The regular ~1nm variations in ETM coating thickness cast a diffraction ring onto beam tube baffles that may produce excess noise through modulation of retro-reflected light (https://dcc.ligo.org/T1300354). We are considering making an estimate of the noise produced by this effect by shaking the beam tube at one of the baffles with large enough amplitude to produce noise at the current LLO sensitivity (here). In making this noise prediction, it is important to know if the shaking moves more than the one baffle. To answer this, I measured the relative motion and phase of every baffle in a 550m stretch of the beam tube centered on the baffle by the shaker.
The relative motions of the neighboring baffles are shown in Figure 1. The relative motions can exceed 0.1 even for baffles a couple of hundred meters away from the shaker. The motion is also not symmetrical about the shaken baffle. I had a hard time believing this at first and returned to many of the baffle locations, remounting an accelerometer and re-starting the shaker, several times. The relative amplitudes were consistent, as were the phases, except for one early set of measurements at one location. Because the asymmetry suggests a location dependence, I think I will have to measure the motion of neighboring baffles when the experiment is done for real at LLO, a significant increase in the difficulty of the experiment.