Low Impedance Over and Under Pickup Design

[Mike Sulzer]

Faraday's law of magnetic inductionn is all you need.

Mike, the classic theory of pickup operation is that the ferrous metal strings disturb a static magnetic field inducing a current in local conductors...the coil. So that would be mode 1

Another mode could be "moving shunt" with the string being a magnetic shunt between two or more poles...for example the North/South of a humbucker. I do think this is one of the more important modes for the micro-humbuckers that make up the Roland GK series of synth controller pickups. They suggest very, very close to string mounting, and clearly when mounted this way, the string is acting as a dynamic magnetic shunt in the magnetic circuit.

And, of course, we have the moving magnet theory whereby either permanent or electromagnet is used to charge the string...but is it a monopole? Highly unlikely! Or does a single coil pickup charge the string as a tri-pole? North or South directly over the magnets and weaker opposite pole on either side of that? Or is the bottom of the string one polarity and the top the other? Hmmmmm....

I've got the questions if you have the answers...

Everything else is just so much BS.

[Rick Turner]

So nice to see things so simplified...

Simple minds, and all that...

[Rick Turner]

Mike, I suggest that you take your training in "science" and watch this:

YouTube - The Ask Dr Science Official National Science Test Part 1

Don't forget that Dr. Science has a Master's Degree in Science, and he knows more than you do!

[Mike Sulzer]

Mike, I suggest that you take your training in "science" and watch this:

YouTube - The Ask Dr Science Official National Science Test Part 1

Don't forget that Dr. Science has a Master's Degree in Science, and he knows more than you do!

Rick, if you understood much about E&M, you would know that the different modes that you mentioned are all examples of how the law of magnetic induction works.

[Mark Hammer]

Before we descend into conflict, allow me to say this is an utterly fascinating thread. Here's a naive question that may either be a nuisance distraction or a productive digression. I'll let you decide.

The axis along which a string vibrates/is deformed most would be precipitated by how it is picked, since the picking applies directional force. Eventually, the string devolves into a sort of entropic vibration, going from a relatively directional vibration, to omnidirectional. Players vary in how they initially apply force in a directional manner. Some, like Les Paul with his broken arm set in a particular fashion, can pick in a more consistent direction, while others can pick in a consistent fashion but very different direction (consider Stanley Jordan's piano-like two-handed style), and still others can pick in a much less consistent style, striking the strings this way and that.

Although the string is certainly most compliant and has the most freedom of omnidirectional movement at the middle of the vibrating zone (which, of course, will move around, depending on where you're fretting) I would imagine that a small part of directionality of string movement - to the extent that there is any bias in that - might be given by what the bridge allows. In other words, is it possible for the structure of where the string sits to "nudge" the string to vibrate in some preferred fashion by damping non-preferred vibrations, or is the amount of pressure applied by the string at the saddle sufficient to over-ride any such "nudging"? I ask this because the discussion has revolved around where the string does and doesn't move around, without any discussion of making the string move in any semi-predictable way, or at least examining what different types of bridge/saddle structures might do in the way of influencing directionality of vibration.

Do ALL bridges let strings "wander" exactly the same way? Alternatively, do strings "wander" differently, and perhaps more directionally, the higher up the fretboard you go? Just wondering.

[bbsailor]

Before we descend into conflict, allow me to say this is an utterly fascinating thread. Here's a naive question that may either be a nuisance distraction or a productive digression. I'll let you decide.

The axis along which a string vibrates/is deformed most would be precipitated by how it is picked, since the picking applies directional force. Eventually, the string devolves into a sort of entropic vibration, going from a relatively directional vibration, to omnidirectional. Players vary in how they initially apply force in a directional manner. Some, like Les Paul with his broken arm set in a particular fashion, can pick in a more consistent direction, while others can pick in a consistent fashion but very different direction (consider Stanley Jordan's piano-like two-handed style), and still others can pick in a much less consistent style, striking the strings this way and that.

Although the string is certainly most compliant and has the most freedom of omnidirectional movement at the middle of the vibrating zone (which, of course, will move around, depending on where you're fretting) I would imagine that a small part of directionality of string movement - to the extent that there is any bias in that - might be given by what the bridge allows. In other words, is it possible for the structure of where the string sits to "nudge" the string to vibrate in some preferred fashion by damping non-preferred vibrations, or is the amount of pressure applied by the string at the saddle sufficient to over-ride any such "nudging"? I ask this because the discussion has revolved around where the string does and doesn't move around, without any discussion of making the string move in any semi-predictable way, or at least examining what different types of bridge/saddle structures might do in the way of influencing directionality of vibration.

Do ALL bridges let strings "wander" exactly the same way? Alternatively, do strings "wander" differently, and perhaps more directionally, the higher up the fretboard you go? Just wondering.

Mark,

Many of the factors that govern the guitar sound that the ear pecceives is based on the interaction of many variables. Let me speak of one factor that is related to your question. "Do all bridges let strings "wander" exactly the same way"?

The short answer is no. Lets look at the transferance of string energy to the guitar/banjo body as sort of a continuous progression from those instruments that suck energy from the strings, banjo, acoustic flat top guitar, archtop jazz guitar, semi solid body guitar to the solid body guitar which sucks the least energy from the string. The bridge assembly on the electric guitar is typically made of metal with mechanical mechanisms to adjust individual string height as well as string length and are designed to only securly anchor the strings.

The banjo has a unique sound as the initial string plunk is very loud and is quickly damped so the banjo has a pretty percussive character which determined the playing styles that evolved. Early archtop jazz guitarists sat in the rhythm section of the band and used the percussive high amplitude of the initial strum to cut through the other instruments to be heard.

These early guitars used very heavy strings and when amplified by early magnetic pickups evolved into the jazz guitar sound the complimented other band instruments, mostly drums, piano and bass when being featured in a smaller set of instruments.

Over time the solid body electric guitar evolved into a popular instrument that in order to be easily played by beginners with little hand strength, used light strings. These light strings allowed for beginners to easily make music, stretch strings in multi-note string bends and develop a modern playing style that continues to this day. But, using light strings has a sound consequence compared to using heavier strings. Electric pickups that sample a string in a small location over the pickup region sound different from an acoustic guitar pickup that responds to wider string harmonics and dynamics.

When you combine the consequences of light versus heavy strings, bodies that absorb string energy versus not absorbing much string energy, and how the pickups respond to the physical mass of the moving string over magnet of various strength and coils of a variery of number of turns, you can see why looking at one factor in isolation may not fully answer your question.

Your question certainly moves the discussion in a civilized and technical direction.

Joseph Rogowski

[Mike Sulzer]

Over time the solid body electric guitar evolved into a popular instrument that in order to be easily played by beginners with little hand strength, used light strings. These light strings allowed for beginners to easily make music, stretch strings in multi-note string bends and develop a modern playing style that continues to this day. But, using light strings has a sound consequence compared to using heavier strings. Electric pickups that sample a string in a small location over the pickup region sound different from an acoustic guitar pickup that responds to wider string harmonics and dynamics.


Joseph Rogowski

Joseph,

I think that is a large part of the reason for moving to lighter strings, but not all of it. The sound of the lighter strings is an important part, also. Since the string on an electric guitar does not need to produce a lot of energy as on an acoustic, you have more freedom in choosing the sound.

[Mark Hammer]

Thanks for the reply.

Yes, banjos are an excellent example to draw from. The high compliance of the banjo head and floating bridge most certainly work to dampen "vertical" (towards and away from the body) vibrations.

I guess the theme that I was moving towards was that, since we are talking about hypothetical pickup designs that maximally sense deformations in the magnetic field produced by the string, we need to know something about where those maximal deformations are more, and less, likely to occur (and there are 360 degrees of movement to search through!). As well, it would be useful to know if there are conditions under which those maximal deformations would occur differently.

Logically, and correct me if the limb I'm crawling out on is about to snap, the location of any "ideal" arrangement of sensing coils around the string would depend on any inherent directionality to those vibrations/deformations.

Of course, if they are inherently omnidirectional, all the time, for all notes, then it really doesn't matter where you stick them, and one couldn't expect to do better than a horseshoe pickup.

Alternatively, let us consider sensing on the horizontal axis by the bridge. That would place coils between strings, although it would mean that you could rest the butt of your hand on the bridge and make Sonny Landreth happy (and having watched him play, I'm ALL for whatever makes him happy). I wonder if sensing horizontally by the bridge (where the fundamental shies away from), coupled with sensing vertically via a horseshoe closer to the neck, would yield some ideal coverage of the full response of the string.

[Rick Turner]

Mike, you can quote laws all day, and they simply miss some of the subtleties here. You can sweep everything under one heading, you can sweep it all under the carpet, but you can't duck issues and questions without my noticing it. You seem to rely strongly on book learning here; I don't read much from you regarding your own pickup winding or sound. The way you present things here, it's all theory, and no experience. It's all up in your head, and I'm not getting any sense of hands-on and listening.

You may say that all I've presented falls under one heading, but you seem to also imply that the modes I'm referring to are either irrelevant or are indeed true.

You've also ducked the issue of aperture quite smoothly. How can pickups work at all if the magnetized section of the string is only over a pole...where the coil isn't? Yet that is what you claimed.

I do not claim to be a scientist as you evidently do. I'm just a guy with about 40 years experience winding pickups and listening to them. I've also worked with some of what I have found to be the best audio engineers in the biz, and they do not come off high and mighty and hide behind textbooks. They listen, and then they try to apply what the books say (which is incomplete at best) to understanding what is going on.



And Mark, yes, the typical design of a bridge predisposes the string to vibrate in a particular way because of the bend over the saddle. It introduces a non-linearity in the (generally) vertical plane by mechanically biasing the string. Many years ago a fellow named Jeff Tripp designed a bridge using Fender bullet strings that held the strings in a manner more like a collet; the "bullets" were swaged onto the string ends. This bridge did not bend the string at the witness point, and the theory was that the string could vibrate more linearly that way. You can really see this string bend on electric basses using standard strings, especially with through the body stringing. This makes intonation even more of a nightmare than it usually is.

Some of the wandering of the plane of vibration is certainly affected by the bridge design and even the underlying top and body structure. For instance, many years ago I noticed that I could get action lower on a 335 than on a Les Paul...same strings, same scale length, same tailpiece and bridge hardware. Why? Because of the natural sustain of the Les Paul. With the 335, the excursion of the strings is more damped, and so when you pluck the string more or less parallel to the top, by the time the fundamental plane of vibration moves around to being at right angles to the top, the excursion has died down more than it would on a heavy solid body.

Also note that string vibration is always along a plane of vibration for a given harmonic. An antinode of a string cannot move in two different directions at once, though different harmonics may be moving in different planes, riding on lower "carrier" harmonics. I'd have to do a lot more research to check that out. Some of the string gurus like the folks at Thomastik-Infeld, Norman Pickering, or the D'Addario engineers probably have that stuff worked out...and they're not talking. I do have a T-I string vibration CD that I'll look at again.

There's yet another interesting issue that doesn't affect us plucked string people much, but is a big deal in the bowed string world, and that's the torsional effect of the bow on the string which wants to twist the string back and forth. The way a bow works is that the rosin on the hair acts like a glue, and the bow is stuck and released at the string frequency. The friction of the bow heats the rosin just enough for it to stick and release, stick and release, and of course, bow position, bow aperture on the string, and the bow pressure all have tremendous effects on harmonic response.

[Mike Sulzer]

You've also ducked the issue of aperture quite smoothly. How can pickups work at all if the magnetized section of the string is only over a pole...where the coil isn't? Yet that is what you claimed.

I explained this to you above, but since this is an issue that could be of interest to others, here is a more complete explanation.

You can read about the law of induction here: Faraday's law of induction - Wikipedia, the free encyclopedia. This is worth doing if you have any interest in physics because you will see that the understanding of this law, and how it covers the effect of not just one, but two, terms in Maxwell's equations, helped lead Einstein to special relativity.

The pickup coil is many loops of wire in series. The voltage induced around each loop results from a changing magnetic field passing anywhere through the loop. It is the changing magnetic flux that counts, that is, magnetic field summed over the area inside the loop.

Repeated for emphasis: passing through anywhere inside the loop. This means that a changing field from a portion of the vibrating string over a pole piece counts just as much as if the changing field passed through the loop closer to the wire. It does not matter where. This might seem amazing, but it is nonetheless true. If it were not true, special relativity would fail and modern physics would fall apart.

It is also true that because the law of induction covers both terms in Maxwell's equations that could result in an induced voltage, it can be used to describe any possible pickup configuration. So what is necessary for a vibrating string to induce a voltage in a coil? Flux from the vibrating string must pass through the coil. This is only possible if the string is magnetized; otherwise there is no varying magnetic field and so no flux. So it follows that only the magnetized portion of the string contributes to the induced voltage.

It is that simple. You do not have to know anything else; you just have to know how to apply this knowledge, which is not so easy sometimes. If you think that there is something else happening, you are proposing that modern E&M is incomplete. If you propose any additions to E&M, you will have to change special relativity and all of physics. Good luck!

[Rick Turner]

By that logic, one could have a coil with, for instance a 6" inner diameter wound around an acrylic coil form, and a 1/4" diameter magnet in the center, and it would work just as well as if the coil were wound tight around the magnet, right? You'd have a moving magnetic field and a coil.

You were saying earlier that the aperture is defined by the dimension of the pole itself, and you were discounting the flux field bending through the coil dimensions. I can't see how you can dismiss coil dimensions when defining pickup aperture. Coil shape, height, and size is a crucial element of the sound of a pickup. My work in rewinding classic pickup forms in low impedance formats has shown me this where I keep the physical size of the low Z coil the same size as the originals but virtually eliminate coil LCR as a tonal factor. You can hear the effects of the interaction among magnets, polepieces, and coil mass, and there are sonic characteristics that remain unique to each bobbin/magnetic structure design.

And it's not like I don't have any time in working pickup aperture. I may actually have been the first to define it from my work of 40 years ago making low Z pickups of differing apertures, making sliding pickups, and doing the whole passive hum-canceling and dummy coil routines.

Your logic completely escapes me and defies my experience with making pickups and listening to them. Perhaps your interpretation of the textbooks is amiss, though the basic information there is accurate. I think you need to spend more time winding and listening...

[Mike Sulzer]

By that logic, one could have a coil with, for instance a 6" inner diameter wound around an acrylic coil form, and a 1/4" diameter magnet in the center, and it would work just as well as if the coil were wound tight around the magnet, right? You'd have a moving magnetic field and a coil.

Such a pickup would work exactly as predicted by the law of induction. It would have a low output. This is because many of the field lines that pass through the coil once coming down from the string would also pass through it again gong back up to the string. They point in opposite directions and so cancel in the sum.

Coil shape, height, and size is a crucial element of the sound of a pickup.

Indeed they can be because they affect the inductance of the coil and its resistance. These are circuit properties that affect the frequency response of the pickup. They are not directly related to the induced voltage, but rather are important in getting the signal to the amplifier.

[bbsailor]

Rick,

Your quote: "Some of the wandering of the plane of vibration is certainly affected by the bridge design and even the underlying top and body structure. For instance, many years ago I noticed that I could get action lower on a 335 than on a Les Paul...same strings, same scale length, same tailpiece and bridge hardware. Why? Because of the natural sustain of the Les Paul. With the 335, the excursion of the strings is more damped, and so when you pluck the string more or less parallel to the top, by the time the fundamental plane of vibration moves around to being at right angles to the top, the excursion has died down more than it would on a heavy solid body."

This is a great explaination by connecting all the dots to explain the subtle interaction of many factors. I had a 335-like guitar and found that it had slightly better action than the solid body Les Paul.


Thanks

Joseph Rogowski

[Mike Sulzer]

Rick,
This is a great explaination by connecting all the dots to explain the subtle interaction of many factors. I had a 335-like guitar and found that it had slightly better action than the solid body Les Paul.

Joseph,

Before praising Rick's explanation of how a guitar string vibrates, one should examine the evidence for and against. I will let Rick support it. Here is the evidence against it:

1. A guitar pickup senses primarily the vertical motion (in the direction perpendicular to the guitar top surface) of the string. This follows directly from the law of magnetic induction in the case of the normal placement of the coils.

2. Rick's model has the fundamental and each harmonic vibrating in its own separate plane . All these planes rotate about the z axis (along the axis of the string).

This should result in dramatic changes in the volume and tone of the sound of the string as the sensitivity to the fundamental and harmonics change at the rotation rate. This is not observed.

On the other hand, if a string is picked in the horizontal plane, there is some time involved for the energy of the fundamental to move into the vertical plane (and a thus execute an elliptical motion). The lowest possible action on a particular guitar could be affected by this depending upon the damping rate as Rick suggests.

[David Schwab]

So the aperture is very much set by the pole piece, not the coil. Remember, it is the total changing flux through the coil that matters. The fact that the flux is mostly in the middle of the area of a loop of wire is not a problem as long as the loop is not so large that the return lines of flux cancel out the ones passing through the pole piece.

How exactly does coil shape change the tone then? A wide coil sounds very different from a narrow coil (like a Strat vs. Jazzmaster).