Ensemble refers to the ability of the members of the performing group to hear each other during the performance, allowing an optimum sense of musical ensemble or the ability of the players to play together effectively. To achieve good ensemble there must not be strong reflections that are delayed longer than the duration of the fast notes in the music being performed. This puts a practical limit on the size of the stage for a variety of musical applications. The lateral distance between the nearly parallel sides of the stage should be less than about 25 m for use with a classical orchstra, one of the larger performing musical ensembles.
Another situation in which the sense of ensemble leads to problems is in the performance of a marching band, particularly on a football field, where the band is spread out over a large area. The time delays for the sound of instruments at one end of the band to reach the other end are large in comparison with the rhythmic pattern. This can be compensated only by having a single reference by which every player sets her timing. By watching a director and playing in time with her conducting, and not with the sounds of other instruments, the band can be kept synchronous. The director's hand signals arrive at all the members of the band essentially simultaneously, because the speed of light is extremely large. Someone in the stands may not hear the different instruments as synchronized, however, due to the varying distances.