In order to estimate whether we can see acoustic coupling in arms or not, we have to calibrate signals to phase noise.
I used the same method as Yuta and Jenne did (6834).
I switched from ETM locking to ITM locking since only ITM actuators are calibrated (5583), and measured the open loop transfer function and the transfer function from ITM excitation to POX/POY error signal. Then I can estimate the calibration value H [counts/m] from POY/POX error signal to displacement.
Yarm; H = 9.51 x 1011 counts/m
Xarm; H = 6.68 x 1011 counts/m
Phase noise in arms:
blue; Xarm, green; Yarm
I will calibrate the acoustic signal and see if it is reasonable that we can see the acoustic coupling signal in the arms.
But I guess it is difficult. Actually I have not seen coherence between ETM feedback signals and acoustic sounds yet. (I measured acoustic noise near POX and in PSL table.)
If I find that it is hopeless, I will create some sounds and try to measure transfer function from acoustic sound to arm cavity signals.
I am interested in how the transfer function calculated by wiener filtering is different from the measured transfer function.
I found that we do not have enough phase margin. This is why the arm locking is not so stable.