As a physical example of how one might measure the energy spectral density of a signal, suppose V ( t ) represents the potential (in volts) of an electrical pulse propagating along a transmission line of impedance Z , and suppose the line is terminated with a matched resistor (so that all of the pulse energy is delivered to the resistor and none is reflected back). By Ohm's law, the power delivered to the resistor at time t is equal to V ( t ) 2 / Z , so the total energy is found by integrating V ( t ) 2 / Z with respect to time over the duration of the pulse. To find the value of the energy spectral density S ¯ ¯ x x ( f ) at frequency f , one could insert between the transmission line and the resistor a bandpass filter which passes only a narrow range of frequencies ( Δ Δ f , say) near the frequency of interest and then measure the total energy E ( f ) dissipated across the resistor. The value of the energy spectral density at f is then estimated to be E ( f ) / Δ Δ f . In this example, since the power V ( t ) 2 / Z has units of V Ω, the energy E ( f ) has units of V s Ω = J, and hence the estimate E ( f ) / Δ Δ f of the energy spectral density has units of J Hz, as required. In many situations, it is common to forget the step of dividing by Z so that the energy spectral density instead has units of V Hz.