"Splitting" of natural frequency
I am studying on a 3 DOF system and I can calculate its three natural frequencies and they match quite well with the experimental result. However, I found a phenomenon that there is “splitting” of the natural frequency (I assume there should be 3 sharp peaks on my frequency response curve but 4 peaks appeared). I wonder what the origin of the additional “natural frequency” is. Would you mind giving me some suggestions?
Thanks a lot.