Above we derived how to handle the external force by direct physical reasoning. In this section, we'll derive it using a more general stepbystep procedure which can be applied systematically to more complicated situations.
Figure F.10 gives the physical picture of a free mass driven by an external force in one dimension. Figure F.11 shows the electrical equivalent circuit for this scenario in which the external force is represented by a voltage source emitting volts, and the mass is modeled by an inductor having the value Henrys.

The next step is to convert the voltages and currents in the electrical equivalent circuit to wave variables. Figure F.12 gives an intermediate equivalent circuit in which an infinitesimal transmission line section with real impedance has been inserted to facilitate the computation of the wavevariable reflectance, as we did in §F.1.1 to derive Eq. (F.1).

Figure F.13 depicts a next intermediate equivalent circuit in which the mass has been replaced by its reflectance (using `` '' to denote the continuoustime reflectance , as derived in §F.1.1). The infinitesimal transmissionline section is now represented by a ``resistor'' since, when the voltage source is initially ``switched on'', it only ``sees'' a real resistance having the value Ohms (the waveguide interface). After a short period of time determined by the reflectance of the mass,^{F.4} ``return waves'' from the mass result in an ultimately reactive impedance. This of course must be the case because the mass does not dissipate energy. Therefore, the ``resistor'' of Ohms is not a resistor in the usual sense since it does not convert potential energy (the voltage drop across it) into heat. Instead, it converts potential energy into propagating waves with 100% efficiency. Since all of this wave energy is ultimately reflected by the terminating element (mass, spring, or any combination of masses and springs), the net effect is a purely reactive impedance, as we know it must be.

To complete the wave digital model, we need to connect our wave digital mass to an ideal force source which asserts the value each sample time. Since an ideal force source has a zero internal impedance, we desire a parallel twoport junction which connects the impedances ( ) and ( ), as shown in Fig.F.14. From Eq. (F.18) we have that the common junction force is equal to
from which we conclude that
The outgoing waves are, by Eq. (F.19),
Since and for this model, the reflection coefficient seen on port 1 is . The transmission coefficient from port 1 is . In the opposite direction, the reflection coefficient on port 2 is , and the transmission coefficient from port 2 is . The final result, drawn in KellyLochbaum form (see §F.2.1), is diagrammed in Fig.F.15, as well as the result of some elementary simplifications. The final model is the same as in Fig.F.9, as it should be.