Descending sounds are created similarly by shifting the partials downward. Over time, the partials' frequencies are shifted upwards. For a given bandwidth, the density determines the number of partials. We determined empirically the bounds of the parameters necessary to maintain the auditory illusion. Six partials and a density around 1.2 give satisfying results.
More partials actually make the sound richer and more intrusive.
We also found that the bell curve can be approximated with a triangle envelope with no perceivable loss in sound quality. This allows for a more efficient synthesis algorithm. We call it an elevator because it conveys the concept of vertical motion and the tone of the sound is not unlike an actual elevator. The bandwidth and density are fixed. Scrollbars are used in graphical user interfaces to allow users to view large documents through small windows.