During yesterday maintenance we performed some acoustic injections in the NEB AHU room using one subwoofer loudspeaker.
At 9:11 UTC we switched off the AHU. The reference quiet time is between 9:12:30 and 9:17:30 UTC. Then we injected a 30s cycling sweep noise with frequency increasing linearly from 10 Hz to 100Hz. The good time of sweeps is between 9:19 and 9:22 UTC. After that euroterm people entered the AHU room and we stopped measuring.
Figure 1 is a spectrogram of the microphone inside the AHU room and the NOISE channel used for the injection.
Figure 2 shows the noise increase in some selected sensors. Seems that the acoustic noise in the hall reacted sensibly to this noise injection, in fact the excess acoustic noise in the hall is more or less of the same magnitude as that injected in the AHU room. A similar excess noise is seen by the SAS microphone and accelerometers on the ducts. It is possible that our injection excited different paths. We have to study better the data, also checking for coherences. We also measure a noticeable increase of AHU floor vibrations: this could be vibrations of the loudspeaker box. Next time we have to better seismically isolate the speaker.
We plan to complete our acoustic injection plan in another dedicated shift.