For EE's: diff btw 'magnetic' noise vs 'electric field' noise?! (pickups, shielding)

[Usable Thought]

I've got a big interest in shielding guitars to reduce noise & have read a lot about it. I've only got humbuckers, but I'm thinking of getting a single coil guitar again (I had one long ago). And I'm curious as to the noise situation for single coils. I know you can get hum canceling passive systems (Ilitch) or noiseless pickups (Kinman) or just shrug your shoulders and live w/it, or get good w/the volume knob. But I'm still curious about a few things. Rather than just hear the usual lore all over, I'm hoping to get some solid info here from EE's or anyone else who know the physics.

To start with, from what I've been reading, single coil pickups are reportedly vulnerable to what some call magnetic" noise. For example here's an excerpt from a page on single coil noise I found on the web site of guitarist/EE/author Ulrich Neumann (who I believe is a member here):

Magnetic fields are created by transformers (like an amp transformer) and power wiring in your walls, among other sources. The AC power grid creates a 60Hz (or 50Hz) mag field almost everywhere. A shield for mag fields is impractical for many reasons, chiefly because the PUs are exposed so you can play. The PUs create a signal from all sources of mag fields they are exposed to. Most of the field is created by the PU magnets and strings (that’s how PUs work) but the 60Hz noise mag field adds to the signal.

And similarly from Ironstone Pickups:

To keep things simple, unwanted guitar noise can be split into 2 categories; hum and buzz are pretty commonly used terms.
Hum is linked to magnetic fields created by mains power circuitry. As such, its dominated by the frequency of the mains electrical system (50Hz here in the UK, 60Hz in N. America for example).

As for noise that gets into the wiring harness and can be blocked by shielding, Neumann says only this:

There are two sources of noise with PUs. One is the electric field and this noise source is addressed by shielding. Again, there are lots of websites and articles about shielding. Ideally, a metal shield surrounds the guitar electronics, including the PUs, and is tied to the guitar cable ground.

And Ironstone doesn't say much more than this:

Buzz is much more varied in its source, but can be categorised as a 'transmitted' source and sounds more like static. So in the basket of buzz sources, you find everything from fluorescent lights, electrical power tools and anything with higher frequency electrical switching going on.

And yet when I search the web for other than guitar-oriented sources, I find pages from government health agencies & power companies that draw little distinction between "electrical" and "magnetic" fields in buildings - they talk instead of electromagnetic radiation. E.g. from the web site of Pacific Power in the northwest U.S.:

An electromagnetic field, sometimes referred to as EMF, is created by electrical charges. Electric charge creates electric fields. Moving charges create both electric fields and magnetic fields. There is an electric field when an appliance is plugged into the wall. When the appliance is turned on, current or charge flows creating both a magnetic field and an electric field. The term electromagnetic field refers to an electric field, or the magnetic field or both fields. Given the widespread use of electricity, electromagnetic fields or EMFs are present everywhere in our daily lives.

Similarly see Electric & Magnetic Fields from the U.S. National Institute of Environmental Health Sciences. Again they talk of EM fields or EM radiation, not just E or just M.

Anyway here are my questions:

1) If shielding for guitars can't address magnetic noise (which we are for the moment calling "hum"), why do some claim success in quieting their single coils with shielding alone? E.g. the GuitarNuts.com site is notorious for advocating shielding alone. And here's a before/after YouTube video with a guy claiming he shielded his noisy single coil per GuitarNuts instructions and it worked for him.

2) Regarding noise that gets into the wiring harness, "buzz," the word used by Ironstone, is not a very scientific word; and when Neumann talks about it he just says it's due to "the electric field." What is this buzz or electric field in physics terms? Is it possibly the "E" component of EM radiation or fields? In other words if EM radiation moves back and forth between electric fields & magnetic fields, is "buzz" just referring to the electric fields? Would this then mean that the magnetic fields that create hum in pickups (question #1) are also due to EM radiation, but this time just the magnetic fields?

3) Possibly this will overlap with question 2: How does the guitarist view of noise (magnetic noise & buzz) fit into the picture of EM fields and radiation in homes and buildings, as sketched by health agencies & power companies?

4) Is it possible that the origin or phase of EM (E vs. M) is not as important as the relative sensitivity of devices that receive such noise? In other words, a pickup is apparently far more sensitive than a wiring harness; so might shielding be more effective for wiring harnesses than for pickups simply because wiring harnesses are less sensitive to noise to begin with, and for no other reason?

5) Or is it that the magnetic phase of EM radiation bothers pickups more than wiring harnesses; but even so the E phase is also important, and this phase can be blocked by shielding? That might account for why persons going all the way back to Danelectro/Silvertone claim success in shielding single coil pickups (see last two paragraphs of this StewMac article).

Please un-confuse me if you can. Apologies if these questions seem simple-minded . . . but it seems to be the simple-minded things that cause the greatest dispute among guitarists and hobbyists.

[pdf64]

My understanding is that electrostatic noise is far easier to deal with, eg 100% coverage with conductive, eg metallic, screening will block it.
But magnetic noise will pass through that, is only affected / reduced by ferromagnetic screening, mumetal being the most effective.
So best to make the signal source resistant to the magnetic noise, eg humbucking pickups.

[WesG]

I'm no EE and certainly not a guitar tech.

I also wouldn't expect guitars in a typical environment to pick up much in the way of magnetic noise, unless you are sitting right on top of your amp while playing. The inverse square rule applies.

Electrical inteference is easy to get rid of. A shield acts an antenna, picks it up like radio waves, and shunts that energy to ground. If you have an old tube amp with a small-value cap across the inputs, that's what it is there for, RFI will blow right through the cap like it was a wire, but the 60Hz won't be affected at all.

Magnetic interference, you can't get rid of it. All you can do is redirect the magnetic field somewhere else. This is why transformer shields, like in DIs and Hammond Organs have a soft-cornered cylindrical shape, they are steering the magnetic field around the transformer so that it doesn't pick up the noise. Like a bridge pilon in a river shields the lee side from the current. Only in 3-space. Permalloy, mu-metal are great at steering magnetic fields. Soft iron can also be used, it is less expensive but way way cheaper.

[WesG]

Also, re "buzz", if they are using the word the same way I do, it's not completely unscientific. Noise at a single frequency is just a sine wave, and it sounds like a hum if you put it through a speaker. The buzz I'm thinking of has all kinds of harmonic content, and is usually the result of rectifying AC. Stick you 'scope on the output of a bridge rectifier, look at the bumps, think about how many sine waves you'd have to add together to get that shape. Those are the harmonics. Connect it to a speaker, it sounds like a buzz.

Now, add caps and smooth it out. Perfectly smooth, no noise. But is your hardware perfect? Doubt it. There is pretty much always noise on your DC. The question is ... how much is there, and how much matters?

[nickb]

I'll have a go. It might, sorry, will not be perfect but hopefully will advance the discussion a little. Also, I'm making some pretty big simplifications.

1) If shielding for guitars can't address magnetic noise (which we are for the moment calling "hum"), why do some claim success in quieting their single coils with shielding alone? E.g. the GuitarNuts.com site is notorious for advocating shielding alone. And here's a before/after YouTube video with a guy claiming he shielded his noisy single coil per GuitarNuts instructions and it worked for him.

His shielding did not reduce magnetic field effects at all. If he placed the pick ups anywhere near a power transformer you would plainly hear the effect. What you hear in the before sample is higher frequency buzz as opposed to low frequency hum.

2) Regarding noise that gets into the wiring harness, "buzz," the word used by Ironstone, is not a very scientific word; and when Neumann talks about it he just says it's due to "the electric field." What is this buzz or electric field in physics terms? Is it possibly the "E" component of EM radiation or fields? In other words if EM radiation moves back and forth between electric fields & magnetic fields, is "buzz" just referring to the electric fields? Would this then mean that the magnetic fields that create hum in pickups (question #1) are also due to EM radiation, but this time just the magnetic fields?

EM radiation is essentially a radio wave and something you get at quite a distance from the electric charges and currents that create them. I believe that here we are talking about near field effects where you are close to the source of the noise.

3) Possibly this will overlap with question 2: How does the guitarist view of noise (magnetic noise & buzz) fit into the picture of EM fields and radiation in homes and buildings, as sketched by health agencies & power companies?

I think we covered this above.

4) Is it possible that the origin or phase of EM (E vs. M) is not as important as the relative sensitivity of devices that receive such noise? In other words, a pickup is apparently far more sensitive than a wiring harness; so might shielding be more effective for wiring harnesses than for pickups simply because wiring harnesses are less sensitive to noise to begin with, and for no other reason?

Yes, the receiver is crucial. Think of a whip antenna - it works using the the E field. Ever hear of a loop antenna? They use the magnetic field. So in the guitar the pickups are coils like loop antennas and respond (primarily) to changing magnetic fields. The high inductance and self capacitance mean they tend to reject high frequency 'buzz'. Broadly speaking, the guitar wiring is more like an antenna and so is sensitive to changing electric fields. So the foil shielding effectively suppresses the E field from being picked up by the wiring.

5) Or is it that the magnetic phase of EM radiation bothers pickups more than wiring harnesses; but even so the E phase is also important, and this phase can be blocked by shielding? That might account for why persons going all the way back to Danelectro/Silvertone claim success in shielding single coil pickups (see last two paragraphs of this StewMac article).

I think others have covered the different kinds of shielding.

[Usable Thought]

Thanks Nick - this is helpful. I am going to look up more info based on what you say (e.g. loop antennas & then again at how pickups work, in light of the comparison). After that I may have a few more questions (just a few!!) for anything that is still puzzling me.

[Usable Thought]

Actually my digging, based on pointers from Nick plus some other Googling I'd already done, is turning up some stuff that I find really interesting - phenomena that are quite well known in other electronics applications, but don't get named or acknowledged as such in the area of guitar electronics. Among other things:

- Near and far EM fields
- Mains hum in a room transmitted to circuits & persons via capacitive coupling
- The role of body capacitance in this transmission - not just "the body is a capacitor" but how this relates to AC noise specifically

For example in radio, apparently it's common to observe that a small (low-power) radio transmitter can be affected by a person's limbs or body being nearby - that is, within the near field. There may be some interesting comparisons that shed light on different sorts of guitar noise.

So all this seems pretty cool and what I was looking for. I'll be back at some point with more info and more questions about that info.

[Fluoroscope 5000]

The more you shield a PUP from magnetic interference, the more you shield it from the string vibrations, no? Bill Lawrence mentioned that ~1/4” thick Aluminum under a pickup will reduce hum a lot, but also reduce signal output a good bit.

There are now several hum-free SC designs that are every bit as good as standard SC designs i.e. strong output and no significant inter-coil cancellations. No need for the Ilitch system. The late Bill Lawrence designed the Wilde NF series and the very low hum Microcoils (of which I have both). The DiMarzio Area series are also excellent, as are Zexcoil and Kinman.

[Usable Thought]

The more you shield a PUP from magnetic interference, the more you shield it from the string vibrations, no?

Thanks, yes, this seems possible. And I know about Kinman, Ilitch, etc. However at the moment my focus is not so much on existing solutions but on the underlying physics of guitar noise due to passive pickups and wiring harness only (thus not including amp, cable, etc. for now). There is a lot of conjecture, common practice, etc. What I find is that discussions of what is effective & why often end up foundering on disagreement or lack of understanding as to cause & effect.

Beyond that, I simply find the topic interesting & want to explore it a bit in its own right.

[rjb]

The more you shield a PUP from magnetic interference, the more you shield it from the string vibrations, no?

Depends on the physical configuration.
The shield can go over the strings.
Think P Bass pickup covers and Tele bridge "ashtrays".
They reduce hum, but players removed them because they "get in the way".
Now those shields only come on vintage reissue guitars, or as "replacement" parts.

[dai h.]

(as I understand it) "buzz" is the higher harmonics of the line frequency (50/60Hz) hum --higher frequencies that are more audible (since they take place in a range the human ear is typically more sensitive to), and hearing a "pure" 50/60Hz tone doesn't sound particularly annoying (to me), so application of copper/alu (capacitive, or electrostatic) shielding could result in a noticable noise reduction even if no extra steps for electromagnetic noise reduction were taken. (At least that would be my (novice) take on it.)

(note: the AC wall voltage coming in doesn't just contain 50/60Hz, it also contains harmonics (multiples of 50/60Hz))

[Usable Thought]

Article says mains hum is electrostatic, not magnetic - implications for single coil

This is an example of some (but not all by any means) of the reading I'm slowly doing. It will take me a long time assuming I keep my interest up!

Mains Hum article by Kevan Hashemi, Open Source Instruments Inc.

Open Source Instruments makes various devices for wiring up small animals (probably for research) to get telemetry from them. Thus the company naturally has an interest in helping clients keep mains hum to a minimum. This article advises that (for example) if you put a small-signal measuring device on an animal, and then connect this via balanced cable to a high-impedance amplifier or other high-impedance instrument, you must also be sure to connect a third line with a low-impedance ground to the instrument's 0V ground; otherwise mains hum will overwhelm your tiny signal. Sound familiar?

Anyway what's of interest is that along the way to this conclusion, Hashemi investigates by experiment the kind of mains hum you find in a typical lab room, i.e. the typical room that his clients will be doing observations in. A typical lab room has mains power lines in the walls, but no odd equipment such as welders, MRI machines, etc.; thus in terms of 50Hz or 60Hz main hum, it ought to be very similar to a typical room in a home or club; but not typical, as he points out, to locations such as factories, etc. So his observations ought to be useful for electric guitar players, in other words.

AND - yes I am getting to the point - he argues that his experiments show mains hum is electrostatic not magnetic in nature; and that further it gets carried about a lab room by capacitive coupling between the power lines & various bodies: human bodies, animal bodies, metal tables, etc.; basically any object that can carry a capacitive charge.

Being electrostatic, this kind of hum can be completely blocked with a Faraday cage (meaning a complete 360 degree enclosure, not a partial enclosure such as typical guitar wiring harness shielding) or siphoned off by proper grounding as described above. (The grounding tactic assumes the object you are measuring with your small-signal probe is a conductive object, e.g. a human or animal.) For example if you want to measure voltage in a human body, say with a biomedical device like an EEG, you'd better ground the body, otherwise the mains hum will overwhelm the signal: the hum is millivolts, the signal only microvolts.

Hashemi doesn't discuss the harmonics that come with mains hum; so I suppose that is a sort of open question; but if these could not be eliminated by grounding, I think he would have covered that point; otherwise you'd still have too much noise when using an EEG, for example, and would need to take other measures. That other measures apparently don't need to be taken suggests that mains hum harmonics are also electrostatic and not magnetic.

Anyway I think the contention that mains hum is electrostatic and not magnetic is something that at least a few guitarists would disagree with - even in this thread there is at least one comment that implies it's magnetic. However there are other comments that specifically point to things like guitar transformers as the source of magnetic noise. So even at this very small scale we find a topic that the electric guitar community is not wholly in agreement on (like so many things!).

How exactly did the author draw his conclusion that mains hum is electrostatic and not magnetic? He made a wire loop 15 cm in diameter and compared the hum it picked up to the hum picked up by a large copper plate. As he says:

If the mains hum we see in our laboratory is magneticaly-induced, then we will be able to pick up a large amplitude with a coil of wire, but not with a copper plate. If the mains hum is electrostatically induced, the converse will be true.

With the loop, he got 4mV of hum "right next to a power supply" (thus magnetic) but less than 1mV elsewhere in the room. Ditto results with a 3 meter loop. Whereas with the copper plate, the scope picked up "thousands of millivolts of mains hum." Hence his conclusion.

Does this mean that if you've got a single coil pickup, and it's only mains hum that you want to ward off, you can get by with some system of shielding and grounding? I can think of quite a few objections:

• A single coil pickup is far more sensitive to magnetic fields than a single 15cm loop of wire, thus his test was not adequate for our purposes.
• Grounding by itself won't work. Right??
• A complete Faraday cage is impossible for a guitar, and a partial cage won't work. So shielding the pickup is out.
• There's no point to only blocking mains hum! Because you'll always have amp transformers nearby contributing magnetic noise.

It would be interesting to do some independent and fairly rigorous tests involving pickups. All you need to start is a pickup - no wiring harness - and a way to isolate it from all but the noise sources you choose to introduce. For example, get the single coil pickup & plug it into a transformerless chipamp run by battery. Take it onto a lawn or other area free of nearby noise sources. Now use a long extension cord to introduce whatever noise source you like. One thing I'd be curious about - does shielding & grounding the wiring harness alone (or basically not having a wiring harness) work by itself for mains hum? After all then the pickup would be grounded too. Lots of "No's!" would be raised to this idea I'm sure. But you could test & find out - w/ no transformers nearby.

With just a pickup by itself, you could even put it in a proper Faraday cage. I'm sure this one has already been done many times - I just haven't Googled enough!

Anyway to some folks this may appear much ado about nothing. I admit this is a very small topic that interests me but wouldn't interest most folks - either they already know this stuff or they don't care because it's not directly practical. What I like in this article is the use of experiment and the attempt to use only precise terms for defining the EM phenomenon. I could draw a comparison between this sort of experimenting and the kind David Collins did for investigating whether cap type really influences guitar tone - many will insist there's no point because they already know the answer, but some will have open minds & be interested.

[dai h.]

It doesn't seem to me to be a question of either/or, mains hum is both (i.e. has both electromagnetic and electrostatic components). So what has been recommended in past threads is to use some sort of hum cancelling scheme to deal with the hum and electrostatic shielding for the buzz (as I recall).

[Mick Bailey]

There's much more to the loop/plate comparison, and I think the conclusion drawn is not convincing. As far as electrostatics go, if this were the case then a guitar pickup could be placed inside an electrostatic shield and the hum eliminated. Any metallic non-magnetic shell would work for a pickup if this were so - such as lipstick cases on Danelectro pickups. Or even a carbon-loaded non-magnetic polymer connected to ground. But in practice this doesn't work.

Mesh is also used for electrostatic screening.

[Usable Thought]

Pot stirred.

If I have time I may still mess around myself. I hear too many competing claims on this topic, always stated as God's truth, for me to take one side or another.