Method for Evaluating Shielding Factor of Magnetically Shielded Room for Uniform Magnetic Field Using Exciting Coil

To shield environmental magnetic noise, magnetically shielded rooms (MSRs) made of ferromagnetic materials are used for biomagnetic measurements. The shielding performances of MSRs are usually represented by the shielding factor, SF. The source of environmental noise is far away from the MSR, so that almost uniform magnetic field is applied to the MSR. Therefore, the shielding factor SF0 for uniform magnetic field needs to be evaluated. To evaluate SF at a construction site, magnetic noise is applied to the MSR by coils. However, the magnetic noise generated by coils is not uniform, and SFs depend on the sizes and positions of the coils and are not unique for the same MSR. Therefore, standardization of a method to evaluate SF0 accurately using coils is being discussed by the Japan Electronics and Information Technology Industries Association (JEITA). We discuss here a method for estimating the SF0s of MSRs by placing a smaller coil near the MSR under the condition with very low frequency, in which the eddy current effect can be neglected, using both 3D linear and nonlinear magnetostatic field analyses and measurements. A method to estimate SF0s by extrapolation using simple numerical expressions representing the relation between the SF and the size and the position of the coil is proposed in the 3D linear magnetostatic analysis. The proposed method is applied to an experiment with a single-layered MSR under ideal conditions. Namely, MSR is placed in a spacious building so that the experiment is not affected by the steel frames and the reinforcing bars of the building. Moreover, the door of MSR does not affect its SFs. In these conditions, we were able to accurately estimate SF0s in the experiment.