Mysterious vibrations

For the past few days I've been trying to figure out whether my lab setup is stable enough to carry out experiments. I've got a vibration isolated 800kg steel table, which measures about 3m long and 1.5m wide, sitting on a concrete block which is something like 7m long. And this whole thing is pumped up in pressurised air just to isolate it from the ground vibrations.

So effectively my experiment which will be sitting on the table and the concrete block will be completely isolated from any ground vibrations. But when I set up a scheme - an interferometer to check this, the table was very good but the concrete showed a small vibration. It's in the scale of micrometers and nanometers, but in optics that's a big issue. I turned off all AC's, exhaust fans and asked my colleagues to keep their voices down, but still I couldn't get rid of the confounded vibration.

It got my supervisor and the other students also thinking.. but finally we managed to find a work-around to it. The vibrations are still there, but I'm going to set up my experiment so that the vibrations will be compensated.

So that done, I'm sitting here blogging, waiting for my big mirror (1m diameter) to be lifted up and mounted for me. That should be done by today afternoon - hopefully!