An effective combination of magnetic and conductive layers of a magnetically shielded room

As magnetic fields generated by the human brain and heart are extremely weak, a magnetically shielded room (MSR) must be installed to reduce magnetic noises. Especially in the case of biomagnetic measurements such as the MEG (magneto encephalogram) and the MCG (magnetocardiogram), of which the signals always exist in the frequency range less than several ten Hz, the following magnetic noises must be reduced by the MSR; noises at less than 1 Hz due to elevators, and noises at several ten Hz due to air conditioners, fans, electric lines and medical equipment. An analytical expression derived by Wills [1] and an approximate formula by Mager [2] have been used to predict the shielding effect of a MSR. Those formulas, however, cannot calculate the contributions of eddy currents and door construction. The popular MSR designed for biomagnetism is composed of a magnetic material such as permalloy (PC) and conductive materials such as aluminum and copper. The expected effects of those materials are to reduce magnetic noises at less than 1 Hz, at several ten Hz and at the radio frequency, respectively [3]. It is pointed out that the shielding effect at above a few Hz depends on the combination of magnetic and conductive layers [4]. In this paper, the 3-D magnetic field analysis is carried out by using the finite element method taking account of eddy currents [5] to estimate an effective combination of magnetic and conductive layers. The consequences of the analysis can be applied to the optimal design of a MSR in the frequency range from 0.01 Hz to several ten Hz. The numerical simulation is verified by comparing results computed with those measured in terms of the shielding factor and the phase shift of magnetic noises generated by the dc electric railcar nearby. The shielding factor of a MSR composed of only PC layers is also compared with that obtained from the formulas by Wills and Mager for the verification. 2 Methods of analysis and measurement