One hertz seismic attenuation for low frequency gravitational waves interferometers

This article describes a mechanical vertical attenuation system capable to provide large attenuation factors above 1 Hz. This system is derived from, and improves, the passive Geometric Anti-Spring seismic attenuation filters minimizing their vertical resonant frequency by means of a tunable electromagnetic spring mounted in parallel with the main spring. The tunable spring is also used to compensate thermal drift in the new arrangement. We found an unexplained deviation from the 1/f2 transfer function at resonant frequencies below ∼100 mHz.