Multi-layer shields improve shielding effectiveness
Figure 1 Metal sheets of shield materials
That non-ferrous layer can be copper or aluminum according to your own choice. Neither of these materials can be attracted by a magnet, but when incorporated into a multi-layer structure, there will be a magnetic effect.
Figure 2 Multi-layer shield
The inner, non-ferrous layer sandwiched between the sheet steel layers will develop a circulating eddy current within itself which will flow so as to oppose the magnetic fields that are not completely blocked by the steel sheet closest to the magnetic field source. Added to the magnetic field blocking action of the second steel layer on the opposite side from that source, the shielding effectiveness of this trio can exceed the effectiveness of using steel sheets alone at equivalent thickness.
Just to mention, I got involved with this topic back in 1992 at which time, my then employer was concerned about a then-recent patent that had been filed in 1990 by a competitor and granted in 1992 (Patent 5,130,697) which seemed to claim this effect as patented. I was tasked to look for prior art predating that patent.
I succeeded. I found a textbook at the Donnell branch of The New York Public Library in Manhattan which described these structures and which analyzed them in terns of transmission line theory. The math itself was way over my head, but the book's copyright date was 1971.Even today though, I still see patent presentations involving this technology which seem to claim it as an inventor's own and original work. I don't think so.
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