How does EMI flux disruption differ from string flux disruption?

[peskywinnets]

This conundrum has always troubled me....it still does!

Whilst I'm cool with a humbucker flipping the phase of the signal such that the signal from the two coils are in phase & the nasty EMI cancels, I'm not cool with why a EMI generated signal is any different to the string generated signal.

guitar signal - generated by the ferrous string disrupting the lines of magnetic flux surrounding the coil of the pickup.

EMI signal - generated by something nearby throwing off a stomg alternating magnetic field, thereby disrupting the lines of magnetic flux surrounding the coil of the pickup.

The commonality being, both the above disrupt the flux lines around a coil & generate a voltage in the pickup.

So how does the humbucker 'know' which 'flux disruption' is which so it can cancel it?!! (much like in the same way ...I put hot water in my vaccuum flask & it keeps it warm, I put cold water in & it keeps it cold....therefore a vaccum flask must have a way of sensing differences too! ;-) )

[Mike Sulzer]

This conundrum has always troubled me....it still does!

Whilst I'm cool with a humbucker flipping the phase of the signal such that the signal from the two coils are in phase & the nasty EMI cancels, I'm not cool with why a EMI generated signal is any different to the string generated signal.

guitar signal - generated by the ferrous string disrupting the lines of magnetic flux surrounding the coil of the pickup.

[B]EMI[ signal/B] - generated by something nearby throwing off a stomg alternating magnetic field, thereby disrupting the lines of magnetic flux surrounding the coil of the pickup.

The commonality being, both the above disrupt the flux lines around a coil & generate a voltage in the pickup.

So how does the humbucker 'know' which 'flux disruption' is which so it can cancel it?!! (much like in the same way ...I put hot water in my vaccuum flask & it keeps it warm, I put cold water in & it keeps it cold....therefore a vaccum flask must have a way of sensing differences too! ;-) )

It is a matter of generating two signals of opposite sign so when they are connected together with reverse polarity, they reinforce instead of subtracting. This is done by magnetizing the part of the string above each coil in the opposite sense. (Magnetic fields coming out of a north pole and a south pole can be thought of as pointing in opposite directions. One is positive, the other negative.)

"Flux disruption" is not a very complete way of looking at things, but it is a start. It is changes in the magnetic flux through a coil that induce voltages. Making the magnitude of the flux greater could make either a positive voltage or a negative voltage, depending upon which direction the field is pointing. For example, if you move a magnet closer to a coil, the sign of the induced voltage is opposite depending on whether the north pole or the south pole of the magnet is closer to the coil.

[ScottA]

I'm not sure if the magnitude of the EMI induced noise that a guitar pickup transmits is independent of the polarity of the guitar pickup's magnetic field, but the phase (or more correctly polarity) is, at least in most real world cases.

This is why humbucking works, the polarity of the EMI interference induced noise is dependent on the current direction and not the magnetic polarity of the pickup, while the polarity of the string generated signal is dependent on both.

You can pick up plenty of EMI interference without a magnetic field associated with the pickup. Heck, you don't even need a pole piece, but the efficiency of the pickup as an EMI "antenna" is greatly increased with a pole piece, especially one that imparts high inductance, although other things also seem to be at play in determining how good a pickup is at coupling interference.

You said it yourself, The EMI is generated by something else creating its own magnetic field external to the pickup, so it doesn't "care" about the polarity of the pickup's field. But the polarity of the signal that the external field sets up in the coil depends on the directionality of the interaction between the external field and the coil. Flip the coil direction, flip the polarity of that externally induced signal.

[J M Fahey]

To make it shorter: both coils are wired in series , *electrically* out of phase, *but* the magnetic fields end up being *also* out of phase; this is the important part.
Let's see the two signal sources that bother you:
1) External electromagnetic interference, (for example as radiated from a transformer) , induces the same voltage on both coils; since they are out of phase and same level they cancel.
This works the same if both (out of phase) coils are wired in parallel.
2) A pickup is a magnetic circuit where part of the path has low magnetic resistance (reluctance) , meaning the magnet itself and the polepiecesm the iron sleeve in Lace Sensors, etc. ; and a part of very high reluctance (air).
Providing a low reluctance path through the air is the iron-alloy string.
When the string vibrates the distance between it and the pickup varies, this distance variation becomes a reluctance variation (the air path length vibrates) and the actual magnetic flux varies in the coils (which have magnetic line forces going through them) and induces a small voltage.
Imagine the string getting closer, and left and right coils; on the left coil you will have an induced voltage because magnetic field is rising thanks to the approaching string ; on the right coil the same happens, you get the same signal because right and left halves are symmetrical, and they *should* cancel as before, but the magnetic field has the opposite polarity which inverts the phase of the right signal.
You know what happens when you invert phase two times (same as multiplying by "minus/negative") twice: you are again in phase!!!
So external magnetic fields cancel, and internal reluctance variations (string vibration) add up.
A similar thing happens on those Strat wirings where the center PU is wound out of phase, but is reversely magnetized too: you do not have that thin out of phase sound but a thicker one, and humbucking to boot.
In fact I have my cheap Strat copy wired like this; I had to invert coil phase (easy) and re-magnetize the PU (I have a *speaker* magnetizer) so it was easy for me.
Really don't know how it might work with those "passive" PU magnetizers used by many.
PS: I think that pert of your confusion came from calling both fenomenae "interference"; in a way they are both so, but through different mechanisms.

[Joe Gwinn]

This conundrum has always troubled me....it still does!

Whilst I'm cool with a humbucker flipping the phase of the signal such that the signal from the two coils are in phase & the nasty EMI cancels, I'm not cool with why a EMI generated signal is any different to the string generated signal.

guitar signal - generated by the ferrous string disrupting the lines of magnetic flux surrounding the coil of the pickup.

EMI signal - generated by something nearby throwing off a strong alternating magnetic field, thereby disrupting the lines of magnetic flux surrounding the coil of the pickup.

The commonality being, both the above disrupt the flux lines around a coil & generate a voltage in the pickup.

So how does the humbucker 'know' which 'flux disruption' is which so it can cancel it?!!

Humbuckers know nothing. It's that word disruption - it give a completely erroneous mental picture. Nothing of the kind happens.

Following is a top-level description of what is going on. I am ignoring many practical details of pickup implementation, to allow the basic principle to be seen more clearly. However, one nonetheless could build and test the below described system.

Consider two identical coils, for now being air coils, that are oriented in the same direction.

In a uniform hum field, both coils will generate a hum voltage. Because the coils are identical, the voltages will be equal. One can connect these coils together in one of two ways, series-aiding and series-opposing.

If series-aiding, the voltages add, so we get twice the hum voltage as from either individual coil. Not desired, but bear with me.

If series-opposed, the voltages are equal and opposite, and so cancel to zero.

So far so good, but where's the music?

If we put magnets in the coils and a vibrating steel string nearby, music voltages will be induced in the two coils. If we have arranged the coils in the usual humbucker manner, the music voltages will be equal, and so just as for hum, we can connect the coils to either double or cancel the music voltage.

Now, we would greatly prefer to double the music while cancelling the hum. Turns out that it's easy to do this, for a simple reason: the hum field does not care which way the magnets are oriented, while the music does care. Said another way, if one reverses the magnets in a coil, the polarity of the music voltage will also reverse, while the hum voltage will remain the same polarity.

So, if the two coils are side by side and connected such that the hum cancels, if one installs the magnets in one coil in the opposite direction from that in the other coil (for instance one coil being north up while the other coil is south up), the hum will continue to cancel while the music will be doubled.


There is another key idea implicit in the above, superposition in linear systems. In a linear system (and all passive electromagnetic pickups qualify), one can analyze each signal and its effect independently, as if each signal were alone in the system, and add the individual results to obtain the net effect of all the effects on all the signals. In the above discussion, both music and hum are considered signals. I implicitly used superposition in the above, analyzing music and hum signals independently and then assuming that the answer is simply the sum of zero hum voltage plus double music voltage. One can always do this.

Superposition principle - Wikipedia

[Rick Turner]

Lucky for us pickups don't care whether you're cool with how they work or not. They couldn't care less about what you think! They still work!

Esteemed and considerably less snarky members than I have chimed in. They know of what they write...

:-)

[peskywinnets]

Lucky for us pickups don't care whether you're cool with how they work or not. They couldn't care less about what you think! They still work!

Esteemed and considerably less snarky members than I have chimed in. They know of what they write...

:-)

Thanks for your comprehensive input - it really helped.

Anyway, enough of this pickup theory melarkey getting in the way of reminiscing - can we talk about you again now?

[David Schwab]

Whilst I'm cool with a humbucker flipping the phase of the signal such that the signal from the two coils are in phase & the nasty EMI cancels, I'm not cool with why a EMI generated signal is any different to the string generated signal.

It's easy to see how it works when you think about the magnetic polarity in each coil is also opposite. The strings use the magnets to be sensed by the coils. The EMI does not use the magnets. So the signal from the strings is being sensed in opposite polarity in each coil because of the opposite polarity magnets. When the two opposite polarity coils are summed the string's signal is in phase. But the magnetic polarity doesn't matter for EMI pick up, so it's out-of-phase.

[edit: pretty much the same thing Joe said, but I hadn't read down that far]

[Mike Sulzer]

....The strings use the magnets to be sensed by the coils.....

But let's complete that thought: the parts of a string over the two pole pieces are magnetized in opposite directions. Therefore, when the string vibrates, the flux changes through the two coils (and thus the voltages around the coils) have opposite signs.

[peskywinnets]

But let's complete that thought: the parts of a string over the two pole pieces are magnetized in opposite directions. Therefore, when the string vibrates, the flux changes through the two coils (and thus the voltages around the coils) have opposite signs.

I think this hits the nail on the head - I've seen you use the term a lot & initially I thought it was bollocks, but I can see clearly (now the rain has gone?), that the fact the string being magnetically charged to a given polarity by the magnet immediately below is key to all of this working.

[Mike Sulzer]

I think this hits the nail on the head - I've seen you use the term a lot & initially I thought it was bollocks, but I can see clearly (now the rain has gone?), that the fact the string being magnetically charged to a given polarity by the magnet immediately below is key to all of this working.

Thanks; I am glad the description is useful.

[Rick Turner]

Pesky, you were the one who said you weren't cool with how this works, and yet how humbuckers work is so elemental, so basic, that someone declaring that they're "not cool with it" is just mind boggling to me. I guess you were trying to be cool about saying that you just don't understand how humbuckers work.

It comes down to the old grammar thing, two negatives make a positive.

[David Schwab]

I think this hits the nail on the head - I've seen you use the term a lot & initially I thought it was bollocks, but I can see clearly (now the rain has gone?), that the fact the string being magnetically charged to a given polarity by the magnet immediately below is key to all of this working.

Yes, but it's both the opposite magnets and opposite coils. You can have same polarity magnets and opposite coils (or vice versa) and the strings will be out of phase, and in the case of same polarity coils with opposite magnets, it wont hum cancel.

So the opposite polarity magnets put the strings back in phase when the opposite polarity coils are summed.

[Rick Turner]

0

180

360

That's the simple version of the math...

[salvarsan]

I'm not cool with why a EMI generated signal is any different to the string generated signal.

Here's another way to look at it.

Each coil is an antenna, picks up EMI signals.
If you wind them in opposite directions,
they generate out-of-phase signals.

If they are near each other, and you connect them in series,
the received EMI signals cancel out.

If you put magnets in these coils, they become guitar pickups.

If you orient the magnets opposite each other, and connect the coils in series,
the guitar signals reinforce each other.

Voila: a PAF humbucker.

Hope it helps.