I check the performance of the enclosure box for input optics. It neither improves the beat signal that much.
==Is the acoustic box good? No==
To check how good the acoustic shield can be, I measured the beat signal and feedback signal to ACAV AOM when the lid were on and off. There were no much improvement in both signals, see fig1 below.
fig1, beat signal and ACAV feedback, converted to frequency noise. Beat signals between the lid close and open (red, purple) are very similar. Feedback signal to AOM are also the same (blue, cyan). I plot the 4 traces together to see if there are any coincided peak, so I can know where it happens (beat path or input optics). Note the peak at 280 Hz in Cyan trace is not real, it pops up after ~50Avg. I could not find its origin yet.
==Is it really acoustic coupling? Yeah, kind of==
The results were so similar between the lid open and close, so I wondered if those were really acoustic. To test this, I turned off the two computer (PC and fb2) and remeasured the beat. Those computers' fans are quite loud when they are on. For fb2, the fans still work even when it is shut down, but definitely much quieter. The beat signal was improved a bit, see figure 2. The results were real, I repeated them twice. Note that the room are still not totally quiet with the two computers off, sounds from sun machine and electronic rack are still there, and they are as loud as the two computer and closer to the beat setup as well.
fig2: beat signals when the computer are on (blue) and off (red), several peaks are obviously reduced when the computers are off.
==discussion and plan==
Since the computer are sitting on the floor, it is not certain if the peaks due to the computers are from acoustic transferred through air or vibration transferred through the ground. But the peaks in question are at high frequency (almost 1kHz), and we have 3 stage seismic isolation on (except floating table). It is very likely that these peaks are caused by acoustic. To make sure that they are really acoustic, I'll float the table and repeat the measurement again.