One of the topics in classical network theory is the reflection and transmission, or scattering formulation for lumped networks [33]. In this formulation, forces (voltages) and velocities (currents) are replaced by so-called wave variables
where is an arbitrary reference impedance. Since the above wave variables have dimensions of force, they are specifically force waves. The corresponding velocity waves are
Dropping the time argument, since it is always `(t)', we see that
and
These are the basic relations for traveling waves in an ideal medium such as an ideal vibrating string [297]. Replacing force by pressure, we obtain the traveling-wave relations for acoustic tubes [275]. Using voltage and current gives elementary transmission line theory.