Hjm. Terbrake et al., NEW RESULTS IN ACTIVE NOISE COMPENSATION FOR MAGNETICALLY SHIELDED ROOMS, Measurement science & technology, 4(12), 1993, pp. 1370-1375
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.