Delayed fill speakers must be time-aligned electronically so their sound arrives no earlier than the main cluster's sound

In distributed or delay fill speaker systems, a loudspeaker positioned close to audience members it serves (e.g., under a balcony) may reproduce the audio slightly earlier in time than the main cluster’s sound (which takes longer to travel to those seats from the stage). If the fill speaker arrives first, the sound appears to come from the fill speaker rather than the stage — an unnatural ‘pre-echo’ effect that is jarring and fatiguing. Electronic delay applied to the fill speaker feed ensures that its reproduced sound arrives coincident with (or slightly after) the main cluster’s acoustic wavefront at the audience position. The Haas/precedence effect means that the listener will localize the source toward whichever sound arrives first even if the late-arriving source is a few dB louder; correct time-alignment maintains stage localization.

Examples

A balcony fill speaker is 60 feet from the stage; the main cluster is also 60 feet from stage. The fill speaker is only 15 feet above the rear seats. Without delay, fill sound arrives ~125 ms (15 ft / 1130 fps) before main cluster sound. Adding ~13 ms delay to the fill corrects this.

Assessment

A fill speaker under a balcony is 20 feet from the audience and the main cluster is 80 feet from the same audience. How much electronic delay should be added to the fill speaker, and in which direction? (Speed of sound ≈ 1130 ft/s)