I have a small bedroom recording studio full of electromagnetic fields (mainly 50hz+harmonics up to ~3,5khz from power cables in walls).
Here is how it looks on Adobe Audition's spectrum analyzer:
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I'm planning to build a 2m x 2m Faraday's cage/booth to protect active electric guitar pickups from external electrostatic and electromagnetic noise 20hz-96khz
Please suggest best material (copper ? aluminium ? iron ? or maybe a combination of different metals), how thick should it be and how to get rid of the current in a situation where no true ground is available.
How many dBs of attenuation should i expect ?
Thanks in advance.
ps. If this is not the right newsgroup to ask this question please suggest the best one.
You need two cages inside each other connected at a single point and earthed there to get a decent attenuation. There have a filter plate where the signals are passed throuigh the walls. The door and possible windows can be a problem but are solvable. For the lower frequencies, iron would be the choice. I'd expect an attenuation in the order of 150dB or better. Note that the quantum limit is at 180dB, there is no better faraday shield.
After looking at your spectrum, I'd take a look for switching ballasts for flouresent lights around you. Studios, and laser labs, should only have incandescent.
I'm a complete ignorant in this matter - can you please explain why two not one and what material and how thick (copper is pretty expensive here) ?
How thick ?
Whoa ! I need only 20dB or something like that hehe :) i just need to remove that ugly peaks you can see on the picture i posted earlier - i want them to drown in the noise floor.
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I believe i have good enough cable. As i said in a different branch of this thread the dirt disapears when guitar volume is turned down to zero so i think it must be pickup picking all this unwanted frequencies - it is shielded properly inside guitar but it can't be shielded outside - it has to "look" at the strings ;)
The three field types are magnetic, electrostatic, and electromagnetic.
Forget magnetic shielding. Enough iron to give any useful shielding to a room would collapse the structure of a house.
Electromagnetic shielding, aka RFI screen room, is difficult and only keeps out high-frequency fields. Google "emi screen room" or somesuch.
You achieve good electrostatic shielding (block capacitively-induced hum) with any conductive coating on the walls: foil, screen wire, metal sheets. Just connect and ground them all. Some modest electromagnetic shielding will result, too. A water pipe is a good low-frequency ground.
But none of this should be necessary if cabling and grounding are right.
20dB is not really a faraday shield, it is a toy. And thus a single shield should be sufficient. Low frequency is coupled magnetically, and magnetically only. Thus iron should be sufficient. Iron means iron and not steel, nor stainless, chrome nickel, or such. By the way iron is better than copper at these frequencies. Magnetic nearfield attenuates as r^3 and not as r^2, I guess 1 mm or so should be sufficient.
I note the 50 Hz fundamental is about 20 db higher than the next highest harmonic. If it's constant in amplitude and phase, you might be able to null it by injecting an adjustable amplitude and phase 50 Hz signal. It might be cheaper to do that until you can do your rewiring. On the other hand, if you thrash around a lot while recording, the amplitude and phase might not stay constant!
Another thought occurred to me. Could you mount another magnetic pickup on the guitar in the same orientation, but not under the strings? It should pick up exactly the same fields as the first, for fields originating more than a few feet away. Subtract its output from that of the first. Then it wouldn't matter if you moved around while recording. I don't know what these things cost, but it might be cheaper than a construction project.
I have noticed that with my guitar when I play I can reduce the noise to a minimum depending on the orientation of the guitar. It would depend on if the noise is omnidirectional or not but it might work.
With the faraday cage you might try some copper mesh and see if that works. It should be relatively cheap to build. I seen them do it on Myth Busters. I'm not sure if it will get the low freqencies though.
Another thing to do is to insulate the guitar's electronic cavities using copper foil or a special paint. These will reduce the hum.
Besides those things you probalby know you can buy noise reducers, power conditioners, etc...
You mgiht try to replace your AC with a HF AC then convert back down using a transformer at the wall outlets. Using about 20khz AC then convert down at the outlet should help. I'm not sure how big the transformer would be though and how efficient it would be. It might not need converting though if the devices hooked up to it use only DC(but convert from AC). The problem here, I think, is when the devices use the frequency of the AC to determine "time" or for some type of modulation.
You might even be able to do this without replacing the current power sources. Just turn off the breaker to them then run you an extension cord but have this HF AC run through it. If you are drawing more than 15 amps though you would need to get a higher powered connection. (maybe from the
230V washer/dryer connection or something similar). All you would need is two transformers that can convert to the 120V mains and some type of frequency converter.
With transformers thrown in to convert voltage if needed.
If your interested in doing this it might be a quick fix. You would need to get the specifics though and might cost a few hundred dollars. You could then insulate the cabling and conversion devices so they reduce the noise even more. You would need to ask about the specifics of freq conversion and such to get that info. There might be a good way to do this efficiently and cheaply.
Any thickness is a solid electrostatic shield. Aluminum foil would work, too... just staple the seams fairly well; if you use copper, you can solder, which is better at RF. Either will kill the annoying harmonics of 60 Hz which are capacitively induced from power lines and fluorescent lamps and such. Thin metal used like this is a moderate RF shield and useless for magnetic fields, which aren't usually the problem here.
Use conduit, either rigid or the much easier MX (flex) stuff.
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