Temporal aspects of rolling sounds: A smooth ball approaching the edge of a plate

We measured the sounds of smooth metal balls rolling over medium density fiberboard (MDF) plates. In the spectrograms of these sounds we observed gradually varying ripples. These ripples were more closely spaced for the sound generated in the middle of the plate than for the sound generated closer to the edge. Furthermore, the spacing for lower frequencies was somewhat closer than for higher frequencies. It is shown that this pattern arises from the interference between the sound directly generated at the point of contact between ball and plate, and the sound reflected at the edge of the plate. This effect was added to synthesized rolling sounds which resulted in a more natural sound. A discussion is presented concerning the perceptual relevance of this pattern.