On February 15th, a significant acoustic noise reduction up to a factor 2 between 10-50 Hz is observed in the NEB hall until February 16th morning, Fig 1, Fig 2. Looking at the different signals, we were triggered by the sudden reduction of the overpressure in the experimental hall (INF_NEB_PRES), of the AHU machine air pressure (INF_NEB_PRES_OUT), and by a slight increase of the air flux velocity (INF_NEB_VEL_OUT) in the supply duct, Fig 3. We thought that such reduction of the overpressure could be due to an accidental opening of the door between the SAS area and the experimental hall.
Yesterday, we performed a test opening the same door for about 10 min and we reproduced the same effect, Fig 4. A possible explanation is that the outlet air pressure decreased because the impendence of the air circuit decreased. Consequently, we can suppose that the air turbulence in the ducts decreased and so did the acoustic noise.
On July 18 we repeated the test of opening the door between the NEB hall and the SAS. We did it also at WEB. Some small reduction of the acoustic noise inside the halls is noticeable, maily at WEB, in the range 10 to 60 Hz.
Figure 1 shows the mean-subtracted spectrogram of NEB microphone, Figure 2 is the same for WEB, and Figure 3 is the same for NEB on Fab 15th. The NEB sound reduction effect is much less on July 18 than on February 15th.
Figure 4 compares some signals of the test done at NEB, on Feb 15 and July 18. The meteo conditions are different: in particular the external temperature. The atmospheric pressure is not so different, at least for part of the test. On July 18 there is more wind. Also the machine operation parameters are a bit different: the NEB hall overpressure was 10Pa in July and 15Pa in February.
We notice, for example looking at Figure 1, that a small reduction of acoustic noise in the same range occurs daily in correspondence of high wind speeds. Figure 5 shows better this correspondence.
We also noticed that the probe INF_NEB_VEL_OUT (air velocity sensor on the supply duct of the NEB AHU right after the fan case) is not working properly: it measures an air speed value of around 1m/s which seems too small (Figure 5). The INF_NEB_VEL_OUT and INF_WEB_VEL_OUT sensors needs to be checked.
