Here is a spectrum of the OMC_REFL PD with 2 stages of whitening. Looking at the violin modes around 500Hz the signal is about a factor of 10 stronger (in Watts) in the OMC-DCPD_SUM compared to OMC_REFL_A.

Clearly visible are the 32Hz and 40Hz lines as well as 2 lines around 100Hz, see also alog 68078. Maybe these lines are added by the AM stabilization circuit for the 45MHz sidebands?

A lot of acoustic coherence with PSL accelerometers.

Some comments to the possible coupling mechanism:

• The RIN of the 32Hz signal in OMC-REFL_A is a few ppm. So, this would be a relatively small effect at the EOM.
• Since changing the modulation frequency doesn't change the location of the 32Hz line, this is not RFAM that is produced indpendent of the EOM.
• One coupling mechanism would be that the polarization is modulated by acoustic noise which in turn modulates the modulation index through the polarization dependence of the Pockels cell.
• Maybe more likely is that one of the electronics components forming the tuned circuit (coil?) are acoustically driven at the 45MHz electrode. This in turn may modulate the modulation index directly, or change the load impedance seen by the RFAM stabilization circuit. The control signal of the 45MHz RFAM stabilization circuit sees the 32Hz line indicating that it adjusts to the change of the load impedance and could be partially or fully responsible for what we see in the fluctuations of the modulation depth.
• The control signal of the 9MHz modulation does not see any acoustic lines.

The EOM mount was changend on Sep 6, 2022, see alog 64882. Looking at the pics one can see the coils just sitting there.