Spectra estimated from structural impulse tests via fast Fourier transform (FFT) algorithms are susceptible to data acquisition related difficulties. The problem stems from vibration response signals which decay sufficiently slow that it is impractical to capture the waveform in its entirety. The problem is further compounded by the data acquisition and processing constraints imposed by FFT equipment. This paper reviews and investigates various aspects of FFT based signal processing in relation to commonly encountered situations in structural impact testing. The analysis is based on time and spectral relationships developed for a single degree-of-freedom (SDOF) system excited by a half sine force impulse. The truncation problem is investigated and the results show that provided at least six time constants of data are captured the error in the estimated frequency response function magnitude and phase is less than 0.5%. Next. the coloring effects of exponential windowing on spectra from an idealized impact test are examined. The results indicate that the windowing effects can be usually compensated for and accurate modal parameters extracted. Finally, the "double hit" phenomena is examined to illustrate the effects on the spectra and the implications with respect to modal analysis procedures.

by ASL