Optimal structure of magnetic and conductive layers of a magnetically shielded room

Although the method of optimal design of magnetically shielded rooms (MSRs) for dc magnetic noise seems to be well established, that for ac magnetic noise at a frequency range from 0.01 to 1 Hz is also required for measurements of extremely weak magnetic fields such as biomagnetic measurements. However, such a method cannot easily be established due to eddy current effects. In order to establish an optimal design method of an MSR for low frequency ac magnetic noise, we have investigated an optimal structure of the magnetic and conductive layers using three-dimensional (3-D) linear magnetic field analysis taking into account eddy currents. We show why a structure which sandwiches the conductive layer between double magnetic layers gives high shielding effect. Moreover, we found that the spacing between the layers does not affect the shielding effect for ac magnetic noise. This is useful knowledge from the standpoint of MSR production.