Nonlinear energy sink (NES) study

I am always in the lab, and I sometimes watch and notice the NES starting oscillate on its own. Actually, it is because of the ambient vibration given my office is next to a large building ventilation fan which produces a lot noise and vibration. And, this noise and vibration is transmitted to my testing apparatus that causes the NES to oscillate.

The NES is mounted on a primary system which is mounted to the electromagnetic shaker. And for this instance, the primary is fixed to ground. The ambient noise and vibration is transmitted through the primary beams to the NES. On the NES, there are two piezoelectric energy harvester installed for energy conversion. From this ambient vibration, it sometimes generates 0.1 or 0.2 volts.