Delayed fill loudspeakers should lead the acoustic signal by fifteen milliseconds.

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 that is fed into the reverberant field. Electronic delay of the signal will often be required in order to match the acoustic delay. When this is used, it should be set at about 15 ms greater than the acoustic delay. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2538

Categorization

• SHARE: A precise, nerdy number (15 ms) that only a live-sound or AV person would relish.
• QUESTION: Why 15 ms rather than matching the acoustic delay exactly — presumably the Haas effect’s fusion window; worth chasing in the precedence-effect literature.