Some results:

• Figure 1 shows and overview of microphones and accelerometers on the LB and EIB benches during the test period. A significant reduction of vibration noise on the LB occurs in correspondence of the switch off of the NEOVAN head chiller at 13:09. There is also a correspondent reduction of acoustic noise on LB. No effect is seen on the nearby EIB.
• Figure 2 is the same but up to 10kHz, showing that both the NEOVAN chiller and the HVAC also contribute noise up to some kHz. At 14:50 about the Neovan chiller has been turned back on (as confirmed by the INJ team).
• Figure 3 zooms on the region around 50Hz which shows a noise associated to the Neovan electronics chiller (on-off sequence 13:39-13:52)
• Figure 4 zooms on a drifting vibro-acoustic noise line starting at approx  500Hz, which never disappeared during the test. Its frequency drift visually correlates, with some delay, with the temperature inside the Lab and the EIB (Figure 5).
• Figure 6 is the same set of spectrograms in the low frequency (below 5Hz): acoustic spectra are "polluted" by some burst noise which might also be associated to people who was working on the platform at that time (nobody was entering the Lab instead) 
• Follows a set of spectra comparing the noise associated to single devices which were swithed off:
  • HVAC noise - Figure 7 (the off time corrsponds to also the CEB HVAC off): we remind that during this test the Towers HVAC in CEB was on and its noise was sensed inside the laser lab.
  • Neovan head chiller (Termotec) - Figure 8: the noise contribution is evident between 100 Hz and  1kHz. It could be the noise noticed in https://logbook.virgo-gw.eu/virgo/?r=68639.
  • Neovan electronics chiller - Figure 9: it contributes with a line at about 47 Hz (also in Figure 3). This noise is sensed louder by NN geophones on the EEroom floor (look at the red colored spectra which are in units of acceleration - m/s2). 
  • Main chiller (OMI) - Figure 10 - no noise evidence from this chiller.

One of the performed tests consisted in closing the air valves of the inlet and outlet ducts while the HVAC was off. Acoustic modes of the ducts themself,  associated to air pressure waves resonating inside the ducts, would be affected changing their frequency or reducing. A similar test has been done for DET: https://logbook.virgo-gw.eu/virgo/?r=69058

The plots compare microphones inside the inj lab (and the horizontal accelerometer on LB) at two times with HVAC off: ducts open (blue) and ducts closed (purple). No significant change is observed.

The purple curve shows extra noise due to the fact that on May 19 the Tower HVAC was on (NI open). The blue line in Figures 2 and 3 was taken on April 14 2025 when the ambient noise outside the lab was much quieter (also the CR HVAC responsible a 18.5 Hz line was turned off at that time). 

Figure 3 zooms in the bumps region. Acoustic bumps peaks at 12.07 Hz and 18.20 Hz. The seismic bump present in the laser bench peaks at 18.06 Hz.

In 2021we did tests of mechanically stimulating the LB and measured a mechanical mode of the bench around 17Hz: https://logbook.virgo-gw.eu/virgo/?r=50496. This seems not present now... did it move to 18.06? Could be tested by tapping the bench.