In the case of a spring with stiffness , we have the impedance
which gives the reflectance
As before, we may eliminate by choosing to get
under the bilinear transform. So we have the digital reflectance
and corresponding difference equation
Again the delay-free path has been eliminated. The wave flow diagram is shown in Fig.F.3.
Thus, the WDF of a spring is simply a unit-sample delay, which is just the negative of the WDF mass. If we were to switch to velocity waves instead of force waves, both masses and springs would again correspond to unit-sample delays, but the spring would become inverting and the mass non-inverting.