Sistematically every night, at 00:09:13 LOCAL time (23:09:13 UTC), an infrasound noise appears in the CEB hall microphones (the standard ENV_CEB_MIC and the Hungarian ENV_CEB_MIC_HR).
The energy is located below 1Hz, and the "sound" lasts for about 2 seconds.
Looking at all the CEB microphones, the one that sees that kind of noise (but at a lower level) is ENV_LB_MIC
Looking at many days, the sound always occurs at the same time (error <1S)
An update: looking more carefully on more sensors (see attached plot), a sharp signal is visible also in many magnetometers, and in some air pressure sensors.
One one these (INF_CEB_PRES_OUT that is maybe non well calibrated? Or has an offset?) show a negative pressure (but it should read only between 0 and 100 Pa - to be investigated).
If we think this sensor has a (negative) offset, the normal working pressure could be about 30 Pa (very low indeed), and for a moment the machine stops, bringing the pressure at zero.
Every night at 00:09:12 Local time.
Saturday we positioned one accelerometer on top of the CEB HVAC fans container (ENV_CEB_HVAC_ACC_Y). Looking at last night data, at 00:09 LT we clearly see a pulse in the accelerometer syncronous with those in the infrasound and magnetic probes: Figure 1
Figure 2 shows an insight of the 2 second around the pulse: the 1s decrease of magnetic noise is typical of an electric load that switches off, and the magnetic spike is typical of a load that starts; correspondingly the IPS voltage increases (load off) and deeps (load on); the seismic noise associates to the restart as well as the air pressure deep (by the way, as reported in the channel database, this probe [INF_CEB_PRES_OUT] is Differential pressure probe in CEB outlet air duct).
So... here is the GHOST: Figure 3 !! ...or should it instead belive it is the CEB air conditioner ?!!
At this point we are quite convinced that the operation of this machine is regulated by a timer that restest at midnights, similarly to what found for NEB and WEB ... please find it!
Massimo and Davide ideed found the timer that was responsible of the daily 1s switch off of the CEB hall air conditioner. After an attempt to reprogram it which failed (the conditioner continued with the periodic switch off) today the timerhas been electrically bypassed.
