Similar tests for studying acoustic modes of the laser bench room were made back in 2024 but with the HVAC on and no much effect was seen:  https://logbook.virgo-gw.eu/virgo/?r=65126

The microphone added externally of the laser bench room is named ENV_CEB_LLR_MIC, the microphone insided the is named ENV_LLR_MIC.

We had a look to the effect of the sound wave pulses produced by repeated door opening and closing.

In the morning, while the HVAC was on, we performed a set of pulses with the door between the laser bench room and the minitower room. Figure 1 shows ASD of the microphones inside the laser bench room. They all see a bump at approximately 20Hz growing above the noise backround noise. We remind that the "LLR" microphone was positioned in the N-W corner of the room, at aproximately the beam height.

In the afternoon we repeated the same test, but the HVAC was off (also those of CEB) and we acted with the door between the Atrium and the laser bench. In Figure 2 (and Fig.3) the purple spectra correspond to the quiet condition. Two bumpy structures at approximately 12 Hz and 18.5 Hz are visible, in particular in the LLR microphone. In the excited condition (blue) microphones see again a peak at about 20Hz and also, less pronounced, growing bumps at roughly 40, 45, 55, 85,135 Hz (Figure 2).

Figure 4 (also 5 and 6) is a spectrogram of LLR mic over the time of this test. After the test is finished (about 13:13:30 UTC) additional pulses around 20Hz are seen (actually here the excited frequency is a bit lower, it looks more around 18.5Hz). These are due to Irene exiting the lab and opening/closing the doors of the atrium, the SAS and the piscina.

We would interpret these frequencies as acoustic modes of the room. We are also tempted to think that the 18.5 Hz bump seen in quiet condition is the same 20Hz mode excited by the door.  We have excluded already it to be a resonant mode of the air ducts ( https://logbook.virgo-gw.eu/virgo/?r=69148). The frequency mismatch (20Hz instead of 18.5Hz) however needs an explanation.

The attached shows the excitation produced by opening and closing the doors of the SAS and piscina. The excited bump looks well matched to the 18.50 Hz one. Some small excitation of the 12Hz bump is noticeable as well.

we report on additional results of the analysis:

About the acoustic bump at 19 Hz - we made two additional observations that support the hypothesis that is is an acousting mode of the INJ lab room (aka, laser lab bench room):

• Figure 1: while the atrium door was open,  the 19 Hz acoustic mode shifted to 20 Hz. The shift-up is a bit counter-intuitive. A similar observation was done when we in Sept 2024 we did the test of opening this door, but with the HVAC on (https://logbook.virgo-gw.eu/virgo/?r=65126). It seems that  this is the acoustic mode that we excided when when repetitively closed and opened opened the atrium door. It is interesting to check if this up-shifting behaviour is reproduced by the ANSYS simulation that Lorenzo is implementing.
• Figure 2: shows that this peak's amplitudes changes by a factor 2 at the location of the three microphones inside the INJ room. It is the larges largest in the NW corner of the room. This change of amplitude within the room is a characteristic of acoustic modes, which also could be cheched with a simulation. We can also notice that the amplitude of the 12 Hz instead is very similar in the 3 positions.  

About acoustic peaks associated to the INJ and DET HVAC fans: 24 Hz and 27 Hz

•  Figure 3 shows the switch off of "just" the INJ and DET HVAC. The two lines disappearing at 24 Hz and 27 Hz are associated to the frequency of the outlet fan of the INJ and DET HVAC, respectively. Interestingly, they have roughly similar amplitude inside the respective labs, e.g. of the order of 0.01 Pa/sqrtHz. But, the 27 Hz is heard clearly inside the CEB HALL (also by the ENV_CEB_LLR_MIC that we positioned close to the EER). Figure 4 shows the time of disappearing lines.

About the acoustic peak at 12 Hz - this peak is associated to the main hall, it could be an acoustic mode of the hall

• Now we look at how the noise further progressively reduces inside the INJ room and in the DET lab when switching off the HVAC of the CEB Clean Rooms and finally that of CEB main hall. The INJ and DET HVAC are already off.
  • Figure 5: switching off CR HVAC. No significant effect is seen in the main hall and inside the DET lab. Some reduction of noise below 10 Hz meaning that CR is actually the main source of noise below 10 Hz (after the INJ HVAC itself). Also loud (aroud 0.01 Pa/sqrtHz) is the 18.6 Hz line associated to the CR HVAC outlet fan.
  • Figure 6: switching off HALL HVAC. The noise reduces below 10Hz in the hall, in DET lab and in INJ Lab. Noticeably an acoustic peak at 12 Hz reduces significantly in all microphones (but it does not disappear). The microphone close to EEroom clearly show a set of well shaped peaks at roughly 2.5, 4.5, 7 Hz, whose amplitude reduced, but did not disapear. These might be also acoustic modes of the hall (?)
  • Figure 7: the 12 Hz peak is coherent between EDB, EIB and the Hall.

Sound transmission measurement (a rough look) - we injected white noise with one loudspeaker in DET terrace while all HVACs were off.

• Figure 8 - shows the injected noise. It overcome the quiet level starting from about 30 Hz.
• Figure 9 - compares ASD spectra during the injection of microphones inside and outside DET lab (left) and inside and outside INJ lab (right). Also shown is the coherence. Roughly we see that effective isolation occurs above 100 Hz. These data should be further analyzed, and possibly injections repeated adopting standard techniques, as done for example in  https://logbook.virgo-gw.eu/virgo/?r=67440.