We have many peaks below 40 Hz that couple, at least partly, through input beam jitter. Last summer Sam and Genevieve determined that a wide variety of site equipment produced these peaks, including the office area air handler, mini-splits in the CER, and chiller compressors for the main HVAC (LIGO-G2402140). The figure shows that there is coherence between DARM and the IMC WFS that might be used to clean these low frequency peaks, but that they are currently not being cleaned.
Eventually, we would rather not have peaks that need cleaning, but instead, reduce the source vibration and/or the vibration coupling to DARM. I think that the best plan is to reduce the source vibration of the largest peaks, but to mainly focus on reducing the coupling, because many of these peaks are just 2-5 times the vibration background at the coupling sites, so even eliminating the vibration of the sources will not be enough to get us to our design sensitivity.
The coupling of relatively low amplitude vibrations at low frequencies seems to be associated with coupling resonances. For example, when one of the frequencies of the office area air handler drifted into the 35Hz peak frequency of one of these coupling resonances, the peak in DARM was huge, but was greatly reduced by changing the operation frequency of the air handler (82986). Ill try to map out these low frequency coupling resonances during commissioning periods as a step in understanding their cause. But for now, it would be nice to see how much we can reduce the peaks with cleaning.
