Reducing the Virgo site infrastructure noise in preparation of the O4 observing run

Abstract

The heating, ventilation and air conditioning systems serving the experimental halls of the Virgo gravitational wave interferometer generate low-frequency noise - namely below 100 Hz - of seismic, acoustic, and electromagnetic origin. Such disturbances have repeatedly affected the interferometer sensitivity throughout its operational history, with particularly notable impacts during the third observing run. In preparation for the fourth run, a comprehensive investigation was carried out to identify the most critical noise sources within this infrastructure and to trace their transmission paths into the experimental areas. This manuscript presents the methodology and results of the noise characterization campaign, together with the design, implementation and assessment of targeted mitigation measures. The technical solutions adopted, along with the operational best practices developed, provide valuable guidance for the design of low-noise environments in future gravitational-wave observatories.

Figures (24)

• Figure 1: Acoustic spectrum of the NEB experimental hall. The blue and red curves correspond to the acoustic noise during the O3b run and O4b run, respectively. The black curve indicates the acoustic noise level when the HVAC system is switched-off.
• Figure 2: Seismic spectrum (vertical channel) of the NEB experimental hall floor. The blue and red curves correspond to the seismic noise during the O3b and O4b runs, respectively. The black curve shows the seismic noise level when the HVAC system is switched off. The vertical axis of the seismometer was used for the plot.
• Figure 3: Left. Plan view of North end building and the technical area which hosts HVAC devices. Right. Sketch drawing of an AHU.
• Figure 4: Left. 3D model of the supply (purple) and return (magenta) air ducts within the NEB experimental hall Lionel2023. Right. Plan views of NEB experimental hall showing the same air duct network (top - supply, bottom - return) and the technical area housing the AHU. Also indicated is the temporary installation of sensors inside the AHU room and on the air ducts: triangles represent microphones and rectangles represent accelerometers. The location of the permanent sensors in the experimental hall is also shown: one microphone (red triangle) and one triaxial velocimeter (blue circle). The tower platform is shown as a grey rectangle, the vacuum chamber and pipe are indicated in blue.
• Figure 5: Pictures of the light textile sleeves connecting the supply (left) and return (right) ducts to the AHU enclosure.