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Old 9th October 2012, 04:12 PM   #1
diyAudio Member
Join Date: Aug 2012
Default EMI mesh and its practical usage

Hi! I finally hit that point where I started designing the enclosure to my preamp that I am going to construct and wanted to cover all my bases to assure a problem free yet beautiful setup.

Being that the enclosure is partially wood, I want to counteract the surrounding radio noises in my house with obvious shielding.

-So the proper method is to use Mu foils... expensive and difficult to manipulate without damaging its structure (but works great!).
-Another lousier method is to use copper foil and do the same thing but with much much less magnetic absorption / deflection decibel capabilities?.

first question is: regardless of mu foil or copper foil, one must properly ground all sheets to earth correct?

Also, check this material out:
Stainless Steel Fabric - Electromagnetic shielding
my buddy has a bunch of this laying around and said I could use it. Expensive... but I can get some. These guys claim that this stainless lattice provides 70 dB
of Electromagnetic Shielding.
Do I also need to ground this mesh? or is it deflective and I need not worry... or is there no harm in grounding it regardless.

and a bonus question... it says here on this page "maternity
clothing and specifically in maternity dresses
and work aprons."
If you are wearing a stainless fabric dress... and its not grounded, will this not work? or is its EMI deflection that damn good.

Thanks ahead of time! I'm curious!
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Old 9th October 2012, 04:52 PM   #2
DF96 is offline DF96  England
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Join Date: May 2007
It says "up to 70db". It does not specify the frequency range over which it is particularly effective or ineffective. At low frequencies it will work mainly as an electrostatic screen so grounding will help. At higher frequencies it will work more as a lossy sheet of conductor so may still be OK without grounding.

Electrostatic screens need to be grounded as otherwise they can act as a capacitor plate and increase interference. One exception: if the screen has no holes so the circuit within it has no external connections then it need not be grounded.
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Old 9th October 2012, 04:55 PM   #3
jcx is offline jcx  United States
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mu metal is very specialized, rarely used - takes lots more cheap steel but you can afford lots more, fab it easily

cheap steel sheet (probably Galvanized) is available at many hardware stores, preforated sheet would be better for air circulation if heat removal is an issue

but steel is really only needed for magnetic field shielding so if you don't put your preamp on top of another component's power supply you may not need mag shielding

radiated EMI from cell phones, ect is easily blocked by any conductor - Al or Cu sheet stock (roof flashing) - even Galvanized metal screen material from the hardware store
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Old 9th October 2012, 06:04 PM   #4
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Join Date: Aug 2012
very interesting tidbits here! This is all good info.
I always had some good confusion with this subject when there is the mu crowd that argues that anything but mu metal provides no EMI protection and the more economic crowd says the simple grounded tesla cage will suffice for the most of everyones issues.
The mu crowd says that aluminum provides little to no EMI protection. Copper is better but in comparison to the mu shields, everything else is considered a waste of time.

Is this true? or is this an audiophile inflation.

In my mind, I just want to scrim out nearby general humming from surrounding equipment and radio station pickup. Will copper foil be good enough for this purpose? I can find copper foil super easily.
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Old 9th October 2012, 06:14 PM   #5
DF96 is offline DF96  England
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The confusion arises when people talk about EM and don't specify E or M or frequency.

Mu metal 'short circuits' magnetic fields. Any good conductor short circuits electric fields. At sufficiently high frequencies any good conductor will also oppose a magnetic field by generating surface currents which generate a cancelling magnetic field.

Decide what you want to do, then choose the appropriate type of screen.
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Old 9th October 2012, 06:37 PM   #6
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Join Date: Aug 2012
Thats true, the term does get lumped.
Knowing that all the above metals are great electrical conductors, the real danger to outside signal noise would be the pulsing (M) magnetic fields from nearby noisy components. Frequency wise, im not quite sure what I am looking for, all I know is that I would prefer a wider range of frequency coverage.

At what point would one need to select a desired frequency range when constructing shields for audio equipment?
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Old 9th October 2012, 09:32 PM   #7
DF96 is offline DF96  England
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A thin sheet of metal will keep out anything from low RF upwards. The exact point depends on the skin depth. At audio frequencies magnetic fields are harder to stop. That is why people usually don't try, but instead concentrate on reducing loop areas so the magnetic field has nothing much to link to.
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Old 10th October 2012, 04:41 AM   #8
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Makes sense about the level of tolerance. As for mitigating loop areas for magnetic fields, what exactly is loop areas, and how would one go about doing that reasonably?
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Old 10th October 2012, 10:59 AM   #9
DF96 is offline DF96  England
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Join Date: May 2007
All circuits consist of loops, as current has to return to where it came from. To take a simple common-emitter stage, the input loop starts at the source (wherever/whatever that is) then you have (in order) cable signal/live/inner, PCB track, coupling cap, PCB track, transistor base lead, transistor emitter lead, PCB track, emitter decoupling capacitor, PCB track, cable return/ground/outer, then finally back to source. If you trace a line around all those places you define a closed loop in space. The area of that loop tells you how much it will couple to magnetic fields. Note that the area contributed by the cable could be very small if it is coax or twisted pair - they use two different methods to reduce loop effective area.

You can reduce loop area by doing simple things like putting the input coupling cap near the emitter decoupling cap. You can even achieve some degree of cancellation by inverting part of the loop - this is how twisted pair wiring works.
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Old 10th October 2012, 12:54 PM   #10
marce is offline marce  United Kingdom
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Location: Blackburn, Lancs
A good source of info regarding electromagnetic compatability engineering, in fact probably the best:
home page
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