Trying to understand how EMI works and how to shield devices

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
Hi, I want to understand exactly how EMI works and how I can prevent them. The exact thing I'm doing is a work in my electric guitar, I want to shield it against EMI/unwanted noises (common practice), I've seen thousands of videos about how to do it, I know how to do it, but I really don't understand it theoretically, or better, the parameters that might affect its purpose, I don't know if the shield must be 100% enclosed without any gap, like air tight closed, or if a conductive mesh would do the work just as fine (as in any Faraday cage experiment), or how thick the shield must be. I was watching the other day videos about Faraday cages, and there's one where a man puts a radio inside an aluminum bin which had a lid. If the lid was slightly opened, like leaving 1 mm open, the radio would just work. So that "scared" me because it made me think to shield properly anything you must seal it very precise and tight.

Anyway, as you might correctly guess, in guitar building there are not a lot of electrical or electronic engineers, luthiers are artisans, they might explain what shielding does but they really don't have the slightest idea of physics, magnetism, electricity, let alone electromagnetism, so they will use terms they don't understand and repeat what they've been told. They will cover every area with copper tape, check continuity and that's it, job's done.

Before proceeding to do so, I'd like to understand how to do it properly from the physics perspective. First, here's how I understand an electric guitar works. Please, feel free to correct me if I'm wrong in my explanations or if they are a little lousy:
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There are 2 basic elements: a string and a coil. I simplified the insides, in reality there are more cables, 3 pickups instead of one, pots, etc...

The coil is not powered, first of all, so I believe it simply acts as a receptor. There's a permanent magnet inside it (green), and I know you do that to strengthen the magnetic field produced by the coil. However, since we are not powering the coil to produce a magnetic field, I believe the magnet serves another purpose. I believe it needs to be there to create a permanent, constant magnetic field, which does not induce current in the coil per se, but that is "disturbed" by the string, which makes the magnetic field not constant, hence varying magnetic field value, hence you induce current in the coil, and voila, you get your electric wave in your coil terminals.

I think the string must be ferromagnetic (and not necessarily conductive, right?), that is, forms a permanent magnet or is attracted by one, or both. In this case, I believe the string is simply attracted by a magnet. So, once you make the string vibrate, it oscillates at a certain frequency. The magnetic field gets "disturbed" by the string vibrating, it changes its value, and a varying magnetic field induces a current in the coil. The coil cables go to a volume potentiometer, then to a jack plug, and that to an amp with a speaker via cable.

So, that's the basic idea of how an electric guitar works. To be honest, I don't really get how the string oscillating at 440 Hz for example can disturb the magnetic field, how can you read that and how that can create a perfectly defined 440 Hz electric wave which I hear through the amp. If somebody could explain it to me better...

Now, the real problem here: shielding all that.
First, from what? I don't even know. I don't really have a clue about what can a guitar pick up, I'm not an expert on interference at all, all I know is vaguely what I've read from magazines and broad stuff like "everything causes an EMI". What I know is that sometimes guitars "hum", or sometimes they emit a "buzz", which stops if you touch a string (you ground it?). I want to stop that, as much as I can.

I don't really get what is an electromagnetic wave, it's hard to believe it doesn't need a medium. I don't know if an ultra thin copper foil is going to do anything, I guess it transforms the EM wave in Eddie currents?, I don't know if the pickups should be shielded from behind (no one does it), so an EM wave can penetrate through the coil inside, etc...
 

MrChips

Joined Oct 2, 2009
21,666
Why do guitar pickups hum? Because they don't know the lyrics.:)

"Hum" is caused by AC mains interference. Depending on your country's AC supply, line frequency is 50Hz or 60Hz.
Hence if your AC supply is 50Hz, hum frequency can be 50Hz and 100Hz.

Hum penetrates into the system in all of the electrical wiring of the circuit. In order to minimize the hum you need to shield all of the wiring and circuit, especially parts of the circuit where the impedance is high, the higher the impedance, the greater the induced voltage since V = I x R.

Humbucker pickups were invented as far back as 1930.

How do guitar pickups work
In order for the coil to respond to the vibrating guitar string you need a changing magnetic field. The permanent magnet produces a constant magnetic field. The field penetrates into the string made of ferromagnetic material. It would not work with nylon strings on a classical guitar. When the string is struck, it vibrates and disturbs the magnetic field. It is this changing magnetic field that is sensed by the pickup coil.

Humbucker pickups work by having two pickup coils. Hum induced into each coil is subtracted so as to reduce or minimize AC hum.

This is how differential instrument amplifiers (and balanced XLR microphones and XLR cables) work. By having paired signals and paired connections, a differential amplifier takes the difference between the two input signals. Any common-mode signal picked up along the wires of the inputs are cancelled. The purpose of a differential amplifier with high CMRR (common-mode rejection ratio) is to reduce the common signal (AC hum) and amplify the difference signal (signals from two coils).

If you want to invent and patent a modern hum-free pickup, place the sensor, differential pre-amp, amplifier, and analog to digital converter right at the pickup. From here on you have a hum-free digital signal. I would also use a MEMS vibration sensor instead of a coil pick up. Make sure you include me as co-inventor in your patent application.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
It's 50Hz. By the way, those 50Hz are in the wall outlet, how are they going to go inside a guitar, if the only cable that goes to the guitar is the one that is sending FROM the guitar to the amp the pickup signal?

Anyways, you didn't answer the core questions about shielding. Its thickness, how tight it must be, if you need a double layer, if I must shield below the pickup, etc...
 

MrChips

Joined Oct 2, 2009
21,666
EMI is all around you. You do not need a wire from the wall outlet to the guitar to pick up EMI.

I did not go into the details of EMI shielding because I outlined better ways of mitigating the problem.
 
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