Prompted by me noticing on-off behaviors in the daily strain spectrogram for today at around 20.2 Hz, I've done some additional investigations into the source and behavior of this line:
The 20.2 Hz line, which is currently prominent in DARM, first appeared in accelerometer and microphone data from the corner station on June 9. The first appearance of this line that I found was in the PSL mics, as shown in this spectrogram. This line then appeared in DARM in the first post-vent locks a few days later. The summary of work from June 9 does not show anything obvious to me that would be the source of this new noise.
This feature also turns off and on multiple times during the day. An example from today can be seen in this spectrogram. Most corner station microphones and accelerometers exhibit this feature, but it is most pronounced visually in the PSL microphone spectrograms. I was unable to identify any other non-PEM channels that showed the same on-off behavior, but this does reveal many change points that should aid in tracking down the source. Almost every day, this line exhibits abrupt on-off features at different times of the day and for varying durations. Based on my initial review, these change points appear to be more likely during the local daytime (although not at any specific time). When the line first appeared, it was usually in the "off" state and then turned on for short periods. However, this has slowly changed, so that now the line is generally in the "on" state and turns off for brief periods.
Looking into past alogs, I noticed that I reported this same issue last summer in alog 79948. Additional discussion about this line can be found in the detchar-requests repository (requires authentication). In this case, the line appeared in late spring and disappeared in early autumn of 2024. No source was identified before the line disappeared.
Going back further, I also see the same feature appearing in late spring and disappearing in early autumn of 2023. The presence of the line is hence correlated with the outside temperature, likely related to some aspect of the air conditioning system that is only needed when it is (roughly) hotter outside than inside. This also means that we can expect this line to remain present in the data until autumn unless mitigation measures are taken.
I looked briefly into the 20 Hz Noise without much success. Comparing the floor accelerometers, the noise is louder in the EBAY than the LVEA (although the signal of the EBAY accelerometer doesn't look good since the vent). The next closest is HAM1 followed by BS. So the noise is around the -X-Y corner of the LVEA, likely in the EBAY, Transition Area or Optics Lab because HAM6 sees less motion than HAM1 and EBAY sees the most.
