A question about resonance curve

[haisydinh]

Searching through Wikipedia (http://en.wikipedia.org/wiki/File:Resonance.PNG), I found this graph about resonance. I do notice from the graph that as the driving frequency gets closer to the natural frequency of the system, the peaks of the curves (i.e. the amplitudes) of the curves increase. However, I don’t understand what the symbol δ stands for in the graph. Also, what does it mean by transmissibility? And finally, how can we generate this graph (I mean from what data?)

Thanks in advance!

 

[maajdl]

δ is the damping factor.
When δ=0, the oscillations go on without attenuation.
When δ>0, the oscillation decrease in amplitude, for example because of friction in the system.
When δ<0, the system is unstable, due to an additional input of energy without external excitation.