Considering that the noise injection on IB tower is well visible on the accelerometers located on SIB1 suspension top stage, we took in occount the possibility that the ground shaking passes through the suspension and move the bench enough to produce the disturbance on RFC error signal. In support of this idea is the fact that a 101 Hz peak is visible when the SIB1 is shaked directly from the angular control actuators.

We injected a line at 101.1 Hz on the top stage actuators, in order to produce about the same suspension motion as when the tower is shaked (fig 1). The effect on RFC was visible, but 10 times lower (fig 2). Moreover, a certain excitation was visible on the accelerometer located on IB tower, meaning that the back action from the suspension actuators and the tower was not negligible.

The first conclusion is that the suspension is not the main path through which the tower shaking can produce an effect on RFC.

The second conclusion is that the transer function from top stage actuation to bench motion cannot be measured because the effect on RFC sensor is covered by a different path.

In fig 3 we see that during the tower shaking also an accelerometer located on EIB measures a motion. We should investigate the possibility that the effect on RFC is generated by this motion.