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We always thought that the daily tractor-noise and the noise seen after an earthquake are caused by oscillating baffles, but so far no 'arches' have been seen in the spectrogram to confirm this idea. Last Wednesday, Paolo and Matteo discovered a very nice example of such an event, in which the bump stayed stable around 70 Hz for half a minute, see fig 1. Fig 2 shows a zoom with high temporal resolution, in which no arches can be seen, but there is something periodic. Calculating a BRMS signal in the band 60-80 Hz, however, one can see pairs of bumps, see fig 3. With some imagination, this resembles the pattern caused by an oscillating baffle, which can be easily simulated. Playing a bit with the parameters yields an amplitude of 10 micron and a frequency of 1.14 Hz. This frequency would correspond to an ideal pendulum with a length of 19 cm. Looking at the ring-down after the earthquake, the oscillator should thus have a Q of around 1000.

According to Julien, there is only 1 suspended baffle left, which is covering the cryotrap from the side of the SR tower. The guilty one can be seen in this image, and should have roughly the right pendulum frequency. (Note that there were similar baffles in the detection tower, which caused problems and were first displaced and later removed.)

If this is true, the trigger of a noise-bump should be a burst of seismic noise around 1.1 Hz in the central area. According to Irene, the most appropriate sensors to watch should be Em_SEBD04 and EmSEBD13. These are located near the beam-splitter and on the base of the detection tower, both measuring along the beam direction. No clear correlation can be observed, see fig 4.