This afternoon we injected some mechanical noise to the laser bench by using the small shaker installed on the leg of the bench (see 47083). The mechanical effect was monitored by means of the accelerometer on the Laser bench, recently moved from vertical to horizontal (see 47078).
The noise injection parameters are reported below. In the attached plot the effect of the largest injected line at 20Hz, where there eems to be a clear resonance, is clrearly visible on both PRCL and DARM.

Analysis and projection of quiet condition will be performed shortly, but it seems likely that the bump in the quiet DARM at that frequency is correlated to the Laser bench resonance. If confirmed, mitigation actions will be studied.

Injection times:

quiet condition
GPS = 1254588112  08 Oct 2019 16:41:34 UTC  (08 Oct 2019 18:41:34 CEST)

small shaker injecting broad noise freq = 15-25Hz; amplitude: 0.0001
GPS = 1254588756  08 Oct 2019 16:52:18 UTC  (08 Oct 2019 18:52:18 CEST)

small shaker injecting a line freq= 20Hz 
GPS = 1254589862  08 Oct 2019 17:10:44 UTC  (08 Oct 2019 19:10:44 CEST)

small shaker injecting a line freq= 20Hz, amplitude x5
GPS_start = 1254590327  08 Oct 2019 17:18:29 UTC  (08 Oct 2019 19:18:29 CEST)
GPS_end = 1254590891  08 Oct 2019 17:27:53 UTC  (08 Oct 2019 19:27:53 CEST)