The Reduction of Bias Error in Transfer Function Estimates Using FFT-Based Analyzers

The susceptibility to bias error of two methods for computing transfer (frequency response) functions from spectra produced by FFT-based analyzers using random excitation has been investigated. Results from tests with an FFT analyzer on a single degree-of-freedom system set up on an analogue computer show good agreement with the theoretical predictions. It has been shown that, around resonance, the bias error in the transfer function estimate H₂ (S_yy/S_xy*) is considerably less than that in the more commonly used estimate, H₁ (S_xy/S_xx). The record length, and hence the testing time, required for a given accuracy is reduced by over 50 percent if the H₂ calculation procedure is used. The analysis has also shown that if shaker excitation is used on lightly damped structures with low modal mass, it is important to minimize the mass of the force gage and the moving element of the shaker.