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Guitar Myth? Guitar shielding

by alsamuef
Tags: faraday, guitar
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alsamuef
#1
Jan14-13, 06:37 PM
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Ok, is doubt this actually works well.
Some guitarists shield their guitars in the cavity of the pickup and the pick guard with conductive foil. This is supposed to work like a faraday cage.

The pickup is basically a solenoid that "pick ups" the magnetic field from the magnetized string. So is desirable to protect the pickup from external magnetic fields.




But how is this going to work if part of the coil is outside the "cage"?



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davenn
#2
Jan14-13, 07:22 PM
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hi there
welcome to PF

Well it has the advantage of shielding the majority of the pickup and assoc electronics/cables from external fields.
Yup, that tiny part of the pickup may get something induced into it, BUT the level is going to be substantially smaller than if the whole assembly was exposed to external fields.
Those fields wich would probably mainly be either mains 50/60Hz hum or maybe nearby RF transmitters

Dave
alsamuef
#3
Jan14-13, 07:31 PM
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Quote Quote by davenn View Post
hi there
welcome to PF

Well it has the advantage of shielding the majority of the pickup and assoc electronics/cables from external fields.
Yup, that tiny part of the pickup may get something induced into it, BUT the level is going to be substantially smaller than if the whole assembly was exposed to external fields.
Those fields wich would probably mainly be either mains 50/60Hz hum or maybe nearby RF transmitters

Dave
Thanks for you response. :)
But this still puzzle me. For example, a microwave oven works under the same principle, since is a faraday cage that is filled with radiation that cannot scape so easily. In the coil case, wouldn't the shielded cavity work in a similar fashion to a microwave, since the cavity have a great antenna in the hole of the cavity? If not, then could i considerer the metallic plate of the coil and the coil wire part of the cage?.

I am just curious.

davenn
#4
Jan14-13, 07:54 PM
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Guitar Myth? Guitar shielding

the size of the end of the coil protruding from the cavity is very very small in relation to any of the external interfering frequencies that may impinge on it

As I said earlier.....
that tiny part of the pickup may get something induced into it, BUT the level is going to be substantially smaller than if the whole assembly was exposed to external fields

its all a matter of scales (area that is /isnt being exposed to external interference) and induced signal magnitudes


Dave
alsamuef
#5
Jan14-13, 08:21 PM
P: 5
Quote Quote by davenn View Post
the size of the end of the coil protruding from the cavity is very very small in relation to any of the external interfering frequencies that may impinge on it

As I said earlier.....
that tiny part of the pickup may get something induced into it, BUT the level is going to be substantially smaller than if the whole assembly was exposed to external fields

its all a matter of scales (area that is /isnt being exposed to external interference) and induced signal magnitudes


Dave
Hum!! i see. Do you an idea about the frequency spectrum that is going to effectively cancelled inside a cavity like this?. Also, what happens if we send to ground the cavity. Most guitar coils are grounded in the metallic plate at the bottom.
sophiecentaur
#6
Jan15-13, 04:34 AM
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What's being described is hardly a Faraday Cage. It is open along one side, whereas a Faraday Cage needs to be totally closed - with no cracks or slots in it. This is the only structure that will ensure that currents flowing in the 'outside' of the cage will not flow to the 'inside' of it. The foil may reduce the level of interfering electric fields around the pickup because of the Capacitance, which could be an advantage, but it won't have much effect on magnetic (hum, in particular) fields.
Often, someone will modify a guitar and manage to achieve an improvement in the susceptibility to interference but why it worked may be simply due to the re-routing of some of the wiring or the changed position of an Earth connection. Many times, though, people tinker around for days and don't achieve any improvement. If it were as simple as putting some foil in the wood cavity, would not all guitar makers do that and charge for it?
ImaLooser
#7
Jan15-13, 04:45 AM
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Quote Quote by sophiecentaur View Post
If it were as simple as putting some foil in the wood cavity, would not all guitar makers do that and charge for it?

The do. Usually they use metallic paint for the cavity. Maybe the foil works better. Some Fender guitars have a metal pickguard under the plastic one. As I recall I shielded a cheap bass with foil and it made no difference.

Many expensive guitars have terrible shielding. It's tradition. Often the amplifier is even worse than the guitar, so improving the guitar won't do you any good.

Common are hum-resistant pickups. They have two coils with opposite polarities and current. The hum cancels out while the induced current reinforces.

Over many years I got completely fed up with the hum from electric guitars and basses and now I won't even use them. Usually the only way around it is to play fairly loudly. If you try play quietly its intolerable. I used to use little battery powered amplifier into headphones and that was better, but I still picked up 60 cycle hum. Now I just play with no electronics at all.
sophiecentaur
#8
Jan15-13, 06:16 AM
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I reckon it's time that a new form of pickup was designed for vibrating strings. Something optical, perhaps, or RF based. A guy was posting recently about an optical sensor for harmonica reeds.
I guess that would trigger a huge revolt amongst electric guitarists along the same lines as the Valve Amp and Vinyl records enthusiasts. "It just isn't the same." yada yada. Lets all have black and white tellys and carbon microphones too! What about Daguerotype film and solid tyres on our cars? They aren't "the same" either.
alsamuef
#9
Jan15-13, 08:42 PM
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Quote Quote by sophiecentaur View Post
I reckon it's time that a new form of pickup was designed for vibrating strings. Something optical, perhaps, or RF based. A guy was posting recently about an optical sensor for harmonica reeds.
I guess that would trigger a huge revolt amongst electric guitarists along the same lines as the Valve Amp and Vinyl records enthusiasts. "It just isn't the same." yada yada. Lets all have black and white tellys and carbon microphones too! What about Daguerotype film and solid tyres on our cars? They aren't "the same" either.
There's a good excuse for that , for once!, since the signal coming from the electromagnetic pickup is different than the optical one. For instance, the electromagnetic pickup sends even-harmonics to the amplifier. And for some reason , most people love even-harmonics sounds, like a violin.
yungman
#10
Jan15-13, 08:59 PM
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I am very involved in hum cancelling for guitar. I did extensive experiment with copper shielding and proved that it really don't help the pickup hum. Yes, it shield the hum that picked up by the bare wires under the pickguard, not the hum produced by the pup. For a non shielded guitar body, you have two source of hum, one and the major one is from the pup that shielding don't help. The other is the hum from the wiring that shielding absolutely help. Case in point, Gibson use coax for all wiring, they never need conductive paint or copper shielding.

For old Fenders without conductive paint in the cavity, copper tape shielding help. For new Am Std that already has conductive paint, you don't need copper tape. I put copper tape in both of my Am Std strat with conductive paint, it did not improve anything. I can't speak for the MIM, Squire or the other copy of strat as I don't know whether they have conductive paint. I guess, when in doubt, it's cheap to put the copper tape in.

The reason why shielding like this don't do a thing is because at audio frequency is so low that the skin depth is very thick. The thin copper foil is almost transparent the the EM wave at audio frequency. If you use heavy gauge copper to shield, you might change the sound as you block the loop of the EM wave. It's a no win.

The idea of what you drawn is done by Lace Sensor. If you open the pup, you'll see the coil sitting inside a thick metal tub that shield all sides except the top. Based on result, it is much easy to move to a position where you don't get hum because hum get blocked by the metal. The thickness is over 1mm. But one reason or the other, Lace never sounds like a true strat single coil and it is not that popular. People that are into Strat want the true single coil sound and passive at that.
yungman
#11
Jan15-13, 09:16 PM
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Quote Quote by sophiecentaur View Post
I reckon it's time that a new form of pickup was designed for vibrating strings. Something optical, perhaps, or RF based. A guy was posting recently about an optical sensor for harmonica reeds.
I guess that would trigger a huge revolt amongst electric guitarists along the same lines as the Valve Amp and Vinyl records enthusiasts. "It just isn't the same." yada yada. Lets all have black and white tellys and carbon microphones too! What about Daguerotype film and solid tyres on our cars? They aren't "the same" either.
I am very involved in Strat Talk forum and I design music electronics. I can tell you those people don't like new stuff. After all these years with numerous innovations and break through, they insist on going back to the "vintage"60s pups and force the winders to reproduce the retro 60s sound.

There are a few active noise cancelling patents and numerous passive noise cancelling patents, so far, none make it popular. Anything you put in the signal chain change the sound. You think if you put an opamp in the signal chain inside the guitar will not make any difference, it does, day and night difference. I yet to be able to put any active electronics in the signal chain inside the guitar or in a pedal and not significantly change the sound. Nobody that I know of manage to do that. Anyone that can buffer the guitar and reproduce the EXACT sound deserve a patent.

I just submitted a patent application on noise cancelling for string instruments, I'll see how's my luck.
alsamuef
#12
Jan15-13, 09:33 PM
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Quote Quote by yungman View Post
I am very involved in Strat Talk forum and I design music electronics. I can tell you those people don't like new stuff. After all these years with numerous innovations and break through, they insist on going back to the "vintage"60s pups and force the winders to reproduce the retro 60s sound.
Have you ever heard about the "cargo cult?" phenomena? I think this one of the causes for such conservatism.
Andrew Mason
#13
Jan15-13, 09:47 PM
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Quote Quote by alsamuef View Post
There's a good excuse for that , for once!, since the signal coming from the electromagnetic pickup is different than the optical one. For instance, the electromagnetic pickup sends even-harmonics to the amplifier. And for some reason , most people love even-harmonics sounds, like a violin.
I agree. It is not sound that it picks up but the electromagnetic effect of the vibrating string interacting with the magnetic field of the pickup magnet. The waveform shape of the pickup signal very different than the acoustical waveform, which is what makes the electric guitar sound so distinctive.

AM
yungman
#14
Jan15-13, 10:05 PM
P: 3,898
Quote Quote by alsamuef View Post
Have you ever heard about the "cargo cult?" phenomena? I think this one of the causes for such conservatism.
I originally thought it's because of the old farts dominates the opinions. But to my big surprised, the young guitarist sort after the 60s sound too. Pickup winders are forced to use the old materials, old techniques to make pickups......if you want to sell anything!!!

Do you know the biggest thing about guitar amps? They want back the tweed ones from back in the 50s!!! The simple straight forward design. They want the formerly cheapest alnico magnet speakers that you only found in the cheapest of the cheap radios. You should check how expensive are those alnico speaker by Jensen and Celestion.


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