There is some confusion within the klog about the frequencies of violin modes.  In this klog I use temperature difference of ETMX in 2023 to identify the ETMX modes.  The purpose is to set up notches for the ETMX IM drive to try to avoid excitation of the violin modes.  Also to set up active damping of the ETMX violin modes.  The ETMX modes that are identified are 173.15, 178.82, 180.60, 182.88, 184.74Hz.  Modes that may be ETMX modes are at frequencies 168.29 and ,174.25, 181.34 and 181.84Hz.  If there is data of long lock stretches where the ETMX temperature is changing looking for the associated change in violin mode frequency  like 32060 may be a more accurate way to identify violin modes. Same for other test masses if temperature changes occur.

The first attached figure shows a sample of spectral data that was used to identify ETMX modes.  The time periods used were:  ETMX 86K, others 250K - 22/5/2023 14:00 UTC. All ~80K - 22/1/2025 1:45:30UTC. All 250K - 11/12/2024 10:00:00 UTC.  All measurements have measurement BW 0.01Hz, 50 averages, 50% overlap.  The full list of mode frequencies is given in the second attached xls file.  Many ETMX modes do not have the same frequency Δ~0.1Hz now as 1.5 years ago. The average change in mode frequency between 250K and 80K is about 0.15Hz or ~1mHz/K.  There should be 8 violin modes, 2 for each fiber. The splitting into two fiber modes would result from fiber asymetry.  There are no clear doublets like observed at LIGO. If the mode frequencues identified here are indeed some of the 8 fiber modes I think the fiber must deviate from circular by ~1% to explain mode frequencies.

Previous klogs report ETMX violin mode frequencies (klog25876 and klog26113) and klog27845, klog32133(from  klog31977), klog32548.  The fact that there are 3 numbers in common (green) between this measure and previous measures makes me confident these modes have been positively identified.   Generally the previous techniques appears to be to drive broadband noise in L or P in the IM and look for increased DARM noise or the ring down (in DARM I think).  There are multiply identified peaks using the previous technique (marked reds) as reported previously.  I wonder if DARM is leaking back to the ETMs, resulting in excitation of ETM modes and misidentified modes?

ETMX

I now understand that the violin modes may be being excited by DAC glitches due to VIS model overruns. This may have been fixed with the model split. If the violin modes continue to be excited I have set up notches in FM5 of K1:VIS-ETMX_IM_DRIVEALIGN_L2P to determine if notching the L2P drive signals to the IM can reduce mode excitation.  The filter history is always on, has a few second step response and is ramped on over 10 sec. 
I have setup violin mode damping filters in K1:VIS-ETMX_NBDAMP_P4 and K1:VIS-ETMX_NBDAMP_P5. 184Hz is set up (damping gain -40phase +60).  The others will need some careful tuning before use.

Comparing previous violin mode identification.
ITMX

ITMY

ETMY