This paper is a report on a series of physical experiments made with a steel sphere vibrating while trapped between and impinging against two end plates, as in an impact damping device, while the plates are shaken by an electromagnetic shaker. Three materials were used for the end plates. It was found that, for the range of frequencies (0-60 Hz) and amplitudes (0-5 mm) investigated, the ball generally hits several times on one boundary before passing over to strike the other, per cycle of the shaker. Observed data are presented on these modes of vibration and on the duration of the contacts. Objective of the tests is to provide a datum for a mathematical model of a single impact. This is pursued further in a sequel paper  in which certain hypotheses are tested by simulation and one is found to match the character of these observed phenomena. © 1975 by ASME.