Do bode plots account for magnetic interaction?

[gpdb]

I was discussing some plots I created with a well-known figure in the pickup world about bode plots. He mentioned that since the drive coil is creating its own signal and isn't being impacted by the pickup's magnet (in the way that a guitar string would be magnetized differently with different magnets) that the bode plot really is only showing the inductance shift created by magnet, but not accounting for the difference in magnetism. This seems to make sense, but I'd like to ask this group if that seems to really be a problem? I want my charts to accurately reflect a pickup's character, and if there's a massive piece missing that seems problematic. I still think bode plots provide a lot of information that's important. 

[stratotarts]

It's easy to determine. Just run back to back plots, one with and one without a string. You can simulate the worst case by laying the string directly between the test coil and the pickup. Please try it and report.

[gpdb]

Jul 8, 2022 10:03:11 GMT -5 stratotarts said:

It's easy to determine. Just run back to back plots, one with and one without a string. You can simulate the worst case by laying the string directly between the test coil and the pickup. Please try it and report.

I could do that test, but how would that change the plot? The coils would still be getting the signal from the drive coil, the string wouldn't be moving.

[aquin43]

In the basic pickup model, the flux returned to the pickup generally and the coil in particular from the magnetised string varies because of the movement of the string. Replacing the string with an appropriately shaped and dimensioned exciter coil simulates the effect of an infinitesimally small string movement but with the result magnified by an arbitrary amount since we can make the exciter field large.

The response of the pickup to the magnetic excitation depends to a greater or lesser extent on the location of the exciter. This is because the response has two components. One is the resonant effect of the coil and capacitance and the other is the result of the multiple paths that the magnetic signal takes to get to the coil.

Some pickups, such as the low loss strat ones are hardly affected by the magnetic signal's path. Others, such as humbuckers with twin coils and shielding covers can be sensitive to the exciter location, giving different responses from each coil for example.

The overall resonant effect seems to be relatively independent of the placement of the exciter but the shielding losses from covers and some of the eddy current losses can vary to a degree.

[antigua]

Jul 8, 2022 9:47:47 GMT -5 gpdb said:

I was discussing some plots I created with a well-known figure in the pickup world about bode plots. He mentioned that since the drive coil is creating its own signal and isn't being impacted by the pickup's magnet (in the way that a guitar string would be magnetized differently with different magnets) that the bode plot really is only showing the inductance shift created by magnet, but not accounting for the difference in magnetism. This seems to make sense, but I'd like to ask this group if that seems to really be a problem? I want my charts to accurately reflect a pickup's character, and if there's a massive piece missing that seems problematic. I still think bode plots provide a lot of information that's important. 

Most pickups of a given type only vary by inductance and magnetic strength. Like most of the Seymour Duncan product line a PAF type bobbin with varying sizes and lengths of wire on the coils in order to end up at a different treble attenuation profile, and then give it a cool name. They use a few different magnets, but they all tend to sit underneath, so the magnetic path is mostly determined by the screws and pole pieces, so the metallurgic properties of the magnetic itself are that much less important. There's a slight difference in the remnant flux between the pickup and the string, which affects "string pull", but aside from that, nearly homogenous factors cab be ignored, because they're homogenous. Fender style pickups are a little less uniform insofar as some use AlNiCo pole pieces and some use steel, but the overall geometry is still the same, and the biggest consequences of those variations do turn up in the bode plots, for example the lower peak frequency and higher eddy currents when steel poles are used in place of AlNiCo poles. 

[gpdb]

What I'm more so trying to get at is if I change the magnet in a humbucker and create a bode plot, am I actually seeing the correct representation of that pickup's resonance? Since an Alnico 2 magnet has much less gauss than a ceramic, is the shift in Q factor and resonant peak all that's really happening, or is there something more? Here's an example I did a while ago of a Dimarzio PAF 59, but with every different kind of magnet. Are these really representative of the change one should hear? Does the magnetism affect the string really all that different?

[stratotarts]

Jul 8, 2022 10:39:23 GMT -5 gpdb said:

Jul 8, 2022 10:03:11 GMT -5 stratotarts said:

It's easy to determine. Just run back to back plots, one with and one without a string. You can simulate the worst case by laying the string directly between the test coil and the pickup. Please try it and report.

I could do that test, but how would that change the plot? The coils would still be getting the signal from the drive coil, the string wouldn't be moving.

I got caught with a misunderstanding of your acquaintance's comment. Such comments have to be fully fleshed out in order to really address any issues that they raise. I assumed by "the difference in magnetism", it was meant, the effect of the presence of the string on the static field produced by the magnet. In fact, the description is pretty open ended and doesn't specify what differences should be taken into consideration.

It is well understood and proven that a stronger magnet produces a stronger magnetic field in the string. This in turn, produces an increase in output for a given string excursion, as a simple result of Faraday's law. But the fundamental information in a Bode plot is not the absolute amplitude, but the relative amplitudes at different frequencies. A Bode plot can display different output amplitudes, as you can see from your experimental plot (evidenced in the flat line portion of lower frequencies).

There are only two accepted additional factors that differ from the application of test coil (there are probably more I can't think of right now, but I believe these are the most significant) - magnetic string pull, and non-linearity due to distance. But can you see, those are almost equal in different pickups of the same general type and physical position in the guitar. They are also not fixed but vary according to pickup height adjustment. Experiments that cover both of those are hosted somewhere on this sub-forum but it's become quite long and hard to find stuff (which isn't completely bad).

It's true that to consider, say, the sonic difference between an A2 and an A5, it would be perfect to somehow measure the two previously mentioned effects, in addition to the Bode plot. This would give you a more complete picture. But before it should be taken as a serious flaw in testing, it should be adequately demonstrated (and it is really in the court of this well-known figure to do so instead of postulating) that those effects are significant compared with other effects.

The main value of this forum, is that rather than being a melting pot for theories, postulates, and rhetorical discussions, it has focused on the design and implementation of actual and reliable tests. These can then be incorporated into a larger theoretical understanding of the devices, which does involve some synthetic thinking and discussion. It's always been a problem (in my view) that public discussions in some other forums, are mostly verbal generalities and theories that depend on plausibility to gain acceptance. Most of the actual progress in understanding pickup physics has been achieved not through the development or promotion of theories alone, but on carefully designed and performed experiments.

You said, "I want my charts to accurately reflect a pickup's character". You should think of character as a multifaceted result of many factors, which differ greatly in their significance. You will never find a single factor that 100% determines the sound. But the Bode plot response encapsulates a few of the most important factors. The main advantage of the plot is that it reveals inductance and Q in a way that has an almost one-one correspondence with the way it will respond in the guitar circuit. Breaking that down, inductance and Q (the most influential factors) can be measured and assigned values recorded with a DCR meter. But the results can't be seen in such a graphic way. Also those parameters vary with frequency and so a single inductance or Q measurement will never represent a complete measurement such as you have with a Bode plot.

An analysis of "character" also has to consider the perceptual side of things, how changes in the actual response change the perceived sound. Looking at things from this end, and attempting to connect with the features of a Bode plot, it becomes obvious with some testing and comparison (and consideration of general opinions about different pickup types), that there are three main aspects that do correspond directly with the plot

• low pass response (-3dB cut off frequency)
  
• Q
• output amplitude

All of those are directly or indirectly seen in the plot. I've seen some attempts to 2-d map pickup types by the first two factors above, that produce "tone zones", for example

• low cutoff, low Q = humbucker
• high cutoff, high Q = Strat
• medium cutoff, high Q = Tele
  

and so on, if you map individual pickup measurements to this map, you will definitely see them cluster and recognize the patterns that develop. But because the perceptual axes for those can only be two, they can't rationally express characteristics that are not expressed in such a map. Many such characteristics are controversial for the reason that they can't be measured in this way, but it doesn't paint the Bode plot as unreliable, only incomplete. So you can say, "a chart can not fully reflect a pickup's character". But a Bode plot never does, or at least never should stand in isolation. Knowing the type of pickup, therefore knowing more about its other physical characteristics as a group, adds meaning to the plot because you can confidently compare members of the same group.

[antigua]

I think one of the dishonest many less than honest things about how pickups are marketed is the fact that there is an equivalence between magnetic strength and how close the pickup is set to the strings. For example, Fender will say, literally on their website "[alnico 2] is especially good for middle-position pickups, and it contributes to a very sweet and musical “vintage-y” sound" , but they never, afaik, will use that sort of terminology to describe the difference with pickup height settings, in that case the commentary is especially trite, like, it's the preference of the musician, or set the pickup one nickel's width away from the guitar strings. The truth of the matter is that the stronger or weaker magnet will vary how much proximity between the guitar and pickup is required for a given result, short of causing the guitar strings to magnetically saturate. I think a lot of guitarists like to have their pickups set to a certain height for aesthetic reason, and so magnetic strength becomes a way of getting both a desired look and a desired sound at the same time.

So when people say "but what about the magnets???" in response to the bode plots, to some extent they've been mislead by the likes of Fender as to significance of the magnets in the first place. Some times people even ask whether or not scatter winding is revealed in the plots, because I've explained more than once that if the pickup's coil is especially sloppy, it will have a lower inductance and a higher peak frequency, so yes, scatter winding manifests in the bode plots as well, but it's not as interesting as the pickup makers who tout scatter winding would have had you believe.