This morning, we installed a microphone (ENV_LLR_MIC) between the two benches in the laser lab (Figure 1). In Figure 2 we compare this microphone with the two microphones on the LB and EIB benches. We noticed that the large bump structure around 20 Hz is not present in this microphone. We think about two possible explanations:
1) the LLR_MIC is in a peculiar position where there is a node for this noise;
2) the 20 Hz noise might be associated with resonance modes of the bench cover structures (plexiglass panels). We noticed that this structure is not present in the microphones inside the DET lab (EDB and EQB1), the bottom plot of Figure 2.
This morning, we moved the microphone ENV_LLR_MIC from the previous position between the two benches (Fig.1) to the left side entering the laser lab (Fig.2).
The bump structure around 20 Hz is now present in the microphone *LLR_MIC (Fig.3).
