The basic idea of a one-port network  is shown in Fig. 7.5. The one-port is a ``black box'' with a single pair of input/output terminals, referred to as a port. A force is applied at the terminals and a velocity ``flows'' in the direction shown. The admittance ``seen'' at the port is called the driving point admittance. Network theory is normally described in terms of circuit theory elements, in which case a voltage is applied at the terminals and a current flows as shown. However, in our context, mechanical elements are preferable.