Flux is diverted, never stopped
A magnetic field cannot be absorbed. A ferromagnetic shield works because its permeability offers the flux an easier path than the air around the protected volume — the field is guided around the zone rather than eliminated.
This is why shield geometry, continuity and seam quality often matter more than raw material thickness. A 1 mm gap in the wrong place can undo half the attenuation of an otherwise perfect enclosure.
Two physical mechanisms
Ferromagnetic shunting uses high-permeability alloys such as mu-metal, permalloy or electrical steel. It is effective at very low frequencies, including static fields, and is the dominant mechanism below a few hundred hertz.
Induced-current shielding uses highly conductive materials such as aluminium or copper. Eddy currents in the plate create an opposing field. Its effect grows with frequency, which makes it attractive at 50/60 Hz and above.
Most real installations combine both: a conductive layer for the power frequency and a permeable layer for the low-frequency and static components.
What attenuation is realistic
A well-engineered five-sided room shield typically achieves 10 to 20 dB (a factor of 3 to 10). Closed mu-metal enclosures for instrumentation reach 40 to 60 dB. Claims beyond this in an open architectural context should be treated with scepticism.
The only reliable way to know before building is a 3D simulation calibrated against measurement of the real source.