Why delay speakers need delay
Sound from the main speakers travels at about 343 m/s, while the signal to a delay or fill speaker arrives almost instantly. Without delay, people near the fill speaker hear it first and then an echo from the mains. Delaying the fill speaker lines the two up.
Delay (ms) = distance (m) ÷ speed of sound (m/s) × 1000
At 20 °C that is about 2.9 ms per metre (0.89 ms per foot). Example: a fill speaker 10 m in front of the mains needs about 29.1 ms.
Add a little extra so the sound stays at the stage
Thanks to the precedence (Haas) effect, our ears place a sound where it arrives from first. Adding 5–15 ms on top of the calculated delay makes the audience hear the sound as coming from the main speakers, while the fill speaker still adds level and clarity. The tick box adds 10 ms. Keep the fill speaker from being much louder than the mains at the listening position, or the effect breaks down.
Measuring the distance
For the most accurate result, measure from the main speaker to the listening area the fill speaker covers, and subtract the distance from the fill speaker to the same spot. When the fill speaker is close to its listeners and in line with the mains, the distance between the two speakers is a good shortcut.
Common questions
How many milliseconds per metre?
About 2.9 ms per metre at 20 °C, or 0.89 ms per foot.
Does temperature really matter?
A little. The speed of sound rises about 0.6 m/s per °C. Over 30 m, the difference between 0 °C and 20 °C is about 3 ms, which matters for outdoor events and large halls.
Why does the calculator show samples?
Some DSPs set delay in samples instead of milliseconds. At 48 kHz, 1 ms is 48 samples.
Should I delay ceiling speakers in a meeting room?
Usually not. In small rooms the distances are only a few metres, so the delay is a few milliseconds. It matters most in long rooms, auditoriums and spaces with under-balcony or zone speakers.