Fill speakers should aim only at uncovered zones to avoid spraying stray energy.

Evidence

Sometimes it is necessary to use more than one loudspeaker, for example to “fill in” a shaded under-balcony area, or cover a very wide area. In these situations the speakers should be as directive as possible so as to cover only the area required. This will minimize the amount of extra energy Figure 7. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2522