Wh~n balls are dropped from a certain height onto a plane surface, a collision event with a characteristic time structure is produced. It starts with an initial sound burst. Then the balls jump back and forth with decreasing amplitude. During the final phase they roll on the surface until they stop. Beside other factors sound levels and time structure depend on the numbers of balls falling. This is shown for the instances of one and four balls in Figure 1: For more than one ball (a) the whole event takes longer, (b) the initial sound burst is louder, (c) the number of discriminable peaks in the whole event is enlarged, and (d) the intervals between successive collisions are shorter. Thus, there are physical factors, that provide the particular information about the numbers of balls dropping on a surface. In principle this information is available in the acoustic array.