A complex structure (a typical room in this case) considering the heterogeneousness in its geometry and construction materials is exposed to some kinds of EM pulses and the response of electric and magnetic fields is obtained by introducing the concept of transfer function for the structure as a linear system. To do this, the FDTD method is used to calculate the temporal response of electromagnetic fields inside the structure due to a Gaussian incident plane wave and then a frequency domain response is obtained via discrete Fourier transform. In the next step, we assign a transfer function to each point by dividing its frequency response by the frequency response of the Gaussian incident wave. This transfer function can be used to predict the response of the system to any other temporal waveforms with the frequency content near to the input's frequency content. It is shown that if the spectrum of the Gaussian input is extended to a specific frequency, the calculated transfer function can predict the response of the system -within a good estimation, to any other waveforms that their spectrum has no significant component beyond that frequency. Thus once we pay the cost of a long-time memory-consuming numerical calculations to obtain the exact solution of the field distribution inside the structure due to a proper input (Gaussian pulse), we can come up with a very simple and fast way to predict the answer to the other similar inputs.