Re: Building a Faraday's cage ?

On Mon, 26 Jun 2006 15:25:41 -0700, Eric R Snow <etpm@xxxxxxxxxxx>
wrote:

On Mon, 26 Jun 2006 08:40:55 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Mon, 26 Jun 2006 15:08:58 +0200, "CS" <address@xxxxxxxxxx> wrote:

Hi :)

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:
http://img158.imageshack.us/img158/4522/dirtysilence7th.jpg

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.


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.

John
Assuming the noise is coming from the wiring in the walls, could the
noise come from fluorescent fixtures, or other noise generating
devices, connected to the same circuit but in another room? I know
little about electronics so this question is because of curiosity.

Yes. Fluorescents and triac dimmers generate harmonics of the line
frequency, and they not only couple better into sensitive electronics,
they are much more audible.

Also, do the guitar strings act as antennas and transfer whatever they
receive to the guitar pickup?

They could, if the pickup was poorly electrostatically shielded or not
grounded, but string-pickup capacitance is small so the effect will be
minor. One easy way to tell would be to touch the strings; if the hum
changes...

One gotcha is that preamps can rectify RF. So a nearby TV station can
produce a buzz that sounds like noise from the AC line, but it's
actually the field rate of the video (which is almost the same as line
frequency.) This is a problem in my house, a few miles from a big
antanna farm. RadioShack telephones and computer speakers are
especially bad about picking up RF. The strings could definitely be
involved here.


John


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