I have a pretty nice home recording studio. I'm happy with most of it. What I'm not happy with though is EMI/RFI (electromagnetic/radiofrequency interference).
Guitar pickups and microphones can be highly susceptible to this. Guitars in particular are problematic (even when humbuckers) because when run through heavy distortion, a "buzz" from EMI/RFI at -30-50 db will come up to -10-20 dB and become horribly audible.
It has become clear to me that a guitar pickup (even the best humbucker) is only as quiet as the electrical environment it is in. For the record, I am NOT talking about AC hum or poor grounding.
Lots of recording studios are built top to bottom as Faraday cages for this reason, but that is not practical. For this reason, I want to build a small Faraday Cage like this:
It's a pretty easy task. I just need to decide primarily: Which frequencies of EMI/RFI do I need to block to get audio clarity?
Ie. When I am hearing a buzz from EMI/RFI in a recording at 100 Hz or 5 kHz (audio frequency) - what frequencies EMI/RFI are most likely responsible?
Which higher frequency kHz, MHz or GHz range EMI/RFI inducing that lower frequency audio range buzz?
Ie. For audio purposes what range of EMI/RFI am I trying to block? Any and all of it?
Thanks for any theory or thought.
Guitar pickups and microphones can be highly susceptible to this. Guitars in particular are problematic (even when humbuckers) because when run through heavy distortion, a "buzz" from EMI/RFI at -30-50 db will come up to -10-20 dB and become horribly audible.
It has become clear to me that a guitar pickup (even the best humbucker) is only as quiet as the electrical environment it is in. For the record, I am NOT talking about AC hum or poor grounding.
Lots of recording studios are built top to bottom as Faraday cages for this reason, but that is not practical. For this reason, I want to build a small Faraday Cage like this:
It's a pretty easy task. I just need to decide primarily: Which frequencies of EMI/RFI do I need to block to get audio clarity?
Ie. When I am hearing a buzz from EMI/RFI in a recording at 100 Hz or 5 kHz (audio frequency) - what frequencies EMI/RFI are most likely responsible?
Which higher frequency kHz, MHz or GHz range EMI/RFI inducing that lower frequency audio range buzz?
Ie. For audio purposes what range of EMI/RFI am I trying to block? Any and all of it?
Thanks for any theory or thought.
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I think I'm just gonna get a prefab bed canopy made from Swiss Shield fabric. Should work well enough and very easy to handle and move with in a "studio" environment. If I get a queen size I can even put it over my drums if needed (not that I do - drums are loud enough to drown out the EMI at low preamp levels).
Bed Canopy: Cotton Fabric To Use In Shielding Sleeping Areas
Cool video showing it actually works here:
https://www.youtube.com/watch?v=nuy3hZJQfzw
Bed Canopy: Cotton Fabric To Use In Shielding Sleeping Areas
Cool video showing it actually works here:
https://www.youtube.com/watch?v=nuy3hZJQfzw
Hi, It seems to be a rather ambitious project, but then I don't know what sort of EMI you are experiencing. You could make the bed canopy out of metal window screen. Copper would probably be the best, but likely to be difficult to obtain. Aluminum would be the lightest in weight and probably good for most frequencies. I question the conductivity of common steel and galvanized steel mess though. The screen material comes in some pretty wide widths (up to about 3 meters) and a simple frame made of poles to hold it up. Be sure to cover the floor. A ground to an "earth" ground at a single point should do. Just be aware not to use anything with exposed or potentially exposed electricity inside the cage. 
Hi, It seems to be a rather ambitious project, but then I don't know what sort of EMI you are experiencing. You could make the bed canopy out of metal window screen. Copper would probably be the best, but likely to be difficult to obtain. Aluminum would be the lightest in weight and probably good for most frequencies. I question the conductivity of common steel and galvanized steel mess though. The screen material comes in some pretty wide widths (up to about 3 meters) and a simple frame made of poles to hold it up. Be sure to cover the floor. A ground to an "earth" ground at a single point should do. Just be aware not to use anything with exposed or potentially exposed electricity inside the cage.
Thanks. I ordered a 1 foot sample of the Swiss Shield cotton/copper/silver fabric they use for the prefab bed canopies I linked above.
Swiss Shield Naturelle - Swiss Shield Fabric
It's a very easy thing to test.
Just hook a humbucker up to an active preamp, then wrap the humbucker in the shield and see what happens. If the residual buzz and hum disappears it is successful at blocking the EMI. If not, it's not adequate.
I'm highly skeptical this stuff will work sufficiently, but if it does it will be much easier than to deal with a wire mesh solution.
The EMI is no different than any house I think. It's just guitar pickups are highly sensitive to this stuff. As soon as I wrap a pickup in copper foil it goes dead silent. Take it away with any amplification and it's BUZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ.
That stuff is intolerable in recordings. As I said, that's why most fancy million dollar recording studios are built as faraday cages from the start.
As for grounding, I have been reading that grounding a faraday cage is not necessary for it to work, as it will primarily be reflecting any signals that come to it, and grounding is more important for protecting against electric surges that might hit the cage like lightning (not relevant here).
grounded and not grounded Faraday cage - what's the difference? - Straight Dope Message Board
Is that not the case?
Either way, the Swiss Shield fabrics cannot be grounded as they are "Nonconductive" due to their primarily cotton component.
Again, we'll see when it arrives ...
Before embarking on an expensive and complex journey try to find the source. I have chased a buzz in a guitar amp design only to find that the buzz was coming from the shop light above the bench.
I am in the basement of a house, and slowly building a small recording studio. I own an RF spectrum analyzer and can "see" the EMI up to 2.7 GHz. We have known for years that fluorescent lights and incandescent lights with solid state dimmers can emit EMI. It is generated by the ballasts, and modern solid state ballasts are particularly nasty. I recently found out that the new LED light bulbs are even worse! I had 11 of them in the basement. The 4 foot LED shop lights that I got at Walmart radiate crud up to 5 or 6 MHz. The guitar amp would pick it up with nothing plugged into its inputs.
I found the light bulbs with an ordinary portable AM radio. There was nasty EMI coming from those things out beyond 2 MHz. It gets into the wiring of your guitar, and even directly into the amp. I could turn the little amp up full with nothing more than a guitar cord plugged in and hear the buzz. It disappeared when I shut off the lights.
Other sources include modern "energy saving" appliances. The controller for the blower motor in our furnace radiates a bit, so does the refrigerator and washing machine. All found with an AM radio held to the power cord. The WiFi router puts out some noise, visible with the spectrum analyzer around 40 MHz, but it doesn't affect my tube guitar amps. A solid state amp may pick it up.
There is still a repetitive clicking sound in the radio with the whole house power shut off, and we live out in rural West Virginia where there is very few RF signals, no cell phone coverage or TV signals......I'm starting to suspect leakage from the cable TV system.
All lighting is being converted to DC fed LED's with an EMI compliant commercial grade power supply. The $4 Chinese light bulbs must not be tested, or the EMI regs are being ignored.
I will probably include some wire mesh or screen in my ceiling treatment, but that's a long way off.....still building.
There is another reason to ground the cage. I used to work in the Motorola plant where we built two way radios and cell phones. We had over 50 Faraday cages that were big enough for two or three people to work in. Large copper screen booths about 10 foot square and 8 foot tall. If they were not grounded people would get shocked. A large ungrounded metal box will accumulate static electricity. It there is any electrical activity inside, the equipment should also be grounded to the same ground. One of my duties as a factory "Mr. Fixit" was to test the screen booths regularly for RF performance and safety.
I am in the basement of a house, and slowly building a small recording studio. I own an RF spectrum analyzer and can "see" the EMI up to 2.7 GHz. We have known for years that fluorescent lights and incandescent lights with solid state dimmers can emit EMI. It is generated by the ballasts, and modern solid state ballasts are particularly nasty. I recently found out that the new LED light bulbs are even worse! I had 11 of them in the basement. The 4 foot LED shop lights that I got at Walmart radiate crud up to 5 or 6 MHz. The guitar amp would pick it up with nothing plugged into its inputs.
I found the light bulbs with an ordinary portable AM radio. There was nasty EMI coming from those things out beyond 2 MHz. It gets into the wiring of your guitar, and even directly into the amp. I could turn the little amp up full with nothing more than a guitar cord plugged in and hear the buzz. It disappeared when I shut off the lights.
Other sources include modern "energy saving" appliances. The controller for the blower motor in our furnace radiates a bit, so does the refrigerator and washing machine. All found with an AM radio held to the power cord. The WiFi router puts out some noise, visible with the spectrum analyzer around 40 MHz, but it doesn't affect my tube guitar amps. A solid state amp may pick it up.
There is still a repetitive clicking sound in the radio with the whole house power shut off, and we live out in rural West Virginia where there is very few RF signals, no cell phone coverage or TV signals......I'm starting to suspect leakage from the cable TV system.
All lighting is being converted to DC fed LED's with an EMI compliant commercial grade power supply. The $4 Chinese light bulbs must not be tested, or the EMI regs are being ignored.
I will probably include some wire mesh or screen in my ceiling treatment, but that's a long way off.....still building.
There is another reason to ground the cage. I used to work in the Motorola plant where we built two way radios and cell phones. We had over 50 Faraday cages that were big enough for two or three people to work in. Large copper screen booths about 10 foot square and 8 foot tall. If they were not grounded people would get shocked. A large ungrounded metal box will accumulate static electricity. It there is any electrical activity inside, the equipment should also be grounded to the same ground. One of my duties as a factory "Mr. Fixit" was to test the screen booths regularly for RF performance and safety.
It's best to fix noise and interference problems at their source.
A legacy guitar can make a good noise probe.
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Jim Brown being an audio system EMI/RFI expert and a Ham radio operator has many papers on interference problems.
Audio Systems Group, Inc. Publications
A legacy guitar can make a good noise probe.
*****************************
Jim Brown being an audio system EMI/RFI expert and a Ham radio operator has many papers on interference problems.
Audio Systems Group, Inc. Publications
Where do you work?
.......I designed transmitters for a living and that dwarfs everything we had except the "indoor antenna range." After the shielding it was stuffed with RF absorbing foam for zero reflected RF. A "perfect dummy" holds a two way radio while standing on a turntable. There is an array of antennas focused on Dummy that record the RF radiation pattern.
It is a operating room-MRI suite. The doctors can get almost real time pictures when operating. This is one room of two, the MRI has it's own room but can be rolled into the OR, that is why the ceiling looks as it does, the MRI is suspended.
I did work for an aerospace company and was suppose to come up with a better EMI-RF filter for the ignitor circuit for a 2.75" rocket. Protect from 100 kHz to 20 GHz, EMP pulse and lightning strikes. Oh yeah, make it low mass, the filter ejected from the rocket dents the aircraft when it fires. Also cost is an issues so keep within $20-30.
In the end I did come up with something I thought would work and gave them the sample, was laid off a week later. Seems our satellite project was running late and they needed a person to cover for one of the engineers that would have gone to the rocket plant after the satellite work. Not like they told me that when I went for the posting. I did come back to the test lab (mechanical) over a year later on contract, asked the engineer about the circuit and if it worked.
I had nothing for test equipment and was going to rent a spectrum analyzer. I needed a RF field and rather than play around with antennas I decided to build a TEM (transverse electromagnetic) cell. For the others, a TEM cell is a piece of coax that expands in size of the work area and then reduces to a small size and is terminated at the far end. It sets up a even field in the work area. Because management did not know what I was getting into (the posting asked for someone with a strong sense of dc? I was into process control, RF was far out of my field.) I had no funds set aside for me. I ended up building a TEM cell out of cardboard which I covered with aluminum foil as the shield. The center conductor was a flat sheet of copper.
I never had the chance to use it but asked the engineer if the circuit and the cell worked. He said it did and the cell was good up to 10 GHz. It was nice to get conformation that my half year there was not wasted (did do other things), I did learn a lot about EMI testing though, forgot most of it by now.
I did work for an aerospace company and was suppose to come up with a better EMI-RF filter for the ignitor circuit for a 2.75" rocket. Protect from 100 kHz to 20 GHz, EMP pulse and lightning strikes. Oh yeah, make it low mass, the filter ejected from the rocket dents the aircraft when it fires. Also cost is an issues so keep within $20-30.
In the end I did come up with something I thought would work and gave them the sample, was laid off a week later. Seems our satellite project was running late and they needed a person to cover for one of the engineers that would have gone to the rocket plant after the satellite work. Not like they told me that when I went for the posting. I did come back to the test lab (mechanical) over a year later on contract, asked the engineer about the circuit and if it worked.
I had nothing for test equipment and was going to rent a spectrum analyzer. I needed a RF field and rather than play around with antennas I decided to build a TEM (transverse electromagnetic) cell. For the others, a TEM cell is a piece of coax that expands in size of the work area and then reduces to a small size and is terminated at the far end. It sets up a even field in the work area. Because management did not know what I was getting into (the posting asked for someone with a strong sense of dc? I was into process control, RF was far out of my field.) I had no funds set aside for me. I ended up building a TEM cell out of cardboard which I covered with aluminum foil as the shield. The center conductor was a flat sheet of copper.
I never had the chance to use it but asked the engineer if the circuit and the cell worked. He said it did and the cell was good up to 10 GHz. It was nice to get conformation that my half year there was not wasted (did do other things), I did learn a lot about EMI testing though, forgot most of it by now.
For guidance:
The largest hole you can have in a shield / chassis etc. and still maintain adequate sheilding is 1/6th of a wavelength at the highest frequency you want to shield against.
Example:
To sheild up to 2.4 GHz
Wavelength is 0.125 meters
Largest Hole = 0.125/6 = 0.02 meters, that is 20mm or 0.8 inches.
So things may not be as critical as you might think when you start to think about critical sheilding.
Printer2 - We all dodge RF stuff when we have a choice. For RF you need to be a good plumber in addition to your electronic engineering knowhow. The above was from my notes from a specialist Industry hosted EMI/EMC course. When I did my EE Degree we had a choice between RF Techniques and Control Theory for a final year subject. I went with the Control Theory.
Cheers,
Ian
The largest hole you can have in a shield / chassis etc. and still maintain adequate sheilding is 1/6th of a wavelength at the highest frequency you want to shield against.
Example:
To sheild up to 2.4 GHz
Wavelength is 0.125 meters
Largest Hole = 0.125/6 = 0.02 meters, that is 20mm or 0.8 inches.
So things may not be as critical as you might think when you start to think about critical sheilding.
Printer2 - We all dodge RF stuff when we have a choice. For RF you need to be a good plumber in addition to your electronic engineering knowhow. The above was from my notes from a specialist Industry hosted EMI/EMC course. When I did my EE Degree we had a choice between RF Techniques and Control Theory for a final year subject. I went with the Control Theory.
Cheers,
Ian
Two comments on that:gingertube said:
1. 1/6th of a wavelength seems very large; I would go for 1/20th wavelength or smaller.
2. This assumes a single hole. If you have an array of holes then they can add up, and you can get a diffraction grating effect so ideally holes should be smaller and randomly placed.
H.Ott states 1/10th and 1/20th wavelength at various points in his "Electromagnetic Compatibility Engineering" for screening hole effectiveness.
The screen in a door of our microwave cookers have holes about 2.5mm to 3.5mm
Assuming 1/20th wavelength, that gives us protection upto ~5GHz
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IME most of the guitar pickup is lowish frequency, think DC to a few hundred KHz or so.
Now the thing is, quite a bit of it is electric field which is easy to screen (In fact the guitar cavities should really be already screened with copper foil to avoid E field pickup), but the rest is magnetic.
The magnetic pickup is a problem because for a shielding enclosure to work the metal must be thicker then several skin depths at the interfering frequency, not a problem at RF, but something of a showstopper when the problem is 100Khz SMPSU noise from some shonky LED lighting supply.
I would be very surprised if that cloth did anything worthwhile.
Also note that any electrical cable entering or leaving the cage must be shielded and filtered at the boundary, no good having the axe inside and the amp outside...
73 Dan.
Now the thing is, quite a bit of it is electric field which is easy to screen (In fact the guitar cavities should really be already screened with copper foil to avoid E field pickup), but the rest is magnetic.
The magnetic pickup is a problem because for a shielding enclosure to work the metal must be thicker then several skin depths at the interfering frequency, not a problem at RF, but something of a showstopper when the problem is 100Khz SMPSU noise from some shonky LED lighting supply.
I would be very surprised if that cloth did anything worthwhile.
Also note that any electrical cable entering or leaving the cage must be shielded and filtered at the boundary, no good having the axe inside and the amp outside...
73 Dan.
Biggest screened room I worked in was the Nokia 4G testbed. half the ground floor of a building. The ESA evacuated antenna test range in spain was bigger but never went there. Better to find out you used non-space grade Aerolam honeycomb before it goes into orbit...
Antenna Test Facilities / Test centre / Space Engineering & Technology / Our Activities / ESA pretty pic of one of their other ranges.
Antenna Test Facilities / Test centre / Space Engineering & Technology / Our Activities / ESA pretty pic of one of their other ranges.
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