Why doesn't humbucker cover stop pickup from working?

[waspclothes]

Hey guys,
first post on the pickups forum - this is just a question that's been bugging me for a while, and I figured I'd ask some experts for the answer.

Why doesn't a humbucker's shielding cover stop the pickup from working? if it's blocking electromagnetic fields from causing interference with the pickup, why isn't it stopping the magnetic fields from the moving string being able to be picked up by the coils?

[Helmholtz]

Why doesn't a humbucker's shielding cover stop the pickup from working? if it's blocking electromagnetic fields from causing interference with the pickup, why isn't it stopping the magnetic fields from the moving string being able to be picked up by the coils?

In the low frequency (audio frequency as opposed to radio frequency) and near field domain, electric and magnetic fields exist and act independently. Meaning that there can be electric and/or magnetic interference fields that need to be treated individually.

Thin electrical conductors like copper, brass, aluminum, nickel-silver and even conductive paint effectively shield electric interference fields (when grounded) but are ineffective regarding low frequency magnetic fields. That's why the humbucker was invented.

[waspclothes]

In the low frequency (audio frequency as opposed to radio frequency) and near field domain, electric and magnetic fields exist and act independently. Meaning that there can be electric and/or magnetic interference fields that need to be treated individually.

Thin electrical conductors like copper, brass, aluminum, nickel-silver and even conductive paint effectively shield electric interference fields (when grounded) but are ineffective regarding low frequency magnetic fields. That's why the humbucker was invented.

So the humbucking design is for audio frequency stuff like 50/60Hz mains interference, etc.. and the cover is for high frequency stuff like radio transmissions?

[Helmholtz]

So the humbucking design is for audio frequency stuff like 50/60Hz mains interference, etc.. and the cover is for high frequency stuff like radio transmissions?

Not exactly. The cover is for/against interference from electric fields (all frequencies) and the humbucking design bucks external magnetic interference fields. Electric interference is mainly produced by high AC voltages while magnetic interference is produced by (high) AC currents (motors, transformers, heaters etc.). Generally magnetic interference is more likely to be low (mains) frequency. Dimmers and SMPS produce both: magnetic and electric interference containing a wide frequency band.

[J M Fahey]

The top cover is brass, non magnetic, and does NOT affect magnetic field interacting with strings.

Itīs there to shield from electrostatic interference, a very different thing.

It isnīt specifically a "frequency" problem.

[Helmholtz]

Covers are mainly and preferably made from non-magnetic nickel silver (German silver). Brass is a better conductor than nickel silver and produces noticeable eddy current losses (attenuation) at high frequencies. Nevertheless old Tele lead PUs had brass covers.

While at low frequencies shielding of magnetic fields requires a ferromagnetic shielding material like steel, low resistance non-magnetic conductors do provide effective magnetic shielding at (very) high frequencies (again by utilization of the the eddy effect).

I wouldn't call electric AC fields "electrostatic". To me electrostatic implies zero frequency or DC. DC fields don't produce audible interference.

[J M Fahey]

"Electrostatic" is a heritage word specifically applied to *shielding* and so the proper one to use here and describe the effect of a pickup cover.
Also what a cable shield does and any grounded metallic shielding, from a metal chassis or cover to kitchen paper glued inside a guitar cavity.

From: https://courses.lumenlearning.com/bo...ugh-induction/

Electrostatic Shielding

Electrostatic shielding is the phenomenon that occurs when a Faraday cage blocks the effects of an electric field.

https://sciencedemonstrations.fas.ha...atic-shielding

Harvard Natural Sciences Lecture Demonstrations

1 Oxford St Cambridge MA 02138 Science Center B-08A (617) 495-5824

Electrostatic Shielding

How does shielding work? Is it a two-way street and work both ways? Can electric fields not penetrate metals? What's going on? This sequence of demonstrations addresses these questions.

And soon and on and on.

Although current Science lingo tends to use plain "Electric" field name , to me it can lead to confusion, so I backtrack to the old division between Electro-magnetic and Electro-static which allows for not doubts.

yes, itīs old wording because it started describing electrical field obtained by wiping amber rods and plates, etc., only electrical generators available 200 years ago or more, and attracting small pieces of paper or silk, but the effects described can also apply to "frequency" variable fields.

Not the only one to use the old wording for clarity, many heavyweights continue to do so, as in the examples above.

For a Ton of reading material, just Google "electrostatic shielding" and enjoy

[Helmholtz]

Thanks for the survey on technical uses of the term "electrostatic". Of course I can't ignore an industry established use of the term. Just wanted to avoid misinterpretation.

To me "electrostatics" deals with charges at rest (https://en.wikipedia.org/wiki/Electrostatics) and AC fields require the charges to move. So I differentiate between electrostatics and electrodynamics (which may or may not include magnetic effects).
The terms probably won't help the OP, but your links might.


O.K., here is another explanation (avoiding the terms electric and magnetic field):

Electric(al) interference works by capacitive coupling while magnetic interference works by inductive (magnetic) coupling (between the source of interference and the PU, circuit etc.
Capacitive coupling generally favors high frequencies...