磁気シールドルームの磁気ノイズとなるグラスウールダクトの超低周波共鳴およびその対策に関する研究

摘要

1．はじめに床からの機械的振動に起因する磁気ノイズを低減するために，情報通信研究機構（NiCT）では，アクティブ除振装置（ACMI）が磁気シールドルーム（MSR）の基礎として用いられている．しかしながら，床から伝達される微振動がACMIにより低減されているにもかかわらず，MSR室内が空調された時，30Hz以下の微弱な磁気ノイズがSQUID磁束計により計測された．この磁気ノイズは，グラスウールダクトを通じてMSRに供給される空気のゆらぎ，音圧によるMSRの壁・天井の振動に起因することが，実験的な検討により明らかになった．%This paper describes the experimental investigation on the fluctuation of air currents due to mechanical resonance of ducts that results in relatively weak magnetic noise at ultra low frequencies measured by SQUID magnetometers, in a magnetically shielded room (MSR) for biomagnetic measurements such as magnetoencephalography (MEG) and magnetocardiography (MCG). First, the magnetic noise, vibration of wall and sound pressure in the duct and MSR were measured to investigate the relations between them. Second, the frequency dependences of sound pressure were measured at various points along the duct from the fan to MSR, and compared with those analyzed in order to confirm that the source of air fluctuation is mechanical resonance of glass wool duct. Lastly, as a measure against the mechanical resonance of glass wool duct, a side branch duct made of glass wool was connected to the main duct. The distribution of sound pressure along the duct, and the magnetic noise, vibration of wall and sound pressure in the MSR with and without the side branch duct were compared by analyzing and measurement.