Magnetic shielding with aluminum

[martyh]

I have been digging around in the archives looking for some information on magnetic shielding with non ferrous materials. I saw varying opinions about its effectiveness. Looking for info on the internets in general I came across this paper

http://www.chomerics.com/products/do...ory_of_emi.pdf

Reading through it seems that aluminum can be an effective magnetic shield if it is thick enough. Playing around with the nomographs at the end it looks to me like .25” of aluminum is about as absorptive as .009” of Mu Metal (about 6dB of absorption losses at 60 Hz) the reflection losses look to be about 20 dB but I gather these are to be ignored if you are only worried about the magnetic portion of the wave.

Does anyone have practical experience or a better grasp of the math that confirms what I think I see?

Thanks,
Marty

[Speedskater]

The John Curl "Blowtorch" tread spent some time on the machined aluminum chassis about a year ago.

[tomtt]

Originally Posted by martyh

Does anyone have practical experience or a better grasp of the math that confirms what I think I see?

not much (none)

looks like your on the right track ....







blowtorch photo -





John Curl's Blowtorch preamplifier




transformers mounted on one piece chassis -




http://page11.auctions.yahoo.co.jp/jp/auction/n82058654

[Sheldon]

Originally Posted by martyh

Does anyone have practical experience or a better grasp of the math that confirms what I think I see?

No grasp of the math, but I can confirm that thick aluminum does provide some shielding. I've used a piece between the power transformer and output transformer in tube projects. Noticeable reduction in mains hum. In my case, thickness was about 8mm. As good as 5mm mild steel.

Sheldon

[pointy]

you can turn transformers through 90 degrees to each other which helps stop the fields of the transformers effecting each other.
otherwise the metal work of the chassis is used mainly to block out RF and airborne hash from the small component connecting wires and pcb copper tracks.
there are few other components that are effected by low level magnetic fields such as (j type) wire wound resistors and some say valves and caps.
with the valves all you normally need to do is space them well and caps can be put below and spaced 1"+ from each other to stop them from over heating each other.

[sawreyrw]

Aluminum will not provide magnetic shielding, since it's permeability is one; the same as air.

[martyh]

Originally Posted by sawreyrw

Aluminum will not provide magnetic shielding, since it's permeability is one; the same as air.

Well if I am following the paper I linked to it seems that it will just not as well as ferrous metals for a given thickness.

I have read others who say it will not work at all but I always had this nagging question. If highly conductive metals have no effect on a magnetic circuit why are they used as shorting rings in loudspeakers?

[Sheldon]

Originally Posted by sawreyrw

Aluminum will not provide magnetic shielding, since it's permeability is one; the same as air.

My personal experience is that it does work. The difference is clear with the plate between the transformers. It can be heard as the plate is moved into position, so it's not imaginary.

The material used was not pure aluminum. I believe it was T6.

Sheldon

[Bas Horneman]

Your personal experience would be correct. I did a google and found out that there is such a thing as lossy magnetic shielding.

Magnetic Shielding Solutions

"Lossy magnetic shielding depends on the eddy-current losses that occur within highly conductive materials (i.e., copper, aluminum, iron, steel, silicon-iron, etc.). When a conductive material is subjected to a time-varying (60 hertz) magnetic field, currents are induced within the material that flow in closed circular paths - perpendicular to the inducing field. According to Lenz's Law, these eddy-currents oppose the changes in the inducing field, so the magnetic fields produced by the circulating eddy- currents attempt to cancel the larger external inducing magnetic fields near the conductive surface, thereby generating a shielding effect."

I read somewhere else on the bottlehead forum that this was only about 5-6dB maximum (according to John Swenson)
http://www.bottlehead.com/smf/index....90.html#msg290


Nothing close to mu-metals but it still works a little.

[TheGimp]

If you are at the lower limits of hum, it might provide the extra little bit you need to knock it out.