J. Kissel

I've processed the data from LHO aLOG 38232, in which I was looking to see if the complex, high frequency dynamics found in the UIM (L1) stage of H1 SUSETMY (see LHO aLOG 31603).

Plots are attached; note that I've used the former 2016-11-17 data for ETMY. To guide the eye, I show the calibration group's dynamical model used during O2. Data shown with a coherence threshold of 0.75 or greater. I've not yet compared this against the unphysical, fitted update to the QUAD model that Brett developed for H1 SUS ETMY in CSWG aLOG 11197

It looks like 
- All suspensions see the "unknown" ~165 Hz resonance, though interestingly ITMY's mode appears to be split. ETMX and ITMY's are *strikingly* similar.
- only ETMY and ITMX show what Norna suspects is the "UIM blade bending mode" at ~110 Hz. ETMY's is indeed the ugliest.
- All but ETMX show evidence for resonances between 290 and 340 Hz, which are likely the (Suspension Point to Top Mass) wire violin mode fundamentals weakly excited by the UIM excitation. (see LHO aLOG 24917, T1300876)
- All suspensions show their (UIM to PUM) wire violin mode fundamentals between 410 and 470 Hz (see LHO aLOG 24917, T1300876)
- ETMY and ITMY's wire violin modes are *strikingly* similar; ITMX's modes are surprisingly low Q.
- We don't see the TOP to UIM wire violin modes likely because I didn't excite up to high enough a frequency; they're between ~500-510 Hz for ETMY (modeled at 495 Hz).


So now the question becomes -- what's the physical mechanism for this huge mystery resonance that's seen virtually identically in all QUADs? It seems quite a fundamental feature. Why is it split on ITMX?

It would be nice to have similarly detailed measurement of L1's QUADs, but all evidence from calibration measurements on L1's ETMY UIM stage point to this same feature (see the discrepancy between model and measurement for the UIM stage, e.g. in 2016-11-25_L1_SUSETMY_L1_actuation_stages.pdf LHO aLOG 29899). Further, it's *still there* before and after their Bounce/Roll mode Dampers (BRDs) have been installed (i.e. the same feature is seen in O1 and O2 CAL measurements; the above linked measurement was after BRD install).