MISMATCHING WINDOWS CAN CAUSE FREQUENCY-RESPONSE FUNCTION NOTCHES TO APPEAR AS PEAKS

This paper examines the errors in frequency response function (FRF) es timators H-1(omega) and H-2(omega) caused by improper input and output signal windowing while using random excitation modal tests. These err ors are enhanced when the FRF has resonances and anti-resonances that are closely spaced. An experimental analysis is performed on a free-fr ee beam. The beam is excited with broad band random excitation over a frequency range covering the first five natural frequencies. The drivi ng point accelerance FRF is calculated from both estimators H-1(omega) and H-2(omega). The results show that erroneous FRFs can be obtained when the wrong type of window is employed and these errors are due to digital filter leakage. A theoretical model is proposed in order to ex plain the discrepancies that were found experimentally. Simulated resu lts from 2 dof system are presented that show the influence of differe nt types of windows on the FRFs. (C) 1996 Academic Press Limited.