[Miyo, Hasegawa, Enomoto]

Yesterday Haino-san pointed out that there is a periodic noise around 40 Hz that appears every ~ 15 mins. Then I looked into the detail.

First attachment shows 2 hours long spectrograms of a microphone at center area (4599, 4871), and Michelson error signal. If you focus on ~ 45 Hz, you will find clear coincidence between them.

Then we looked at the spectra and coherences of the microphone and MICH error signal (and optical levers in center area), when ~ 45 Hz is quiet or loud.
See the plots below:

== Quiet ==

== Loud ==

When it is quiet, there is no coherence and there is no peak at 44 Hz.
In contrast, when it is loud, there is high coherence between Microphone signal and MICH error signal.
In addition, there are also coherences between MICH error and PR2/BS oplev signals, which might imply the coupling path from 44 Hz sound to MICH signal.