Damping of the baffle has been shown to have successfully reduced noise in DARM from local vibration injections around the input beam tube (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=36979), but an improvement for site-wide vibration has not been shown until now.
While we have been down for some time following the Montana quake, there was enough data before the quake that I could look into indicators of site-wide coupling. The evidence here suggests that the baffle damping also reduced coupling of global 10-30 Hz vibrations and thus supports the hypothesis that the baffle was the dominant coupling location on site for this band.
The rush hour range drop was thought to be produced by scattering from the Swiss cheese baffle (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=35735), and it was thought that damping might reduce this nearly daily drop in range. Each of the panels in Figure 1 show the 10-30 Hz seismic band and the inspiral range. The rush hour traffic features are the broad ~gaussian peaks (from many cars) at the beginning and end of the work days. The panels from April and last December show that the range dropped during each rush hour (unless the range was already low), and that this has been happening for the entire run. The recent plot does not show similar range drops, suggesting that site-wide coupling in this band has been substantially reduced (as an aside, we do still often loose lock during the morning rush hour, even though the range doesn’t drop, and this bears further investigation).
To test for a global effect when the traffic noise was not elevated, I used Jenne’s jitter subtraction code and compared non-rush hour times before and after the vent. I took 1024 second stretches that were matched in original inspiral range and applied Jenne’s code. Figure 2 shows that the range with subtraction was, for each pair, higher after the vent. The difference was only a few percent but is significant (at p<0.05 ). Because the scattering noise is highly non-stationary, damping may also have improved stationarity (compare minute-scale range variation in Figure 1).
