These ringdown measurements, of the LHO QUAD suspensions violin mode 4th and 5th harmonics, were obtained 2 days after the big Earthquake hitting LHO on the 20170706. For this analysis I used 17 hours of detector data in Low noise state from "20170708 06:30:00" UTC. None of the 4th and 5th harmonics were being actively damped or excited. Although clearly the Earthquake excited these harmonics well enough to get very nice ringdowns.
In this analysis, a line tracker (iWave) was applied over each of the identified 4th and 5th harmonic frequencies, locking onto them.
I give next the results for those frequencies that show ringdowns. I also attach 'png' plots which shows; in each column the mode monitored with the top plot being the frequency tracked as a function of time, the middle plot is the 'log(Amplitude)' (natural logarithm of the mode's amplitude). The red dashed lines are respectively the median of the tracked frequency and the fitted first order polynomial to the 'log' of the mode's ringdown. The plot at the bottom shows the Phase deviation respect to the linear fit.
4th Harmonic
Mode frequency (Hz) Q
1924.678 795280070
1924.919 940157702
1926.236 1013199693
1927.462 963907721
1929.327 590576596
1931.569 1022004204
1932.15 890355597
1932.331 985512132
1932.623 946775350
1941.352 838872691
1942.129 808459376
1942.178 1140538266
1942.384 806784836
1946.727 1153580301
1947.704 1078584113
1954.459 778605954
1956.503 788173599
1957.333 847485779
5th Harmonic
Mode frequency (Hz) Q
2373.487 431860210
2374.656 495634311
2381.893 719623590
2382.987 638328976
2385.441 691971670
2386.924 767976787
2398.523 791021530
2398.529 761223997
2399.509 843982379
2400.302 746568211
2404.642 860932811
2405.621 807844171
2406.763 630423384
2407.992 880751631
2409.266 617892329
2410.265 819561647
2411.371 789591091
2414.764 827720300
2415.259 860571672
2419.552 603552195
2424.416 575422358
2427.241 672293500
2428.109 789905044
These results show that exciting and measuring the Q of higher order harmonics is possible. If we are able to identify these higher order harmonics with specific fibres then this information could be used to enhance suspension thermal noise characterisation.
