Repetitive shock (RS) and electrodynamic (ED) vibration systems produce substantially different vibration conditions at the input point to the test article. These differences are most evident in terms of peak G level and spectrum content. The RS system produces vibration in short bursts which contain extremely high G amplitudes at the leading edge of each air hammer impact. The frequency content of the RS spectrum is nonuniform and exhibits many "holes" in the test spectrum. The ED system produces a continuous vibration time history that contains peak G amplitudes that vary within a moderate, programmable range. The distribution of vibration energy over the test spectrum is uniform and easily programmed using accelerometer feedback (closed-loop) control.