[with many people help]
Since cryo duct shield has non black coated screws and it is faced along the optical axis, it can cause the scatterred light noise.
So, we had hammering test around cryo duct shields today.
We hit the vaccume duct near cryo duct shields and check the DARM signal.
During hitting the EYC cryo duct shields at arm side, we observed large excesses at around 20 Hz (and small excess at around 40, 60, 80 Hz).
Today's hammering test is just a demonstration and we didn't use any accelerometer, inpulse hammer, and so on.
So, we need to do precise hammering test later.
Note: we hit the duct shield at arm side of all cryostat and BS side of IXC and IYC, and obvious excess can be observed when EYC arm side was hit.