Influences of Air Fluctuation and Mechanical Vibration on Magnetic Noise in a Magnetically-Shielded Room

摘要

??This paper describes the experimental investigation of relatively weak magnetic noise at low frequencies, which was caused by the fluctuation of air currents through ducts in a magnetically shielded room (MSR). The noises were measured by SQUID magnetometers in the MSR installed on an active microtremor isolation system. First, the paper clarifies that magnetic noise depended only on air fluctuation at frequencies less than 10 Hz, whereas on both air fluctuation and mechanical vibration via floor at frequencies from 10 to 30 Hz. Second, the major contributor to magnetic noise at low frequencies was identified by evaluating the coherence coefficients between magnetic noises, vibrations of the MSR, and sound pressure inside and outside the MSR. The deformations of the wall and those of the ceiling in directions perpendicular to their planes due to air-fluctuation mainly contributed to magnetic noises. Third, by measurement, we inferred that the peak frequencies of the sound pressure resulting in magnetic noise were generated by the air duct.