On October 4th, we performed a set of acoustic noise injections inside the experimental hall of the North end building with the aim to measure the resonances of the room. As reported in the attached logfile, we injected white noise (20-180) Hz and then a 120s frequency sweep from 5 Hz to 150 Hz, both with the AHU off. Comparing the two kinds of injections, they have similar amplitude above 20 Hz, Figure 1. For the analysis, we focus on sweep injection.
In Figure 2, it is visible that some frequencies (red spots) were excited during the sweep in the microphone (top panel) and also in the seismometer (bottom panel). In Figure 3, we highlight what seems to be room resonances from ~5 Hz to ~120 Hz. It is worth mentioning that, during the sweep injection, we heard sounds emitted from the vibration of various objects that we could feel vibrating touching with the hands (e.g., metal cabinet, scaffolding).
Comparing the wind effect (yellow curve) on the building (elog #57191) with the sweep acoustic injection, we observe that many excited resonances match, Figure 4.
Looking at the NN acoustic array ( 3 microphones), we notice that the resonances of the hall (indicated by arrows) are slightly shifted, Figure 5. This depends on the NN microphone positions, Figure 6.