STUDY AND MEASURE ON AIR FLUCTUATION DUE TO MECHANICAL RESONANCE OF GLASS WOOL DUCT RESULTING IN WEAK MAGNETIC NOISE AT ULTRA LOW FREQUENCIES IN A MAGNETICALLY SHIELDED ROOM

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.