Hi everyone! This is my first post here at PF after years of on & off lurking! So, I'll get straight to it. At work (yes work, not HW) I have a forcing function signal that acts on a spring-mass system. When the spring-mass system "tracks" the forcing function, the curves overlap. However, as frequency is increased to a point, the amplitude of the response signal increases, and then loses "tracking". When it loses tracking, we see a signal that is the same frequency as the forcing function, but two things are evident: it is a composite of some combination of signals and there appears to be a beat phenomena. So anyway, I just purchased a book on vibration because I had sold mine in college for food. Before it gets here, maybe I could get some suggestions as to what I should look for? I am specifically looking to place a function to the response signal. I'll draw up a quick picture in non-other than MS Paint. *The black is the forcing function (simple sine wave) and the blue is the response. Top is tracking, bottom is loss of tracking* Thank you for your help!