NEW RESULTS IN ACTIVE NOISE COMPENSATION FOR MAGNETICALLY SHIELDED ROOMS

New results on an active compensation technique for improving the perf ormance of a mu-metal magnetically shielded room are presented. This t echnique is based on measuring the magnetic field inside the room by a SQUID magnetometer. The output of this sensor is used to feed a curre nt through a coil surrounding the room, thus compensating the field de tected inside the room. After previous experiments on a single walled room, we applied the technique to a room with two mu-metal walls. At l ow frequencies a shielding improvement of more than 40 dB was obtained . Furthermore, the contribution of the intrinsic noise of the compensa tion 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 com pensation set-ups are described and experiments are presented and disc ussed.