How Sensitive is a pickup to horizontal vs vertical string vibration?

[Mike Sulzer]

Think of a guitar sitting flat on a bench. Vertical vibration of the string is along the axis of a pole piece. Horizontal vibration is towards the sides of the guitar. We expect the pickup to be sensitive to the vertical vibrations of the string, but how sensitive is it to horizontal vibration? The answer to this question is interesting because it is useful in the evaluation of claims in at least one patent, and it also is important in determining the degree of non-linearity in the response of a pickup.

First, we construct a model of how a pickup works. If we have a loop (for example, bounded by a piece of wire) and think of a surface enclosed by it, then if there is a magnetic field passing through the surface, when the component of the field perpendicular to the surface changes, a voltage dependent on the rate of change is induced around the loop. You can think of this as happening at each point on the surface, and you add up all the voltages caused by a changing field through each point. If you put many loops in series, the voltages induced in each add.

The program FEMM is used to model magnetic circuits. It has limitations, related to the possible geometry of the circuits, and the accuracy of the results. The latter often shows up as small random variations of the field from point to point. This "noise" can hide small changes in the actual field, and we must construct the model so as to avoid the effects of this noise.

If we construct a complete model, magnet, core, and string, and move the string, the small resulting change in the field through the core is obscured by the modeling errors from the large field from the permanent magnet. On the other hand, if we remove the magnet from the core, and we replace the string with a small magnet, it is possible to measure the change in the field through the core when the magnet moves.

To see how the string can be considered a magnet, consider an electromagnet. Wire is wrapped around a core of iron (or other magnetic material). A current is passed through the wire, making a small magnetic field. The core responds to this field by amplifying it when its small magnetic domains, initially randomly oriented, tend to line up with the applied field; that is, the core becomes magnetized. The result is a stronger magnetic field. The string also responds to the permanent field from the magnet (and pickup core) by becoming magnetized.

This is shown in this figure:http://www.naic.edu/~sulzer/compStringNoString.png. The FEMM model has a 1018 steel pole piece with an alnico magnet on the bottom. The plot shows the field nearly along the axis of the core, up from the bottom. The blue line is the field when a small piece of steel is placed above the pole to act as a string; the red line is the field without the "string." Except at or near the string, the field strengths in the two cases are nearly the same, differing by the random "noise", or errors, mentioned above. The field increases inside the string because the magnetic domains of the steel tend to line up with the applied field, reinforcing it. And this field from the string extends outside of the string, but it becomes weak quickly, and it is obscured by the noise inside the pole piece where we need to look.

Thus, in the analysis to be described in the next posts, the alnico magnet is removed from the pole piece. The string is replaced with a small magnet. The magnet is moved vertically or horizontally to simulate motion of the string, and the changes in the field are examined.

[David Schwab]

Thus, in the analysis to be described in the next posts, the alnico magnet is removed from the pole piece. The string is replaced with a small magnet. The magnet is moved vertically or horizontally to simulate motion of the string, and the changes in the field are examined.

My answer would be it depends on the design of the pickup.

If the information in the two Bartolini patents, as well as others concerned with shaping the flux field in which the strings vibrate, is an accurate demonstration of the way the pickup senses the strings, than moving a magnet over the pickup is not a proper simulation.

The Bartolini patent (as well as others) clearly shows that when you have static lines of flux over the pickup, the shape of those lines determines which string movement directions the pickup is most sensitive to. So if a rod shaped magnet pole piece has it's lines of flux extending in a vertical cone shape over the pickup, the string will mostly disturb the fields with it's side-to-side motion. What Bartolini did with the planar pole tips was to crate more lines of flux parallel to the top of the pickup, to sense more of the up-and-down motion of the string, as to simulate an acoustic instrument's sound board, since most of their movement is from the up-and-down motion of the string.

My own experience with different pole shapes, as well as Bartolini's original Hi-A (high asymmetry) designs does indeed show that the pole shape and resulting field shape have much to do with the resulting tone of the pickup. I've made many of my designs with both rod magnets and blades of different types to see how the tone was affected. There is a subtle (or not so subtle) but tangible difference. Finding the words to describe it is another thing. Rod poles had a "wetter" and somewhat "glassier" top end compared to blades of various thicknesses. Even square poles have a different tone.

You can move a magnet over the pickup, but then you are leaving out the shape of the magnet and it's corresponding field shape, along with the poles or other magnetic parts of the pickup from the equation.

[Mike Sulzer]

You can move a magnet over the pickup, but then you are leaving out the shape of the magnet and it's corresponding field shape, along with the poles or other magnetic parts of the pickup from the equation.

The effect of the permanent magnet and pole piece is to magnetize the string. When you say "disturb the field" you are speaking in poetry, not engineering or physics. The magnetization of the string is the mechanism by which the string disturbs the field, the poetry turned into physics. Therefore, if we accurately simulate the magnetization, we have accomplished the first step in simulating the operation of the pickup.

[Diablo]

The effect of the permanent magnet and pole piece is to magnetize the string. When you say "disturb the field" you are speaking in poetry, not engineering or physics. The magnetization of the string is the mechanism by which the string disturbs the field, the poetry turned into physics. Therefore, if we accurately simulate the magnetization, we have accomplished the first step in simulating the operation of the pickup.

So, has anyone designed a guitar with an electromagnetic coil around the string? Seems like you could control the magnetic strength of the string simply by changing the current in the coil.

[soundmasterg]

So, has anyone designed a guitar with an electromagnetic coil around the string? Seems like you could control the magnetic strength of the string simply by changing the current in the coil.

I would think that would be really hummy and noisy myself....


Rickenbacker did their Horseshoe pickup which surrounds the string, but its not an electromagnet.

Greg

[Mike Sulzer]

The two ways that the magnetized string affects the magnetic flux through the core.

When the magnetized string moves, the magnetic field in the surrounding area changes. Changes in the core, and hence the coil result in a voltage induced in the coil. Thus we must analyze why the field changes. There are two reasons:
1. As the string moves, it moves into space where the permanent field is different. For example, as the string move away from the pole piece, the permanent field decreases, and so the magnetization changes.
2. The magnetic field of the string decreases with distance from the string. Therefore, if the string moves, even if the magnetization remains constant, the field through the core changes because the distance to the core changes.

The fact that there are two effects is potentially interesting. They appear together as a product, multiplication, and therefore cause a non-linearity. Whether the non-linearity is significant is another question. I have not yet computed its effect.

Also, horizontal motion causes an additional non-linearity that we will discuss later.

The next few posts will analyze first reason number one, and then reason number two.

[Diablo]

I would think that would be really hummy and noisy myself....


Rickenbacker did their Horseshoe pickup which surrounds the string, but its not an electromagnet.

Greg

I don't think it should hum because it's DC in the magnetizing coil. Maybe it would be potentially dangerous with the amount of current needed?

[David Schwab]

The effect of the permanent magnet and pole piece is to magnetize the string.

We have been through this before, and I disagree. It's a variable reluctance pickup. The fact that the string becomes magnetized is of little consequence, unless you call a ferromagnetic object in a magnetic field magnetized. The word "magnetized" is defined as: "give magnetic properties to; make magnetic." and this is not the case. If it were, you could remove the magnets from the pickups and they would still work, but they really don't. Any magnetism induced in the strings is minimal at best and it overwhelmed by the permanent magnet's field. It would also probably oppose the permanent magnet's field.

The permanent magnet is where it's happening, not the strings. A static field wont produce current, but a disturbed field (reluctance) will.

So yeah, moving a magnet over the coil will induce current, but that's not how you would test to see what motion of the string is induced in a pickup... that just checks the motion of a magnet over a coil. Try making a mechanical device that moves a piece of metal rapidly in either plane over a magnetic pickup.

So, has anyone designed a guitar with an electromagnetic coil around the string? Seems like you could control the magnetic strength of the string simply by changing the current in the coil.

Some of the original pickups had the coil first act as an electromagnet, to magnetize the strings, and then would switch to a pickup coil. However strings make poor magnets, so it didn't work well, which is why it was never a viable pickup design. The designs also did produce noise.

[Diablo]

We have been through this before, and I disagree. It's a variable reluctance pickup. The fact that the string becomes magnetized is of little consequence, unless you call a ferromagnetic object in a magnetic field magnetized. The word "magnetized" is defined as: "give magnetic properties to; make magnetic." and this is not the case. If it were, you could remove the magnets from the pickups and they would still work, but they really don't. Any magnetism induced in the strings is minimal at best and it overwhelmed by the permanent magnet's field. It would also probably oppose the permanent magnet's field.

The funny thing is there is a Seth Lover interview on the www that I read about how the humbucker pickup works, and he described it exactly in the manner that you say it doesn't work. He said the strings are magnetized by the permanent magnet in the pickup, and that the pickup would continue to work for a while even when you removed the magnet because of the residual magnetism in the string. My understanding of magnetism is too basic to sort this out. All I know is that if you have a conductor in a changing magnetic flux, you generate a current. I don't think you can separate the effect into the string's magnetic field or the string's effect on the reluctance - because you can't get an effect on the reluctance without the string being ferromagnetic. It's just two ways at looking at the same thing.

[Mike Sulzer]

Is it magnetization?

Yes, and it disappears almost completely immediately when the applied field is removed. Yes, we have discussed this before, on this forum and others. Magnetization is the lining up of previously randomly oriented magnetic domains. If you think a change in reluctance is caused by something else, please explain what it could be. In science and engineering we deal with measurable physical effects, not with picky definitions of terms.

[Joe Gwinn]

Yes, and it disappears almost completely immediately when the applied field is removed. Yes, we have discussed this before, on this forum and others. Magnetization is the lining up of previously randomly oriented magnetic domains. If you think a change in reluctance is caused by something else, please explain what it could be. In science and engineering we deal with measurable physical effects, not with picky definitions of terms.

Umm. The two approaches (reluctance, induced fields) are mathematically equivalent.

[Mike Sulzer]

Mathematical equivalence ==> physical equivalence

Umm. The two approaches (reluctance, induced fields) are mathematically equivalent.

Yes, and that mathematical equivalence implies physical equivalence: the two methods model the same reality. The issue is convenience in understanding and computation.

[David Schwab]

My point was about the shape of the field and how it affects the way the string is picked up. That's why I referenced the Bartolini patents. Bartolini references a patent by Zoller (3588311) which does something similar.

Pickups can and have been made to be more sensitive to the up-down motion of the string.

So whereas a moving magnet will simulate a moving string over a magnet, it will probably have a different field shape than a real pickup (depending on the pickup design of course), so unless you are talking about something very simple like a Fender pickup with rod magnets, you wont get proper results, and even then you are leaving the other 5 magnets out of the model. And then humbuckers are a different story.

What this test shows is the sensitivity of a moving magnet to up-down or side-to-side motion over a coil, and not a string moving in a more complex magnet circuit.

[bbsailor]

Asymmetry

My answer would be it depends on the design of the pickup.

If the information in the two Bartolini patents, as well as others concerned with shaping the flux field in which the strings vibrate, is an accurate demonstration of the way the pickup senses the strings, than moving a magnet over the pickup is not a proper simulation.

The Bartolini patent (as well as others) clearly shows that when you have static lines of flux over the pickup, the shape of those lines determines which string movement directions the pickup is most sensitive to. So if a rod shaped magnet pole piece has it's lines of flux extending in a vertical cone shape over the pickup, the string will mostly disturb the fields with it's side-to-side motion. What Bartolini did with the planar pole tips was to crate more lines of flux parallel to the top of the pickup, to sense more of the up-and-down motion of the string, as to simulate an acoustic instrument's sound board, since most of their movement is from the up-and-down motion of the string.

My own experience with different pole shapes, as well as Bartolini's original Hi-A (high asymmetry) designs does indeed show that the pole shape and resulting field shape have much to do with the resulting tone of the pickup. I've made many of my designs with both rod magnets and blades of different types to see how the tone was affected. There is a subtle (or not so subtle) but tangible difference. Finding the words to describe it is another thing. Rod poles had a "wetter" and somewhat "glassier" top end compared to blades of various thicknesses. Even square poles have a different tone.

You can move a magnet over the pickup, but then you are leaving out the shape of the magnet and it's corresponding field shape, along with the poles or other magnetic parts of the pickup from the equation.

David, Mike, and others

The asymmetry of the guitar pickup signal is best viewed on an oscilloscope to observe that one of the poles of the signal , either above or below zero is about 20% to 25% higher in amplitude than the other pole. This occurs for one very simple reason; vertical string motion generates more output than horizontal string motion. This is a complex analysis as strings vibrate in both planes.

If a similar pickup were held/mounted above a lower pickup at the same distance and the coils were put in series, in phase, the output would be more symmetrical as the upper pickup would smooth out the asymmetrical errors that favors the lower motion of the strings producing more output than the upper motion of the strings being farther away and in a weaker magnetic field.

String output and harmonic structure content is pretty sensitive to the closeness of the string to the pickup's magnetic field. Some form of asymmetry will always be present when the strings are mounted above the pickup's prime magnetic field.

The high asymmetry Bartolini patent takes a potential defect and converts it into a feature through the magic of marketing words and some technical ways to sample a wider portion of the string. Sampling a wider portion of the string would tend to minimize the natural asymmetry somewhat.

My low impedance pickup research has revealed this oscilloscope-viewed asymmetry whenever there is a strong and focused magnetic field beneath a string whether it be a high or low impedance coil.

Joseph Rogowski

[Diablo]

David, Mike, and others

The asymmetry of the guitar pickup signal is best viewed on an oscilloscope to observe that one of the poles of the signal , either above or below zero is about 20% to 25% higher in amplitude than the other pole. This occurs for one very simple reason; vertical string motion generates more output than horizontal string motion. This is a complex analysis as strings vibrate in both planes.


Joseph Rogowski

Since the string is vibrating in both planes, how do you know that the vertical motion generates a stronger signal than the horizontal motion? Do you have a way to excite a string in one plane at a time and compare signal strength?