Acoustic noise inside the INJ room has changed significantly between the most recent switch off (July 28, https://logbook.virgo-gw.eu/virgo/?r=69458) and the previous which was in November (Nov. 27 2025: https://logbook.virgo-gw.eu/virgo/?r=68269). The changes are shown in the LAST plot attached comparing mics ASD. We tried to track these changes and found the following:

Reduced 20Hz acoustic bump:

On June 18th the RETURN fan frequency of INJ HVAC was changed from the usual 21 Hz to 14 Hz, and the acoustic bump around 20 Hz reduced significantly: Figure 1 and Figure 2. This is consistent with (but more pronounced then) what observed in our quick slow down in https://logbook.virgo-gw.eu/virgo/?r=67829   (return fan moved only down to 17Hz). We do not know why this reduction to 14 Hz was done, but the HVAC is running in this way since then. Good news is also that the environmental parameters seems not significantly affected (Figure 3).

Acoustic bumps 40-120 Hz changing level:

Examing VIM spectrogram back since Dec 2025, we noticed that in a number of occasions (see Table) some acoustic bumps in the INJ room mics increased or reduced. Bumps are roughly at 40, 60, 80 120 Hz. Figures 4 to 8 show some of these occurrences. The last increase happened on May 7th, and the noise condition is this since then. It looks that these occurences coincide with people entering and then leaving the INJ area (seismic activity).  Examining some occurences, we often find a change in the pressure of the INJ room, yet no change in the fans' frequency: see in Figure 9 and following. 

Excess noise around 490 Hz and above 2kHz: this has to be investigated .... :-)

A list of these occurences: