viewing paper doesn't confirm that many small magnets act like a large magnet, and it is too imprecise to say how it affects the steel, though that part doesn't seem to change.

For argument's sake let's say we're dealing with ceramic or NdFeB, I know alnico can have degaussing problems with varying dimensions.

Normally I'd do experiments on my own to confirm or deny this, but I haven't been able to come up with a good controlled experiment for this. If I made p-90s or something similar I'm sure I could come up with something, but even then I'm too cheap for a gaussmeter.

Sorry to throw stupid questions at you all, but it seems like you're a crowd that enjoys head scratchers and arguments.

[LtKojak]

it seems like you're a crowd that enjoys head scratchers and arguments.

Yes, every opportunity is good for a heated argument, we've been a bit TOO polite for a full two month now?

C'mon... you know WHO you are and you all KNOW you want to!

[Possum]

...

Thats too damn long a question to answer, I'll just spit out one point. A P90 has a wide aperture because the coil is very short and wide. The flux is going from the very bottom outside horizontal edges of the pickup up to the pole screws, so if you look at a P90 sideways from the ends, you can see that the flux will going to the pole stops and that the flux doesn't go in a straight line but arcs up the screws. So you can see that there is flux going horizontally out from the pole screws more than a tall strat pickup where the flux is coming up from the same width bottom pole end. Hope that makes sense, so you ARE getting a bigger aperture for the strings to vibrate in and you don't really have much of a problem with volume dropout when bending a string on a P90. I have seen some real early P90 type pickups that had rows of small magnets with their pole point to and touching the keeper or screws, but its doing the same thing, the other end of the magnet it out at the horizontal bottom edges of the pickup with that big arc up to the pole screws. The ones I saw had all the small magnets side by side against eachother so probably didn't sound any diff than a bucker magnet. I'll leave the rest for others to comment, I think used up every word in my small vocabularly and broke my brain....

[Mike Sulzer]

Here is why a P-90 does not have a wide aperture:

The part of the string that contributes to the signal is the part that is magnetized significantly with the field lines pointing down through the coil. The screw magnetizes the string; the field is strongest right over the screw and it points in the correct direction. The magnetic field falls of quickly with distance from the screw head, and the field points at an angle.

But there is more. When the magnetized string vibrates, the time varying magnetic flux is directed through the coil by the magnetic material of the pole piece. This works most efficiently right over the screw head. So, even if the strong were uniformly magnetized in the correct direction everywhere, the part over the screw head would still contribute most.

Both these effects matter; they multiply together to give an aperature that is tightly confined to the part of the string near the screw head.

[Yves]

I think you may partly answer your questions by doing some magnetic simulations using FEMM sotware that can be found here:
Finite Element Method Magnetics: HomePage

Indeed this program has been discussed here years ago (I think in the old forum).

OK, this isn't real life and this remains in 2D, still I think it can halp getting an idea of the shape/aperture of different magneting fields depending on the assembly.

Here are basic examples I did with it (HB, P90, strat like).

Actually, by the time this was discussed in the forum, SK did quite a number of simulations with different models, you can find them here:
Magnetic fields in pickups

Hope this helps.

[Possum]

...

True but wrong. The string also gets heard away from the sides of the pole screw to some extent, because the flux doesn't shoot straight down to a bottom pole direction below the top pole like a strat pickup does; you're hearing more of the side to side arc of a vibrating string in a P90 because the flux is going out more horizontally to meet the wide space double pole edges on the bottom. The proof here would be to use a single magnet with north and south poles on top and bottom of the magnet with one pole directly on the bottom of the screws. You should hear a distinct difference. Also note that I tell my customers if they want the fattest tone out of a P90 you have to have the strings down real close to the poles tot take advantage of that wider aperture. I believe Duncan or someone else I read also recommends this as well. one note about that also, is that Asian P90 guitar makers don't seem to have a clue about this and almost always build their P90 guitars with the pickups way too far below the strings, so the guitars sound real tinny; these guitars don't sell well and eventually get dumped on Ebay, I have several good ones, that with shims can be real nice guitars....

I see someone posted the simulations, I think a better experiment would be to get some magnetic viewing fluid to see the real deal...if you go by those simulations a strat pickup would have a bigger aperture, which is baloney, drop outs are always a problem with strats and not with P90's nearly as much...

OK, I have some magnetic viewing paper and put it over a strat and a P90, you can plainly see a wider flux area around P90 poles than a strat, the strat has a hotter field around the poles but much smaller radius.....try it for yourself..

[Mike Sulzer]

True but wrong.

It is the vertical component of the permanent field where the string is located that matters. Those plots you refer to are not well-scaled as they could be, but they do show that in both cases:

1. The field gets weaker rapidly as you move horizontally away from the center of the pole piece. (The field lines are diverging; this indicates a weakening field.)

2. The field is becoming horizontal. (This is indicated by the direction of the field lines.) So not only is the total field weaker, but the vertical component (the fraction of it that points vertically) makes up a smaller part of the total.

Then in addition there is the sensitivity of the coil to the changing field. It is much more sensitive to changes over the pole piece (as when you put the string close to the pole piece).

I do not understand how the viewing paper tells you about the vertical component of the field where the string is located.

These pickups are sensitive to the vertical motion of the string since that is what changes the vertical pointing flux through the coil most efficiently. They are only slightly sensitive to horizontal motion.

[FunkyKikuchiyo]

I wish I had a p-90 handy. It seems a great test would be to plug one in raw (or at least with no strings in front of it) and move a tuning fork back and forth to see how it disappears.

I do like Possum's point about flux orientation. A vibrating string is three dimensional and thus has three vector forces, just as pick attack changes the tone, it makes sense that the direction of flux lines would make a difference. I think of the difference between free stroke and rest stroke for classical players in particular. It makes sense with some pickups, bardens, p-90s (and others?) that seem more sensitive to height than other pickup designs.

Any more thoughts? Tone nerds, put up your dukes......

[fieldwrangler]

Is this truly the case?

It is the vertical component of the permanent field where the string is located that matters.

Horizontal lines of force moving coherently through conductors don't count?

[Mike Sulzer]

It is the vertical component of the permanent field where the string is located that matters.

Horizontal lines of force moving coherently through conductors don't count?

I do not know how familiar you are with E&M, but you might want to look here: Faraday's law of induction - Wikipedia, the free encyclopedia.

The key thing is the definition of magnetic flux. Scroll down until you find the equation. It contains the "dot" product between dA, a small piece of a surface, and the field B. A vector perpendicular to the surface dA defines its direction. The result of a dot product is the product of the magnitudes of the two vectors if they point in the same direction, zero if they are perpendicular. In general it is given by the cosine of the angle between them. You have to add up the products over a surface containing each loop of wire. You have a great deal freedom how to draw the surface, but the easiest is to use the flat plane containing the loop. Then it is easy to see that the maximum sensitivity is obtained when when the field points straight down the coil. The sensitivity is zero if it points perpendicular to the axis of the coil. In general it is given by the product times the cosine of the angle, that is, the vertical component.

[fieldwrangler]

Most Interesting!

Mike -

I need to spend a bit of time going step by step through the path you've provided relating to the "dot" product.

However, I think the basis of my misunderstanding may be a simple lack of orientation regarding the meanings of "horizontal" and "vertical" in this context.

Thanks for the guidance here.

[fieldwrangler]

Also...

... I realize that I have a basic misunderstanding regarding the components of the definitions that define the orientation of the "field" and the direction of the "lines of force."

[Possum]

...

Got a new camera today so took a shot of a strat pickup and a P90 with magnetic viewing paper sitting directly on both. You can see the lighter "glow" around each group of poles, and note that you can see the two fields reaching out to the other. I think if you look at this you can see why P90's have a fatter tone, the P90 has a large oval kind of field that is bigger and softer than the strat pickup's more intense smaller field....

[David Schwab]

First question: Does the full length of the magnet affect the overall magnetic field or the sound? For example humbucker mags sometimes are of varying length, will that length make a difference?

Second question: A given magnet 2.5" long will have one effect. Will using two 1.25" magnets or four .625" magnets have a different effect? If you look at magnets placed together like that with viewing film, the magnetic field has a very different characteristic, but I'm not convinced it'll have a practical difference.

I use multiple short neos between two blades for some pickups. I read an even field strength across the blades. I do that just because those are the available sizes. I could have a custom magnet made, but it works fine this way. I can't say that applies to alnico though.

The thickness of a magnet affects the strength. So a thicker alnico should be stronger than a thinner one because of the difference in the pole surface area.

But I'm sure all the other dimensions have an affect as well.

When you looked at them with the magnetic viewing paper, where they touching the poles? Or just loose on a table? You need to look at them in place.

[David Schwab]

Got a new camera today so took a shot of a strat pickup and a P90 with magnetic viewing paper sitting directly on both. You can see the lighter "glow" around each group of poles, and note that you can see the two fields reaching out to the other. I think if you look at this you can see why P90's have a fatter tone, the P90 has a large oval kind of field that is bigger and softer than the strat pickup's more intense smaller field....

The brighter area seems to be the null point between the poles. So with the rod magnets this gives them a smaller area around the top of the magnet. The P-90 has the magnets laying on their sides, so the opposite poles are farther away.

On a blade humbucker you can see the brighter area as a thin line right between the two blades.