New results in active noise compensation for magnetically shielded rooms

New results on an active compensation technique for improving the performance of a mu -metal magnetically shielded room are presented. This technique is based on measuring the magnetic field inside the room by a SQUID magnetometer. The output of this sensor is used to feed a current through a coil surrounding the room, thus compensating the field detected inside the room. After previous experiments on a single walled room, we applied the technique to a room with two mu -metal walls. At low frequencies a shielding improvement of more than 40 dB was obtained. Furthermore, the contribution of the intrinsic noise of the compensation electronics to the noise present in the room was considered. Also the possibility of using a normal conducting coil outside the room as the sensor in the compensation loop was tested. In the paper, the compensation set-ups are described and experiments are presented and discussed.