Is it possible to accurately describe the tone of magnets and/or pickups?

[Antigua]

I measured the values of Alumitones http://guitarnuts2.proboards.com/thr...nalysis-review , it does have an unusual bode plot compared to other pickups.

[Mike Sulzer]

Yes, I also expect a lower cut-off frequency depending on the ratio of loop DCR to loop inductance. The reflected secondary current will help to shift the cut-off to lower frequencies, meaning that the cut-off frequency will depend on secondary load. I did not have a chance yet to measure an Alumitone.

I tend to view the Alumitone as a voltage transformer: The primary circuit consists of a (zero impedance) voltage source (EMF) in series with the DCR and the loop inductance. This primary circuit is also the primary winding of the transformer which steps up the EMF by say 10E4. It as well steps up (reflects) primary impedance by 10E8. A realistic primary inductance of say 50nH will thus be reflected to 5H at the PU's terminals, a value in the typical range of "normal" PU's. Together with the self-capacitance of the secondary winding of the transformer, a typical parallel resonance results.

I think that there are two separate inductances that have to be considered: the inductance of the pickup (the conducting loop), and the inductance looking into the primary of the transformer. When you look into the secondary, you see approximately the two in parallel. To determine the pickup low frequency response, you need the individual values.

[Helmholtz]

Thanks for your reply.

Both effects observed are quite interesting. I have great respect for your experimental creativity and agree that digging deeper would require enormous efforts. After all, these are small second order effects, which always are much harder to quantify than the obvious ones.

Getting back to your post #86:

So it's a given that higher harmonics decay faster, I think we can all agree that any change that shifts energy to higher harmonic levels will therefore result in a net loss of sustain. IIRC, in my own testing I found that putting the magnetic closer to the bridge (or the nut) promoted a transfer of energy to higher harmonics than if you place the magnet over the middle of the string, for much the same reason that if you pluck the strings by the bridge, you also get higher harmonics. Therefore it might be that increased magnetic pull at the bridge could reduce sustain more than magnetic pull elsewhere, by shifting the energy into higher harmonic levels that experience greater mechanical damping within the guitar strong.

If the same magnetic pull produces stronger additional string damping at the bridge position than at the neck position, this in fact points to a damping effect that increases with frequency. It rather excludes PU vibration, but eddy currents (theoretically even in the string) and string hysteresis losses come to my mind. If the magnetic pull actually shifts the string vibration spectrum to higher harmonics, I cannot exclude increased losses by a higher average string bending rate either.
I don't feel able to speculate about which effect might be the dominant one at this point.


From a practical point of view, it seems that all the observed, detrimental effects more or less vanish, if the PUs are adjusted so that no audible "wolfetones" are perceived. And this is what most players intuitively do and many manuals recommend. Except if someone goes for a somewhat rougher sound

[Helmholtz]

If the magnetizing current in the transformer with the single turn primary has a significant effect it would be most at low frequencies, decreasing at higher frequencies. The effect would be to load the single turn pickup at low frequencies, causing a decreases in bass response. Obviously, there is a frequency below which this effect becomes important, but I think it is low enough so that it does not matter for guitar frequencies. Or do certain models of these pickups have a distinct lack of bass?

Yes, I also expect a lower cut-off frequency depending on the ratio of loop DCR to loop inductance. The reflected secondary current will help to shift the cut-off to lower frequencies, meaning that the cut-off frequency will depend on secondary load. I did not have a chance yet to measure an Alumitone.

I tend to view the Alumitone as a voltage transformer: The primary circuit consists of a (zero impedance) voltage source (EMF) in series with the DCR and the loop inductance. This primary circuit is also the primary winding of the transformer which steps up the EMF by say 10E4. It as well steps up (reflects) primary impedance by 10E8. A realistic primary inductance of say 50nH will thus be reflected to 5H at the PU's terminals, a value in the typical range of "normal" PU's. Together with the self-capacitance of the secondary winding of the transformer, a typical parallel resonance results.

[Mike Sulzer]

I disagree. The primary current is determined by the loop EMF, the loop resistance and the loop inductance. This primary current is the magnetizing current of the transformer.
Secondary signal current stepped up with the (inverse) turns ratio of around 10000 will further increase the primary current as with any transformer. Secondary current cannot decrease primary/magnetizing current.

If the magnetizing current in the transformer with the single turn primary has a significant effect it would be most at low frequencies, decreasing at higher frequencies. The effect would be to load the single turn pickup at low frequencies, causing a decreases in bass response. Obviously, there is a frequency below which this effect becomes important, but I think it is low enough so that it does not matter for guitar frequencies. Or do certain models of these pickups have a distinct lack of bass?

[Antigua]

Do you have positive evidence that stronger fields/closer PU distances increase the amplitudes of the higher harmonic relative to the fundamental?

Yes, http://guitarnuts2.proboards.com/thr...-string?page=2 and http://guitarnuts2.proboards.com/thr...scrollTo=82326

Looking back at those threads, the difference made is rather subtle, it's not as though raising the pickups causes the guitar's output to jump an octave or two. But also, there is beating in the harmonics, and the rate of beating would vary depending on the degree of string pull. This research isn't conclusive, the testing should be conducted multiple times with multiple guitars in order to establish trends, but there's an enormous time cost in that. I was hoping that maybe someone with a background in physics might come along and explain what was being seen with the string pull and explain it well enough that further testing might not be critical to gain an understanding of how pickup height effects tone. What I can say for sure is that the effects are complex enough that they can't be summed up with simple adjectives.

Another fact that rarely gets mentioned, and is probably never noticed, but raising one pickup effects the tonal outcome of the others, as it disturbs the movement of the guitar string from the perspective of all of the pickups. That being said, it also seems to be that the way in which a pickup disturbs the string movement is most apparent from that pickup, for example, a bridge pickup's magnetic pull excites higher level harmonics, and the bridge pickup also receives more higher level harmonics due to it's position, and the the same is true for the neck pickup and it's interference and reception of lower harmonics, which would contribute to the mutual effects of pickup height being difficult to notice.

[Helmholtz]

Higher harmonics decay faster, true.

Do you have positive evidence that the same PU at the same distance from the string produces stronger string damping at the bridge position than at the neck position?

Do you have positive evidence that stronger fields/closer PU distances increase the amplitudes of the higher harmonic relative to the fundamental?

[Helmholtz]

It is a single turn pickup operating into a transformer with a single turn primary, a transformer that steps up the impedance something like 10^8 times. I think that the current flowing in that single turn is effectively nearly the same as that of a many turn pickup.

I disagree. The primary current is determined by the loop EMF, the loop resistance and the loop inductance. This primary current is the magnetizing current of the transformer.
Secondary signal current stepped up with the (inverse) turns ratio of around 10000 will further increase the primary current as with any transformer. Secondary current cannot decrease primary/magnetizing current.

[bbsailor]

I think it makes sense to ignore it in this case, because even the rough math, which is to consider the small magnetic coupling, and the tremendous difference in mass, means that this effect will not be in the %5 range, nor the 0.01% range, but somewhere even below that. The difference between mounting rings versus body mount is equivalent to pissing in the ocean and expecting the sea level to rise




The guitar string produces some magnetomotive force, and that force is like a potential voltage, and what stops that voltage from being fully realized is magnetic reluctance between the magnemotive force and the loop(s) of wire where the measurement is made. The magnetomotive force must be rather high, because we know the reluctance also very high (lots of air gap between the string and the pickup), and yet a guitar pickup produces a usable signal. Knowing that the reluctance path is very high, it can also be said that eddy current action between the string and pickup will be very low by the same virtue. I bet if someone could calculate the value of the reluctance path, or the magnetomotive force, then they could probably also figure out the counter electromotive force, which would give some clue as to how able the pickup is to decrease sustain. With a typical eddy current brake, the reluctance path between the magnet and the conductor being brought to a halt is usually small, by comparison.

Antigua,

Here is a quick way to measure any eddy current attempting to suppress a vibrating string in a magnetic field.

First, establish a baseline with the magnet near the string with a controlled pluck timed till it fully decays.
Now, obtain a piece of copper wire that has 10 to 15 times less resistance than the test string. Find a low resistance way to secure this wire across the string, attached behind the nut and bridge to short out the string. Pluck the same string in a controlled way and measure the time for this string to fully decay.

The shorted string will induce more current in the string wire while vibrating in the magnetic field. Since the coupling between the string and magnet is relatively low there will be some measureable decay in time. The variables will be the area of your chosen string, the strength, length and closeness of the magnet to the string and the lowest resistance connection to short out the string.

Based on my experimentation, a vibrating string will induce a voltage in itself depending on the length of the magnetic field. It can range from about 1mv to about 3 mv. Then, when you put a 8 ohm transformer across the string and use the high impedance side such as 10K to 50K side connected to the amp input, you will hear an output in the passive pickup range. This is how the Stringamp for the violin works.

Joseph J. Rogowski

[Antigua]

Yesser. I actually DO get that. But pickup mounting systems aren't all THAT secure really. Surely there is SOME movement. I admit I never thought about it before Helmholtz mentioned it. I just don't think it makes sense to ignore it. After all, we do strive for that extra five percent on this forum. If all we wanted to do was play our guitars we could ignore a lot of what's discussed here.

As for what happens if you hardmount a pickup to a guitar instead of using springs or rubber tubing, EVH screwed his pickups right down to his guitar body with woodscrews.

I think it makes sense to ignore it in this case, because even the rough math, which is to consider the small magnetic coupling, and the tremendous difference in mass, means that this effect will not be in the %5 range, nor the 0.01% range, but somewhere even below that. The difference between mounting rings versus body mount is equivalent to pissing in the ocean and expecting the sea level to rise

Won't the impedance of the circuit that the pickup is attached to affect the string's sustain. A dead short in the coil would quickly stop the string due to eddy currents in the coil, I'm not sure if a 50k ohm load would have any effect but what if you turn the volume way down to where most of the signal is shunted to ground?.

The guitar string produces some magnetomotive force, and that force is like a potential voltage, and what stops that voltage from being fully realized is magnetic reluctance between the magnemotive force and the loop(s) of wire where the measurement is made. The magnetomotive force must be rather high, because we know the reluctance also very high (lots of air gap between the string and the pickup), and yet a guitar pickup produces a usable signal. Knowing that the reluctance path is very high, it can also be said that eddy current action between the string and pickup will be very low by the same virtue. I bet if someone could calculate the value of the reluctance path, or the magnetomotive force, then they could probably also figure out the counter electromotive force, which would give some clue as to how able the pickup is to decrease sustain. With a typical eddy current brake, the reluctance path between the magnet and the conductor being brought to a halt is usually small, by comparison.

[Antigua]

Yes, string bending involved in the vibration means material deformation. Though mostly elastic (non-dissipative), there is also has a lossy component in non-ideal materials. As higher harmonics bend the strings at a higher rate, they lose energy faster.

The way I understand it, having read some papers about acoustic guitar physics, is that the higher harmonics divide the string into smaller segments, and that those smaller segments are more mechanically rigid than are longer segments, and higher rigidity, or less elasticity whatever, means more energy lost in the form of heat within the guitar string. Supposedly this relates to why flat wound strings produce less treble; the flat interlocked winding is more rigid that round wound windings.

So it's a given that higher harmonics decay faster, I think we can all agree that any change that shifts energy to higher harmonic levels will therefore result in a net loss of sustain. IIRC, in my own testing I found that putting the magnetic closer to the bridge (or the nut) promoted a transfer of energy to higher harmonics than if you place the magnet over the middle of the string, for much the same reason that if you pluck the strings by the bridge, you also get higher harmonics. Therefore it might be that increased magnetic pull at the bridge could reduce sustain more than magnetic pull elsewhere, by shifting the energy into higher harmonic levels that experience greater mechanical damping within the guitar strong.

[Helmholtz]

I am not aware of such additional loss processes. Momentum is no energy category and momentum inversion does not consume energy. But I am no acoustics expert like Zollner and cannot exclude other minor loss processes. I am quite sure though, Zollner would have mentioned them if they were relevant.

[Mike Sulzer]

Yes, string bending involved in the vibration means material deformation. Though mostly elastic (non-dissipative), there is also has a lossy component in non-ideal materials. As higher harmonics bend the strings at a higher rate, they lose energy faster.
I just don't see neither a higher bending rate nor a higher bending amplitude in the presence of a magnetic force. The field just minimally shifts the equilibrium or rest position of the string in the plane perpendicular to the fingerboard.

The slightly different frequencies of up-and-down vs. sideways vibration caused by a strong magnetic field produce a beating effect as seen in the waviness of the signal decay. A local minimum in the decay means that the energy content in the up-and down vibration has momentarily decreased while the energy in the horizontal motion has correspondingly increased. This energy exchange repeats at the beat frequency. I can't think of a reason for increased losses produced in the energy exchange between the two vibrational modes. If the amplitude (and thus the bending) of one mode increases, the amplitude of the other one decreases.

Did I adress what you meant?

Put a strong magnet near a string and shine a light on it. When I do that, I see a different effect than without a magnet.This dies away very quickly, and so you have to watch carefully.

Are you sure you can pump energy between different modes with different, but close, frequencies without a loss mechanism to conserve momentum? (I do not know the answer to this, have not done the physics, but it looks like it might be something that needs some thought.)

[Mike Sulzer]

Yesser. I actually DO get that. But pickup mounting systems aren't all THAT secure really. Surely there is SOME movement. I admit I never thought about it before Helmholtz mentioned it. I just don't think it makes sense to ignore it. After all, we do strive for that extra five percent on this forum. If all we wanted to do was play our guitars we could ignore a lot of what's discussed here.

I agree! But is it 5% or .01%?

[Mike Sulzer]

Yes and this very low resistance single turn in the Alumitone is shorted and hence constitutes a perfect eddy current loop. Consequently sustain will suffer if the magnetic flux is strong enough.
It looks like a potent eddy current brake.

It is a single turn pickup operating into a transformer with a single turn primary, a transformer that steps up the impedance something like 10^8 times. I think that the current flowing in that single turn is effectively nearly the same as that of a many turn pickup.