Last night, I injected acoustic noise at POX table and AS table with oplev controls on (LPF is on).
1. acoustic noise at the POX table
I set the microphones and speakers at the POX table and see the acoustic coupling.
I could see slight change around 40 Hz. This can be caused by the oplev feedback loop because the speaker was on the same table as the ITMX oplev.
2. acoustic noise at the AS table
I controlled XARM with AS error signal and set the microphones and speaker on the AS table.
The resonance a 200 Hz seemed to be enhanced. But still we are not sure that it is caused by acoustic noise. Because this resonance is enhanced when the OL gain is high, and the gain adjustment was so critical that this resonance was easily enhanced even when the acoustic noise is not injected. And sometimes it has gone away.