For generation of frequency response functions (FRF), there are two signals, the impulse (i.e. hammer) signal and the response signal. A force window is often applied to the impulse signal. This is just a shortened rectangular window - ones for the first part of the time region and zeros for the rest. This eliminates noise that wasn't part of the impact signal. An exponential window (just an exponential decay starting from 1 at t=0 and decaying to near zero at the end of the region). The idea here is to not throw away any of the data at the begining of the sample (as it probably has the most useful data) and to add some additional damping to the end of the signal. A rectangular window (i.e. no window at all) is also used.
