Looking at recent data when the INJ air conditioning was off (https://logbook.virgo-gw.eu/virgo/?r=68269) we noticed that the microphone on the laser bench measures some, not large but significant, excess acoustic noise with respect to the microphone in the room (_LLR_) and the microphone on EIB.

This noise shows up mostly between about 200Hz and 2 kHz. Most evident peaks are around 370Hz and 635Hz, which are very coherent with LB vibrations.  There seems to be an excess noise also around 30-80Hz, but this is less coherent with the LB accelerometer.

It seems to us an indication for some acoustic-vibration source positioned on the LB. Since the HVAC is off, the source is unlikely related to air turbulence underneath the LB cover.  One suspect might be the chiller water flow. To be investigated.

When the HVAC is on -- second figure -- this noise is less visible because partially covered, but two coherent bumps aroud 370Hz and 635Hz are still evident.

To be noted other two bumps present in all microphones, most evident when HVAC is off. They are  at about 12 Hz and 18.5 Hz. These might be acoustic modes of the room.