Antigua did it again

[LtKojak]

When it's about p'ups, I can only say the following:

"Not everything that can be measured counts, and not everything that counts can be measured"

The few scientific-prone hobbyists got a computer device that can make nice graphic representations of certain parameters, think that those are the only parameters that matter, and make outrageous statements as most lack the most basic understanding of how a p'up works, and even among the most educated members of this very Forum there are irreconcilable differences about some the most basic concepts, hence the inevitable "pissing contest" outcome.

An analogy I just came up with is: arguing that as all the measurements of every single part of their body are the same, Adriana Lima and Candice Swanepoel are the same person!

It's really incredible how some turns of wire around some magnets can generate such heated disputes! It's worse than religion!

I think the P'up Theory sub-forum should be created. That'll be a turning point in the selling of popcorn all over the world!

Food for thought...? I'll let you to be the judge of that!

[John Kolbeck]

You are missing (or not responding to) two key points here:

1. Reluctance implies circuit concepts, That does not cover the geometric complications of a guitar pickup.

2. Many are not embracing linearity; rather they are denying it. Many deny the magnetization of the string but claim it distorts the field of the permanent magnet nonetheless. How is never stated. This is the origin of the "belief" that you have to study the shape of the permanent field throughout the region near the pickup rather than find the field at the string and use it to compute the magnetization.

Sure, you can use the analogue between electrical and magnetic things: J = (sigma)E analogous to B = (mu)H in order to solve for the flux from the string in all space. But you still will have to use the law of magnetic induction to compute the voltage induced in the coil from the time varying flux. Why on earth would anyone look at something in a way that makes computations harder, and encourages others to make wrong deductions about how things work?

And why do you, time and again, claim that the term "variable reluctance" adequately describes a guitar pickup when the circuit concept implied does not cover the complexities?

After reading the different points of view on this, it seems to me that the simplest explanation is probably the most likely, and this is what seems to be the simple explanation to my mind, that magnetic fields follow the superposition principle, so when you have two magnets side by side, their fields sum together. When the string is near the pole piece, the string becomes magnetized, so you have the sum of the pole piece and the string, but when the string moves away, you no longer have the sum of the pole piece and the string, you just have the pole piece, and so as the string moves towards and away from the pickup, there is a rapid addition and subtraction occurring, and that is all that is required in order for induction to occur and for the pickup to work.

[Mike Sulzer]

After reading the different points of view on this, it seems to me that the simplest explanation is probably the most likely, and this is what seems to be the simple explanation to my mind, that magnetic fields follow the superposition principle, so when you have two magnets side by side, their fields sum together. When the string is near the pole piece, the string becomes magnetized, so you have the sum of the pole piece and the string, but when the string moves away, you no longer have the sum of the pole piece and the string, you just have the pole piece, and so as the string moves towards and away from the pickup, there is a rapid addition and subtraction occurring, and that is all that is required in order for induction to occur and for the pickup to work.

Suppose the string gets magnetized by the pole piece. This means that the string produces a field that is stronger closer to it and weaker father away. So if the string vibrates towards and away from the the coil, the field through the coil changes and a voltage is induced. It can be that simple.

One level more complicated: The permanent field of the pole piece changes with distance from the pole piece, and so the degree of magnetization of the string changes when it vibrates. This change also results in a variation of the flux through the coil, but it is less important than the first.

Next you could consider vibration in the other plane, and so on.

[John Kolbeck]

Suppose the string gets magnetized by the pole piece. This means that the string produces a field that is stronger closer to it and weaker father away. So if the string vibrates towards and away from the the coil, the field through the coil changes and a voltage is induced. It can be that simple.

One level more complicated: The permanent field of the pole piece changes with distance from the pole piece, and so the degree of magnetization of the string changes when it vibrates. This change also results in a variation of the flux through the coil, but it is less important than the first.

Next you could consider vibration in the other plane, and so on.

I see what you mean. Since the pole piece is magnetized and permeable at the same time, then when the string is closer, you would sum together not only the string and pole piece's permanent magnetic field, but also a third field, which is the additional magnetic field of pole piece's permeability responding to the presence of the magnetized string. Since the increase and decrease of that third magnetic field is proportionate to the increase and decrease of the second magnetic field, I'd think you could ignore it to some extent. It would serve to increase the signal's amplitude, as it causes a greater sum of flux change to occur, but it wouldn't change the harmonic makeup of the signal, as that has already be determined by the action of the second magnetic field, the string.

[Mike Sulzer]

I see what you mean. Since the pole piece is magnetized and permeable at the same time, then when the string is closer, you would sum together not only the string and pole piece's permanent magnetic field, but also a third field, which is the additional magnetic field of pole piece's permeability responding to the presence of the magnetized string. Since the increase and decrease of that third magnetic field is proportionate to the increase and decrease of the second magnetic field, I'd think you could ignore it to some extent. It would serve to increase the signal's amplitude, as it causes a greater sum of flux change to occur, but it wouldn't change the harmonic makeup of the signal, as that has already be determined by the action of the second magnetic field, the string.

Well, yes, it is quite complicated, but all I was saying after "one level more complicated" is that the string get magnetized to a higher degree when it is closer to the pole piece because the field of the pole piece falls off with distance.

[John Kolbeck]

Well, yes, it is quite complicated, but all I was saying after "one level more complicated" is that the string get magnetized to a higher degree when it is closer to the pole piece because the field of the pole piece falls off with distance.

Probably the bigger question is not so much whether the complications exist, as it's apparent that they do, but do the complications introduce new non-linearities, or do they amplify whatever caused them, in a linear fashion? If the complications all manifest as linear, then I'd reason that they're not going to change the tonal character of the pickup by much, so you could discount their significance.

[Joe Gwinn]

You are missing (or not responding to) two key points here:

1. Reluctance implies circuit concepts, That does not cover the geometric complications of a guitar pickup.

It does, well enough.

2. Many are not embracing linearity; rather they are denying it. Many deny the magnetization of the string but claim it distorts the field of the permanent magnet nonetheless. How is never stated. This is the origin of the "belief" that you have to study the shape of the permanent field throughout the region near the pickup rather than find the field at the string and use it to compute the magnetization.

At the field levels of practical pickups, all the magnetic materials are going to be linear. Nor is the variation due to strumming the guitar all that large a fraction of the steady field. So, superposition is going to work well enough.

Sure, you can use the analogue between electrical and magnetic things: J = (sigma)E analogous to B = (mu)H in order to solve for the flux from the string in all space. But you still will have to use the law of magnetic induction to compute the voltage induced in the coil from the time varying flux. Why on earth would anyone look at something in a way that makes computations harder, and encourages others to make wrong deductions about how things work?

Variable reluctance methods do use the law of induction. As for the confused, cannot help them. Trying to solve that problem is right up there with emptying an ocean with a teacup.

And why do you, time and again, claim that the term "variable reluctance" adequately describes a guitar pickup when the circuit concept implied does not cover the complexities?

Because it's good enough, and far cheaper and easier than the full-complexity approach.

But I'll grant you that large industrial producers of variable-inductance sensors instead use full-up 3D E&M Field solvers like COMSOL.

https://www.comsol.com/comsol-multiphysics Be sitting down when you read the price quote. Nor such tools all that easy to use - one must become a specialist to get anywhere.

[Mike Sulzer]

It does, well enough.




At the field levels of practical pickups, all the magnetic materials are going to be linear. Nor is the variation due to strumming the guitar all that large a fraction of the steady field. So, superposition is going to work well enough.




Variable reluctance methods do use the law of induction. As for the confused, cannot help them. Trying to solve that problem is right up there with emptying an ocean with a teacup.




Because it's good enough, and far cheaper and easier than the full-complexity approach.

But I'll grant you that large industrial producers of variable-inductance sensors instead use full-up 3D E&M Field solvers like COMSOL.

https://www.comsol.com/comsol-multiphysics Be sitting down when you read the price quote. Nor such tools all that easy to use - one must become a specialist to get anywhere.

I did not imply that linearity is not valid; in fact, it is an essential part of the analysis using the moving magnet method. But I did say something about how misinterpretations of the variable reluctance method cause people to deny the validity of linearity. Please read it again.

"Because it's good enough, and far cheaper and easier than the full-complexity approach."

You seem to be saying that somebody actually analyses guitar pickups using some sort of analogy involving variable reluctance. That is not how is is done, and no one would because it is too complicated. Take a look at the two analyses that have been discussed on this forum (from Princeton and UI). They use moving magnet.

[John Kolbeck]

I'm not sure how magnetic reluctance is useful in describing how pickups work in a physical sense. Magnetic reluctance is an abstraction that is meant to aid in computations, effectively hiding the underlying physics.

[Joe Gwinn]

I'm not sure how magnetic reluctance is useful in describing how pickups work in a physical sense. Magnetic reluctance is an abstraction that is meant to aid in computations, effectively hiding the underlying physics.

It does "hide the underlying physics" because this it is essential to do so, because solving Maxwell's Equations in a practical device by hand is intractable, and it can be intractable with a computer as well. So, one always approximates and simplifies. The art here is choosing the correct simplifications and approximations, so what's essential isn't lost. One progresses by comparing the results of the model with lab measurements, alternating between updates to the model and measurements in the lab until a satisfactory model with sufficient accuracy is achieved.

One can use the variable reluctance approach to estimate such things as output given a string motion and frequency. In the pickup world, we more often ar seeking qualitative understanding rather than precise numbers, and for this, simplifications are essential.

But if one has a 3D EM solver, by all means use it. The 2D solver FEMM is also quite useful.

[Mike Sulzer]

It does "hide the underlying physics" because this it is essential to do so, because solving Maxwell's Equations in a practical device by hand is intractable, and it can be intractable with a computer as well.

Counter example: The Princeton solution. Pickup voltages for both up and down and sideways motion are found by first solving for the string magnetization, and then finding the flux through the coil from the resulting field. Approximations are made as required.

On the other hand, reluctance cannot be used because the necessary approximations are not valid.

[John Kolbeck]

It does "hide the underlying physics" because this it is essential to do so, because solving Maxwell's Equations in a practical device by hand is intractable, and it can be intractable with a computer as well. So, one always approximates and simplifies. The art here is choosing the correct simplifications and approximations, so what's essential isn't lost. One progresses by comparing the results of the model with lab measurements, alternating between updates to the model and measurements in the lab until a satisfactory model with sufficient accuracy is achieved.

One can use the variable reluctance approach to estimate such things as output given a string motion and frequency. In the pickup world, we more often ar seeking qualitative understanding rather than precise numbers, and for this, simplifications are essential.

But if one has a 3D EM solver, by all means use it. The 2D solver FEMM is also quite useful.

Earlier you had said "As for the business about strings distorting the magnetic field, that is a very common way to describe the operation of variable-reluctance pickups in industry", I don't believe people who use the former language are referring to the latter, rather they are wholly misunderstanding what is happening, and so the objective is not to calculate anything in this context, but to explain it in a way that is true to the underlying physics, and then worry about the abstractions such as magnetic reluctance only after that understanding has been achieved.

[rjb]

I move to recess.
I come here primarily to learn (and secondarily to make wisecracks).
I don't believe I've gained any new knowledge or understanding since about this point in the thread:

Wars have and continue to be fought over which of these models are correct.
There is a problem - they are both right.

Those who understand both models know that they know.
Those who don't get it just don't get it.
Those who don't care just don't care.
May we leave it at that, and move on?

Thanks,
-Mr. Wisdom Seeker Guy

PS-
Am I more likely to reach salvation through prayer or good deeds?
I'm a busy guy and don't have time for both.

[DrStrangelove]

. . . requires no further comments . . .

I move to recess.

(cough)

[Mike Sulzer]

If you really have interest in understanding how pickups work, then put aside the old ridiculous line "All stories have two sides," and take a look at the article I am referring to (www.physics.princeton.edu/~mcdonald/examples/guitar.pdf).Section 1 is an introduction. (It does not require physics, and so it is accessible to all.) Near the end is this statement "Deduce the magnetic field when the string is displaced by (x,y) from its rest position,and from this deduce the voltage V (t) induced in the pickup coil by the vibrating string. " Then section 2 is the solution; you can easily verify that he is solving Maxwell's equations, albeit with relevant approximations, even if you do not know much physics. The quoted statement indicates that the paper intends to do what Joe says you cannot, and then it proceeds to do it. I do not know what Joe's motives are, or what his mental state might be, but it seems to me that if you want to reduce contention, you might address your comments to the person who is acting as a troll. Purposely inciting confusion should be strongly discouraged.

I move to recess.
I come here primarily to learn (and secondarily to make wisecracks).
I don't believe I've gained any new knowledge or understanding since about this point in the thread:

Those who understand both models know that they know.
Those who don't get it just don't get it.
Those who don't care just don't care.
May we leave it at that, and move on?

Thanks,
-Mr. Wisdom Seeker Guy

PS-
Am I more likely to reach salvation through prayer or good deeds?
I'm a busy guy and don't have time for both.