Yeahbut© they're close enough that sinusoidal is a half decent first order approximation and it explains the phenomenon that rkay asked about.

If you want to get complex about it, the driving forces are two sine wave shaped forces of slightly different frequency and vastly different amplitude. Resistance and impedance are local variables but the whole thang could be modeled quite easily on an analog computer, at least to a second or third order approximation.