Help with guitar noise please

[JohnRoberts]

I challenge the idea of "body capacitance" here. Where is this capacitance "in the circuit" ? Between "Ground" and ????

The significant effect is that your body is a noise source when it is electrically floating.
When you touch "Ground" (typically by being in contact with the grounded metal parts/strings on a guitar) your body is held at the same potential and ceases to be a source of noise. You could also touch any other grounded metal for a similar effect - equipment rack / metal chassis etc. And it can be useful to arrange such a connection to your body to avoid noise when you might lose connection via the guitar due to playing technique. tapewound or coated strings etc.
I have had people doubt, or full out deny the body as a noise source. I tell them to touch the tip of a 'scope probe and observe the distorted 50Hz waveform on the trace. (60Hz if that is your mains frequency but it doesn't sound as good ).
And if you get a periodic noise at one second intervals - it's your watch (yep - been there )

[veer alert]
During the design of my OD-1 outlet tester http://www.johnhroberts.com/OD1.htm I spent a lot of time investigating the electrical characteristics of an ungrounded human body. The classic problem for outlet testers is "voltage relative to what?". To identify hot-energized leads for outlet testing my OD-1 uses the human body connected through a touch probe to be a relative 0V AC reference. For this to work and have any chance of being UL approved, the human touch contact has to be extremely high impedance. I accomplished that with a mosfet input buffer (schematic available at the link). I measured my testers input impedance as >500M Ohm at 500V with a commercial tester.

I abandoned this project even though I believed it "could" pass UL safety criteria, but that would likely involve opening a file with UL and paying them tens of $k to confirm the safety of my design. But after that sunk cost I have a SKU that needs to retail for <$20 and convince inexperienced customers that its OK to touch a metal probe while testing outlets. IMO an uphill battle for only modest reward. Therefore I share the design with the world free to copy.

Long story short and back on topic, I found that the human body served as an adequate 0VAC reference for comparison testing to mains voltage. So indeed the human body is floating but has some apparent capacitance relative to the local environment (ground?. I have not attempted to specifically characterize how much capacitance the human body contains, only that there was enough for my outlet tester to work.

[edit- Capacitive touch switches appear to work using body capacitance so may have done more rigorous research than I did. /edit]

JR

 

[ccaudle]

I have not attempted to specifically characterize how much capacitance the human body contains

There has been some work on that.
The JEDEC human body model testing spec uses 100pF to local ground reference in series with 1k5 Ohms as a simplified model.

How that applies in a macro setting (when not specifically using that model to test device ESD susceptibility) is not clear to me.
Presumably standing barefoot on a concrete slab which is in contact with the ground would be a different situation than wearing rubber soled shoes on the second floor of a wooden building.

 

[Newmarket]

There has been some work on that.
The JEDEC human body model testing spec uses 100pF to local ground reference in series with 1k5 Ohms as a simplified model.

How that applies in a macro setting (when not specifically using that model to test device ESD susceptibility) is not clear to me.
Presumably standing barefoot on a concrete slab which is in contact with the ground would be a different situation than wearing rubber soled shoes on the second floor of a wooden building.

Indeed. There is always some sort of path. Relating to standing on a surface - I work in a high voltage environment (literally - not just an office with excitable people ) Our safety footwear needs to pass an insulation test so that the risk of electric shock to the floor via our feet is minimised.

For clarity, when I wrote "I challenge the idea of "body capacitance" here. Where is this capacitance "in the circuit" ? Between "Ground" and ????" - I didn't mean to dispute that a body has associated capacitance. It clearly does and I have worked with capacitive sensors in several contexts eg Touch Sense on moving faders. What I meant was "here" ie in the context of electric guitar screening. Considering a "standard setup"- earthed amp chassis with hard connection to guitar bridge etc The "body capacitor" is connected from the grounded metal parts of the guitar ("earth") to...errr..."earth" so it is effectively bypassed with a low impedance (ideally zero) impedance connection.

 

[mr coffee]

Out of curiosity, what would shielding a guitar cavity with nickel or mu-metal connected to the output jack ground do?

What about lining the pickup cavities with nickel or other magnetic material. I know it would affect the sound of the pickup in many cases (especially single coils), though it might be for the better (depending on taste).

Anyone tried it to see?

 

[dai h.]

(forgive me I am slow, lol...). So what I gather:

The mental model of a body acting as a plate of a capacitor which helps provide electrostatic shielding when touching circuit ground (via strings, bridge, etc.) is incorrect.

Why is this? Not enough (body) capacitance (apparently around 70-200pF) and/or wrong material (alu or copper would provide some shielding effect even though not completely enclosing the circuit but not a human body)?

Would it be more correct to think of the body more as (sort of) an antenna over which noise voltages are generated (from nearby AC wiring, lights, whatever), then if the human isn't touching guitar circuit ground, these couple in to the circuit capacitively (hence more "buzz")? Or maybe the body as capacitor and when you touch circuit ground you send the noise voltage (if say lights then the power came from the wall) back home (back to wall-breaker power source though safety ground (or "death cap" and neutral)? (Sorry if sounds like crazy speculation--I'm just a total amateur but for some reason I like reading about noise, grounding, and layouts.)

edit: so if barefoot and the floor is a low impedance connection to AC safety ground (I guess dangerous if no safety ground connection and a fault occurs) i.e. body isn't floating (or it's less floating), then maybe less of a difference in "buzz" when touching/not touching?

 

[DarkSky]

... I figured the noise comes from the house and there's something wrong with the electrical installation.
Guess what? The ground terminals of the outlets weren't connected to the spike! The spike was there alright but UNCONNECTED!! I guess somebody "forgot" to connect the house to it

Glad you were able to identify the cause of your friend's problem and correct it!

I've had a somewhat related experience a few years ago when I tried using a "Marshall" branded bluetooth amp as a travel amplifier for my guitar rig. The sound was acceptable (and a step up from the battery powered Fender unit I'd been using previously) however, while in some locations the noise (hum/RFI) it was acceptable, in others it was decidedly not.

Turned out that the amp itself was not grounded via the mains (not sure if that is true of all the Marshall branded bluetooth amps, but it probably is) and in fact, the power input port was a two-prong socket and the circuit 'ground' was therefore floating.

A bit of experimentation led me to conclude that connecting the circuit ground to mains earth solved the hum/RFI issue without introducing any other problems - whereupon I modded the amp to install a standard 3-conductor IEC power socket and wired the internal 'earth' to ground. The amp has been silent ever since.

What's interesting is that the extent (and character) of the noise varied from place to place - presumably depending on the quality of power and presence of RFI interference etc in each location. It reminded me of accounts I've heard/read of studio techs finding that the building earth was not adequate, and solving hum/RFI issues by driving a new (much larger/longer) ground stake closer to the studio mains outlets, and connecting it with the shortest possible span of heavy gauge cable.

Presumably the problems addressed or ameliorated with that method were a variation of what your friend was experiencing --- pointing to the fact that it's not only the presence or absence of the building ground connection that's at issue, but the *quality* of that connection that we need to consider. FWIW.

 

[dai h.]

Glad you were able to identify the cause of your friend's problem and correct it!

I've had a somewhat related experience a few years ago when I tried using a "Marshall" branded bluetooth amp as a travel amplifier for my guitar rig. The sound was acceptable (and a step up from the battery powered Fender unit I'd been using previously) however, while in some locations the noise (hum/RFI) it was acceptable, in others it was decidedly not.

Turned out that the amp itself was not grounded via the mains (not sure if that is true of all the Marshall branded bluetooth amps, but it probably is) and in fact, the power input port was a two-prong socket and the circuit 'ground' was therefore floating.

A bit of experimentation led me to conclude that connecting the circuit ground to mains earth solved the hum/RFI issue without introducing any other problems - whereupon I modded the amp to install a standard 3-conductor IEC power socket and wired the internal 'earth' to ground. The amp has been silent ever since.

What's interesting is that the extent (and character) of the noise varied from place to place - presumably depending on the quality of power and presence of RFI interference etc in each location. It reminded me of accounts I've heard/read of studio techs finding that the building earth was not adequate, and solving hum/RFI issues by driving a new (much larger/longer) ground stake closer to the studio mains outlets, and connecting it with the shortest possible span of heavy gauge cable.

Presumably the problems addressed or ameliorated with that method were a variation of what your friend was experiencing --- pointing to the fact that it's not only the presence or absence of the building ground connection that's at issue, but the *quality* of that connection that we need to consider. FWIW.

I think a "death cap" would've helped to reduce the buzz, but that puts you in peril of connecting the hot side of the AC to the chassis if the plug is reversed (and might be illegal today). (My understanding) safety ground is connected to neutral back at the breaker. So unconnected safety ground connection is like the old two-prong scheme.

re: improved grounding, I guess it can help if the safety ground into actual dirt (or maybe metal frame if up high in a building) connection is a low impedance (caps dumping noise into chassis, safety ground see a lower impedance path--maybe check all those paths and clean connections, tighten screws, etc.).

 

[DarkSky]

I think a "death cap" would've helped to reduce the buzz, but that puts you in peril of connecting the hot side of the AC to the chassis if the plug is reversed (and might be illegal today). (My understanding) safety ground is connected to neutral back at the breaker. So unconnected safety ground connection is like the old two-prong scheme.

re: improved grounding, I guess it can help if the safety ground into actual dirt (or maybe metal frame if up high in a building) connection is a low impedance (caps dumping noise into chassis, safety ground see a lower impedance path--maybe check all those paths and clean connections, tighten screws, etc.).

Yes, undoubtedly the quality of connections in the electrical path to the safety ground is one part of the equation. The other part is the length and quality (composition and gauge) of the wire between the electrical outlets and the ground spike. Even a few ohms of cable resistance over lengthy runs could be a factor.

As to the death cap, I agree that it's not optimal, as there are several failure conditions that could produce unfortunate outcomes (including the reverse polarity one you mention). In all, not a risk I was about to take. ; )

 

[Rusan]

On the subject of guitar shielding, a seemingly little-known technique for shielding single coil pickups is to do just that - shield the pickups, not the cavities. If it's a taped, open coil, apply the copper foil over the tape. If it's an untaped coil with a plastic cover, shield the entire inside of the cover. Well, almost.

I've seen suggestions to make a break in the (back of the pickguard (usually continuous)) shielding so it doesn't form a loop around the pickup,

Yep, the idea is to prevent a circumferential circuit for the induced eddy currents, otherwise the shielding tape acts just like the fully wraparound nickel silver covers on Tele neck pickups to kill the magic sparkle. instead, you must leave a 1mm gap at the round end. If internally shielding a plastic cover, use a hobby knife to cut and remove a 1mm wide strip beginning from the round-end gap, on centerline across the magnet holes to the opposite E magnet hole. Then, solder a ground wire from the foil directly to the pickup's ground lead.

Out of curiosity, what would shielding a guitar cavity with nickel or mu-metal connected to the output jack ground do?

What about lining the pickup cavities with nickel or other magnetic material. I know it would affect the sound of the pickup in many cases (especially single coils), though it might be for the better (depending on taste).

Anyone tried it to see?

I haven't actually tried it, but my postulation here mirrors that of Newmarket.

At a technical guess I'd tend to think that copper or aluminium (ie non-ferrous) shielding would have minimal effect.
Steel would affect the field and hence the pickup response. Indeed the sound of a traditional Telecaster pickup is modified by the metal plate it is mounted on.

In addition to the phenomenon of pickup cavity shielding wiping out the high freq sparkle of single coils, I could only imagine that a ferromagnetic foil shield would contribute additional negative artifacts. After all, guitar pickups are high impedance, reactive elements with a specifically engineered resonant frequency, and placing capacitive and/or inductive elements in parallel with them changes the recipe anyway, but when said reactive element is also extremely magnetic...

 

[Newmarket]

(forgive me I am slow, lol...). So what I gather:

The mental model of a body acting as a plate of a capacitor which helps provide electrostatic shielding when touching circuit ground (via strings, bridge, etc.) is incorrect.

Why is this? Not enough (body) capacitance (apparently around 70-200pF) and/or wrong material (alu or copper would provide some shielding effect even though not completely enclosing the circuit but not a human body)?

You seem to be mixing up two different things.
a) use of a capacitor to filter out noise - as I think I illustrated any such capacitance here would be between the (already) earthed guitar and earth (if we consider the player's body is at earth potential by whatever means / path. ie the capacitor is essentially bypassed so doing nothing.
If we consider the player to be electrically floating when not touching the earthed metal then it would just be a connection to one leg of a capacitor with the other leg not connected. So no effect.
b) screening / shielding - this does not involve capacitance. The screen is at a fixed potential (a low impedance connection to the reference potential).
It does have an effect even if not completely enclosing the circuit - and really absolute shielding seldom happens due to the need to pass cables, connectors etc.
eg hf crosstalk between faders is reduced if they are placed on top of a grounded metal sheet - a ground plane. This effect is more accurately described as "Guarding".

 

[RoadrunnerOZ]

In a lot of guitars that have no apparent shielding around the wiring or pickups there is conductive paint (usually black like blackboard paint) when the scratch plate with aluminium shielding is screwed on it connects the conductive paint to ground. The aluminium shielding on the scratch plate connects via the pot bodies which usually have the cable ground soldered to the back of one or more of the pots. Any guitar with metal knobs will have ground at the knobs if using metal shaft pots.
Any metal arm toggle switches also are grounded via the shield on the scratch plate.
I have done shielding on many guitars and basses and used conductive adhesive copper foil which can be soldered to, the ground carried through to the pickup cavity by either a wire or braid or foil tape for continuous cavities. Like this:

And this:

This shielding has reduced noise from pickups and controls extremely well without any change to pickup output sound or quality.
If you were to use a magnetic material this would influence the performance of the pickups.
For some amps I make a special cable which is twin core plus shield (ordinary mic cable) jack to jack - the outer shield is connected at only one end (usually the amp end), tip goes to one core each end, sleeve goes to the other core each end. This dissociates the shield from signal ground as it only connects at the mains grounded connection at the amp.

 

[dai h.]

You seem to be mixing up two different things.
a) use of a capacitor to filter out noise - as I think I illustrated any such capacitance here would be between the (already) earthed guitar and earth (if we consider the player's body is at earth potential by whatever means / path. ie the capacitor is essentially bypassed so doing nothing.
If we consider the player to be electrically floating when not touching the earthed metal then it would just be a connection to one leg of a capacitor with the other leg not connected. So no effect.
b) screening / shielding - this does not involve capacitance. The screen is at a fixed potential (a low impedance connection to the reference potential).
It does have an effect even if not completely enclosing the circuit - and really absolute shielding seldom happens due to the need to pass cables, connectors etc.
eg hf crosstalk between faders is reduced if they are placed on top of a grounded metal sheet - a ground plane. This effect is more accurately described as "Guarding".

Thanks. I am having trouble understanding a). Does that mean if the body is a capacitor, both sides are connected to earth (when touching circuit ground)?

(found a diagram to try to mentally picture things (source: https://incompliancemag.com/article/human-body-model-and-electrostatic-discharge-esd-tests))

If we consider the player to be electrically floating when not touching the earthed metal then it would just be a connection to one leg of a capacitor with the other leg not connected. So no effect.

I thought this meant noise coupling into the guitar (the body capacitively coupling noise from AC wiring, whatever noise sources nearby. Closer by = more noise?).

b) screening / shielding - this does not involve capacitance. The screen is at a fixed potential (a low impedance connection to the reference potential).

(I could be mistaken on this but) I thought electrostatic shielding was also known as capacitive shielding. (I picture a cap in series with another cap = less C)

It does have an effect even if not completely enclosing the circuit - and really absolute shielding seldom happens due to the need to pass cables, connectors etc.

I was aware of this. (Apparently erroneously) I thought making your body equipotential to circuit ground (by touching it) meant the body was becoming some sort of ground plane/electrostatic shield and reducing "buzz".

 

[Newmarket]

Thanks. I am having trouble understanding a). Does that mean if the body is a capacitor, both sides are connected to earth (when touching circuit ground)?

(found a diagram to try to mentally picture things (source: https://incompliancemag.com/article/human-body-model-and-electrostatic-discharge-esd-tests))

I'd forget about the body = capacitor concept here tbh.
But there is capacitive coupling in the manner shown in the diagram (nice).

I thought this meant noise coupling into the guitar (the body capacitively coupling noise from AC wiring, whatever noise sources nearby. Closer by = more noise?).


(I could be mistaken on this but) I thought electrostatic shielding was also known as capacitive shielding. (I picture a cap in series with another cap = less C)

I've not heard it described as that but YMMV. And I'm not particularly keen on the term "electrostatic" - But I'd describe it as - electrostatic screening protects against capacitively coupled interference. This does not mean that the interference rejection mechanism itself involves a capacitor.

I was aware of this. (Apparently erroneously) I thought making your body equipotential to circuit ground (by touching it) meant the body was becoming some sort of ground plane/electrostatic shield and reducing "buzz".

The dominant effect is that you hold the body at "Ground" (ie there is no dV/dt) and so stop it being any sort of antennae for interference noise.

 

[Newmarket]

For some amps I make a special cable which is twin core plus shield (ordinary mic cable) jack to jack - the outer shield is connected at only one end (usually the amp end), tip goes to one core each end, sleeve goes to the other core each end. This dissociates the shield from signal ground as it only connects at the mains grounded connection at the amp.

One problem with such a cable is that, for high Z passive pickups (typical e guitar / passive bass) a cable should really have a semiconducting layer to minimise noise due to cable movement. Rather than relying on the typical cotton fillers etc of a typical mic cable. Such twisted pair cable does exist or at least it has. (It fooled me recently when I made up a cable that failed test due to not cutting that shield back and it touching a signal core).
In a studio environment where there may be little physical movement of the cable it might not be an issue. Similar to where you might opt for a less flexible and better screened cable where movement and flexibility is not a priority.

 

[RoadrunnerOZ]

One problem with such a cable is that, for high Z passive pickups (typical e guitar / passive bass) a cable should really have a semiconducting layer to minimise noise due to cable movement. Rather than relying on the typical cotton fillers etc of a typical mic cable. Such twisted pair cable does exist or at least it has. (It fooled me recently when I made up a cable that failed test due to not cutting that shield back and it touching a signal core).
In a studio environment where there may be little physical movement of the cable it might not be an issue. Similar to where you might opt for a less flexible and better screened cable where movement and flexibility is not a priority.

I use cable that has the shield braid wrapped in a conductive plastic sheath and has no movement internally. Never had a cable noise problem. I have had issues with foil+braid or foil+ground-wire shielded cables rustling in stage boxes and studio looms when moved - really annoying.

Thanks. I am having trouble understanding a). Does that mean if the body is a capacitor, both sides are connected to earth (when touching circuit ground)?

(found a diagram to try to mentally picture things (source: https://incompliancemag.com/article/human-body-model-and-electrostatic-discharge-esd-tests))

The body is a capacitor when the skin is not touching ground or other items - as in your diagram - it’s relative to proximity.

I thought this meant noise coupling into the guitar (the body capacitively coupling noise from AC wiring, whatever noise sources nearby. Closer by = more noise?).


(I could be mistaken on this but) I thought electrostatic shielding was also known as capacitive shielding. (I picture a cap in series with another cap = less C)


I was aware of this. (Apparently erroneously) I thought making your body equipotential to circuit ground (by touching it) meant the body was becoming some sort of ground plane/electrostatic shield and reducing "buzz".

When the body is grounded it is no longer being an antenna.
In short the human body acts as a nice big antenna for any radiated noise and can induce noise in a guitar pickup or wiring via capacitive coupling by proximity - the moment the body is grounded to the guitar component it’s radiating to like the strings it ceases to be an antenna relative to the guitar.
Conversely the strings in a guitar can act as antennae radiating into the pickups if they’re not grounded and can be damped by a body touching them which sits closer to ground potential. Depends on whether the bridge is grounded or not.
Some pickups are really prone to being noise receivers

 

[Scott Harmonic]

On the subject of guitar shielding, a seemingly little-known technique for shielding single coil pickups is to do just that - shield the pickups, not the cavities. If it's a taped, open coil, apply the copper foil over the tape. If it's an untaped coil with a plastic cover, shield the entire inside of the cover. Well, almost.



Yep, the idea is to prevent a circumferential circuit for the induced eddy currents, otherwise the shielding tape acts just like the fully wraparound nickel silver covers on Tele neck pickups to kill the magic sparkle. instead, you must leave a 1mm gap at the round end. If internally shielding a plastic cover, use a hobby knife to cut and remove a 1mm wide strip beginning from the round-end gap, on centerline across the magnet holes to the opposite E magnet hole. Then, solder a ground wire from the foil directly to the pickup's ground lead.



I haven't actually tried it, but my postulation here mirrors that of Newmarket.



In addition to the phenomenon of pickup cavity shielding wiping out the high freq sparkle of single coils, I could only imagine that a ferromagnetic foil shield would contribute additional negative artifacts. After all, guitar pickups are high impedance, reactive elements with a specifically engineered resonant frequency, and placing capacitive and/or inductive elements in parallel with them changes the recipe anyway, but when said reactive element is also extremely magnetic...

this is about the first post here that i can fully agree with, Anything electrically conductive surrounding the pickup will indeed induce eddy currents (and their related magnetic field) and alter the sound/characteristics of the pickup. (using copper or any non-ferrous shielding material doesn't change that) using steel or any magnetic material nearby may be less conductive electrically, but then creates additional effects by attracting and re-shaping the pickup's magnetic field. -it acts as an additional pole-piece, conducting and re-orienting the magnetic field. After some decades in business, I tend to think that the well-intended folks who don't hear any difference, etc are more likely describing a less acute 'ear' for these things. Aside from these usually undesirable effects, Shielding in guitars has has very little effect at 60hz -compared to the noise of single coil pickups. With hum-cancelling pickups the small reduction in 60hz noise can be more effective. With any metal pickup cover ( or just a metal baseplate) creating these same effects to start with, then the effects from shielding or other conductive parts nearby is less noticeable. (with chrome plated, or so-called 'nickel-silver " metal covers, the Eddy currents and effects are compounded by the covers also being magnetically conductive as well.) (fwiw I'm some guy who's built a small number of custom guitars for famous dudes, a few amps, and tens of thousands of guitar pickups ( most in plastic covers) Most folks I hear from seem to forget that these are not just a passive DC coil and magnet, they are AC devices that actually generate the signal- within a powerful, modulated, AC magnetic field- which will interact with any nearby conductor.- electrical or magnetic. Si? ;-)

 

[RoadrunnerOZ]

It’s more the buzz that comes from dimmers, switch mode power supplies and the like that is the problem - the HF component of 60Hz “noise”, not the LF hum - listen to any guitar amp with a guitar that has single coil pickups and you don’t hear a smooth 60Hz (in my country 50Hz) sine wave but a buzz that although sounding like 60Hz has a lot of HF superimposed on that - just put an oscilloscope probe be against the skin and see how much noise there is. It’s the HF component that a sort of Faraday cage around the electronics will help reduce. Copper being non magnetic acts as a very good sink for stray HF interference as well as not magnetically interfering with the pickups. For a back-feed of induced magnetic field due to eddy currents compared to the strength of the induced AC in the coils the copper would need to be very, very close. They use copper sheet between windings in power transformers to decouple source noise and spiking from the secondary - the copper does not interact with the transformer’s mag field (or it would heat up) as it does not constitute a shorted turn but just a grounded layer.

Anything electrically conductive surrounding the pickup will indeed induce eddy currents (and their related magnetic field) and alter the sound/characteristics of the pickup. (using copper or any non-ferrous shielding material doesn't change that) using steel or any magnetic material nearby may be less conductive electrically, but then creates additional effects by attracting and re-shaping the pickup's magnetic field.

This eddy current back effect is inversely proportional to the square of the distance from the device - the closer the shielding is to the pickups the more the reactive effect of eddy currents will be. If you’re going to completely shield a pickup right on/at the pickup a slit in the shield interrupts the eddy currents - however in cavity shielding the shield is much further away from the pickups and is going to have proportionally less back effect on them and the cavity profile usually provides interruption to a complete surround of copper.
Guitar makers have been cavity shielding for decades without most people being aware that the process has been done - as I mentioned earlier the conductive black paint in the pickup and electronics cavities. Copper is just more effective. If shielding were to be “dulling” the pickups then most guitars would already be “dull”. You just need to put a meter on the paint to see if it’s conductive.

 

[abbey road d enfer]

I challenge the idea of "body capacitance" here.

It is real, though.
Any conductive object has a capacitance. Clouds have capacitance, it's how they carry charges.

Where is this capacitance "in the circuit" ?

This capacitance is between body and the rest of the universe. Actually its a sum of all the discrete capacitances, between the body and the walls, between teh body and the electric wiring, between the body and whatever appliance in the room, dominated by capacitance between body and earth.

Between "Ground" and ??ar

The significant effect is that your body is a noise source when it is electrically floating.
When you touch "Ground" (typically by being in contact with the grounded metal parts/strings on a guitar) your body is held at the same potential and ceases to be a source of noise. You could also touch any other grounded metal for a similar effect - equipment rack / metal chassis etc. And it can be useful to arrange such a connection to your body to avoid noise when you might lose connection via the guitar due to playing technique. tapewound or coated strings etc.
I have had people doubt, or full out deny the body as a noise source. I tell them to touch the tip of a 'scope probe and observe the distorted 50Hz waveform on the trace. (60Hz if that is your mains frequency but it doesn't sound as good ).
And if you get a periodic noise at one second intervals - it's your watch (yep - been there )

The body itself is not the source of noise. The body is an armature that defines two dominant capacitive pathes. One between the actual source of noise (in most cases the electrical wiring, and another between the body and the equipment that is subject to the resultant electrostatic field.
Touching a grounded part makes the body equipotential with it, so it doesn't radiate.
The body is a "secondary source", as in the Huygens-Fresnel principle.

 

[abbey road d enfer]

Out of curiosity, what would shielding a guitar cavity with nickel or mu-metal connected to the output jack ground do?

What about lining the pickup cavities with nickel or other magnetic material. I know it would affect the sound of the pickup in many cases (especially single coils), though it might be for the better (depending on taste).

Anyone tried it to see?

There are two sources of noise in electric guitars: electrostatic and magnetic.
Electrostatic shield can be any conductive material, even not very good. Carbon paint is commonly used to shield cavities with good results.
A magnetic shield that would efficiently divert external magnetic interfernce would have to completely enclose the pick-up, which would result in no signal from the strings.
Attempts at using a magnetic shield on the back of guitars result on very poor attenuation of interference.
The most efficient systems use a coil that is disposed so that it does not pick the strings and reinjects the hum and buzz out-of-phase in the output.
Music Man has that, and there's a Russian guy who oes it for aftermarket install.

 

[RoadrunnerOZ]

The skin of the human body is one plate of a capacitor, the surface of the earth is another (or anything close by that has a conductive skin). We earth houses with a ground-rod hammered into the earth and connected to the Earth/Neutral bus at the incoming power switchboard otherwise mains earth could be at dangerous voltages compared to local ground.