Design, construction, and performance of a large-volume magnetic shield

A cubic magnetically shielded room has been constructed. The room consists of three concentric cubic shields, and the side length of the smallest shell is 2A5 m. It utilizes ferromagnetic and eddy current shielding, active noise compensation, and shaking to achieve a shielding factor of about 400 000 between 1 Hz and 100 Hz. At lower frequencies, the shielding drops gradually and is about 16 000 for a 0.1-Hz disturbance. The residual magnetic induction in the room is less than 5 nT, and the magnetic noise above 0.5 Hz is lower than that of the superconducting quantum interference device (SQUID) magnetometer used in the measurements. When the room was completed, its performance in shielding was better than that of any other room made before.