Some comments about the NEB and WEB AHU selective switch-off (#56630, #56617) are reported below.

• Figure 1, Figure 2 - The involved devices and their shut-down are indicated in the acoustic, seismic, and magnetic spectrograms for WEB and NEB, respectively. At first glance, it is observed a broad band reduction of the acoustic (ENV*MIC) noise level in both terminal buildings when the air handling unit (AHU) is switched off (Action 2). A vibration noise reduction is observed in the experimental room (*ENV*SEIS*) when the AHU is off but also when the water circuit is switched off (Action 3).
   
• Figure 3, Figure 4 - The two figures illustrate the comparison of the acoustic and seismic noise of two buildings when all devices are switched on and off. Most of the frequency lines in the seismic spectrum are due to harmonics of NEB and WEB belts at ~5.4 Hz and ~4.3 Hz, respectively. 
   
• Figure 5 - Looking at WEB environmental sensors in the frequency band ~(10-100) Hz, the shutdown of the water circuit produces a further vibration noise reduction not only in the (Guralp) seismic sensor of the experimental room but also in the accelerometers installed in the technical area for the monitoring of the AHU system. The interruption of water flow inside the water pipes is well visible in the ASDs of the accelerometers installed on the AHU case (*UTA_BOX_ACC), on the frame suspending the fan-motor group (*UTA*MOTOR_ACC), and on the floor (*UTA_FLOOR_ACC). There is a slight noise floor reduction in the microphone and magnetometers of the experimental room when the water pumps are off. The arrows point out a temporary disturbance in the frequency band ~(23-28) Hz.
   
• Figure 6 - The same effect is visible in the NEB environmental sensors. Looking at the acoustic spectrum of the SAS (*SAS_MIC) and the spectra of accelerometers installed along the supply and return air ducts (*SAS_SUP/ RET*  and *UTA_RET_W*), the noise floors seem to be reduced with the water pump switch-off.
   
• Figure 7, Figure 8 - Looking at some geophones (experimental room) and microphones of the Newtonian noise (NN) arrays, it is observed a slightly further reduction of the noise in the frequency band ~(10-80) Hz when also the water pumps are switched off (blue curve). Of course, this effect is more evident in the NN microphones deployed in the technical area, but it is observable in the SAS microphone and in the geophones on the building floor and tower floor. 
   
• Figure 9 -   Detailed view of the WEB water circuit shutdown: hot, cold water pumps and chillers. Magnetic glitch (ENV*MAG_N) in frequency band ~(20-80) Hz and glitch in the chiller current probe disappear when the two water pumps are off. The intermittent lines at ~48.2 Hz highlighted with red circles and arrows in the seismic and current probes are due to the switch on/off cycle of one of two chillers. The seismic frequency lines related to water pumps are identified by correlating the seismic noise with water pressure monitors of the hot (INF*HEATER_PRES_OUT) and cold (INF*CHILLER_PRES_IN) circuits. The line at ~46.7 Hz of the hot water pump (~47.3 Hz for NEB) is weaker than the line at ~47.6 Hz of the cold water pump (~48 Hz for NEB) but both are visible in the (Guralp) seismometer in the experimental room. Instead, only the line of cold water is well visible in the spectrogram of the microphone in the experimental room (same for NEB).

As reported in the ENV-INF technical report VIR-1314B-21, the water pipes of terminal HVAC systems were considered among the possible sources of vibration noise propagating through their wall clamps, acting as mechanical shortcuts. A set of mitigations actions was performed: the vibration isolation of the water pumps and damping; installation of springs suspending the water pipes; the opening of the wall around the water pipes to avoid any kind of contact, see Figure 1 and Figure 2.
Two switch-off tests are taken into account in order to estimate the before and after effects of these mitigation works: elog 49997 for WEB and elog 49565 for NEB. The  ASDs for the Guralp seismic sensors (3 directions) and microphone are attached for both terminal buildings. In each plot, the black and blue curves correspond to the noise when the water pumps and chillers are on and AHU is off before and after noise mitigation, respectively. The yellow and red curves correspond to the conditions where water pumps and chiller are off (also AHU off) before and after noise mitigations, respectively. 
The NEB plots 3, 4, 5 show a reduction of the seismic noise in the range ~(15-80)Hz which can be related to the noise mitigations described above. And for the acoustic noise, Figure 6, the reduction happens in the frequency range ~(30-100)Hz.
In the case of WEB, Figures 7-8-9-10, unfortunately, the switch-off before the mitigations was not satisfactory probably due to a strong wind. Therefore, we can not do the same assessment of noise reduction.