Application and correction of the exponential window for frequency response functions

Applying windows to experimental data is a common practice in modal testing to minimise the effects of leakage, and the exponential window is used for the transient signals measured with impact testing and burst random excitation. Used properly, the exponential can minimise leakage errors on lightly damped signals and can also improve the signal-to-noise ratio of heavily damped signals. The time constant of the exponential window is specified typically by the user, and this paper discusses guidelines for specifying the window for both types of response signal. The effect of the exponential window is to increase the apparent damping of the measured system, and the correction for this effect on the estimated modal parameters is developed by utilising the shift property of the Laplace transform. In addition, the transfer function of a half-period sine pulse, which is a representative model of an impact force signal, is studied to show the need for, and consequences of, applying the exponential window to both the force signal and response signals. Finally, several numerical simulation test cases of an sdof system are presented to demonstrate the issues discussed in preceding sections. (C) 1997 Academic Press Limited