Magnetic Noise Due to Sound of Footsteps on Wooden Free-Access Floor Outside a Magnetically Shielded Room for Biomagnetic and Nondestructive Measurements

In order to realize the ultimate low magnetic field in magnetically shielded rooms (MSRs) for nondestructive measurements and biomagnetic measurements such as magnetoencephalography and magnetocardiography, we investigated magnetic noises due to microtremors caused by sound pressure and mechanical vibration which arose from footsteps applied on the wooden free-access floor. Using superconducting quantum interference device (SQUID) magnetometers in this MSR, we measured the magnetic noises with and without operating an active microtremor isolation (ACMI) system that reduced microtremors transmitted from the concrete slab underneath the MSR. The microtremors of the MSR due to footsteps were divided into two components. One was caused by mechanical vibrations transmitted through the concrete slab at frequencies from 10 to 25 Hz. Its frequency characteristic depended on the mechanical resonance of the MSR. The other was caused by sound pressure transmitted directly from the free-access floor. This component occurred at frequencies from 25 to 50 Hz even with the operation of ACMI when footsteps were applied on the wooden free-access floor.