Loudspeakers and coverage
Under-Balcony Delay Speakers
Quick answer
A fill speaker 15 feet from a listener who is 90 feet from the mains needs about 79 milliseconds of delay: 75 feet of path difference at 0.889 ms per foot, plus a 12 ms Haas offset so the sound still appears to come from the platform rather than from the ceiling above your head.
Under a balcony is the worst seat in most sanctuaries. The main loudspeakers cannot see it, the ceiling is low, and the only sound arriving is reflected energy with the consonants already stripped out. A delay fill fixes it for a few hundred dollars, and the whole job comes down to one number: how many milliseconds late the fill should be.
Get that number wrong in one direction and the fill arrives first, so the sound appears to come from the ceiling above the listener and the preacher seems to be standing behind them. Get it wrong in the other direction and you have a distinct echo. Get it right and the fill is inaudible as a source while making every word clear. Put your own measurements into the delay speaker timing calculator and read on for what to do with the answer.
On this page
The delay calculation, in full
Sound travels at about 1,125 feet per second in dry air at 68 F, which is 0.889 milliseconds per foot. The delay you set is the extra distance the main system has to travel to reach that listener, converted to milliseconds, plus a deliberate offset of 10 to 15 ms.
That offset is the Haas effect, and it is the entire trick. Two arrivals within about 30 ms of each other are fused by the ear into one sound, and the ear localises it at whichever arrived first. By making the fill arrive slightly after the mains, you keep the image up front while the fill supplies the clarity.
A 75 foot path difference between the mains and the fill needs 79 ms of delay once the 12 ms Haas offset is added.
| Distance to mains | Distance to fill | Path difference | Delay to set |
|---|---|---|---|
| 60 ft | 10 ft | 50 ft | 56 ms |
| 75 ft | 12 ft | 63 ft | 68 ms |
| 90 ft | 15 ft | 75 ft | 79 ms |
| 100 ft | 18 ft | 82 ft | 85 ms |
| 110 ft | 20 ft | 90 ft | 92 ms |
| 120 ft | 25 ft | 95 ft | 96 ms |
Figures use 0.888 ms per foot at 68 F and a 12 ms Haas offset. Speed of sound rises with temperature, so a room that sits at 80 F on a summer morning shifts these numbers by roughly 1 percent, which is inaudible. Measure both distances from the same listening position, at ear height, with a laser measure.
Setting delay, level and equalisation
- Pick the design seat. Choose a seat about two thirds of the way back under the balcony, on the centre line. That seat is the one you time to. Seats nearer the front edge will be slightly early and seats at the back slightly late, and both are inside the fusion window.
- Measure both path lengths. From the design seat, measure to the acoustic centre of the main loudspeaker, which is the horn mouth rather than the cabinet corner, and to the fill speaker. Use a laser measure and record both to the nearest foot.
- Set the delay. Delay equals the path difference times 0.889 ms per foot, plus 10 to 15 ms. For a 75 foot difference that is 67 ms plus 12, so 79 ms. Enter it in the loudspeaker processor or the console output delay.
- Set the level 6 to 10 dB down. Turn the fill up until it is just supporting the mains, then back it off. A correctly set fill measures roughly 6 to 10 dB below the level the mains deliver to that seat. If it is loud enough to notice, it is too loud.
- Roll off the lows. High-pass the fill at 120 to 160 Hz. Low frequency from the mains already reaches under the balcony, since long wavelengths bend around obstacles, so low end from the fill only adds mud and comb filtering. The fill exists to supply consonants.
- Verify by walking. Walk from the open seating into the under-balcony area during speech. The change should be a gain in clarity with no change in where the voice appears to be coming from. If your head turns toward the ceiling, add 3 to 5 ms and try again.
How do you know you need a fill at all?
Stand under the balcony and look for the main loudspeaker. If the horn is not visible from the seat, the seat is in an acoustic shadow: everything above about 2 kHz is blocked, and those are the frequencies carrying speech intelligibility. That is the test, and it takes thirty seconds.
The other indicator is depth. An under-balcony area more than about twice as deep as its opening height is always a problem. A balcony overhang of 20 feet with 9 feet of clearance underneath is a tunnel, and no amount of level from the mains reaches the back of it cleanly. The same logic applies to transepts, cry rooms and side chapels, and to any overflow room audio and video feed.
If the balcony itself is occupied, it needs its own consideration. Balcony seating usually sees the mains directly and often sits closer to them than the back of the main floor, which is why balconies are frequently too loud rather than too quiet.
How many fills, and which box
Space fills so their coverage circles overlap at ear height. A small 4 to 6 inch ceiling or surface box with a 90 degree pattern mounted on a 9 foot ceiling covers about 10 feet of diameter at seated ear height, because coverage is measured from the ear plane at 4 feet, not from the floor. Using the floor instead over-states coverage by a third and is the single most common spacing mistake. That means a 30 foot wide under-balcony area wants three or four fills across, not one in the middle.
For a low ceiling, in-ceiling boxes such as JBL Professional Control 24CT 4-Inch Background/ForegroundCeiling Speaker, Black, Sold as Pair or JBL Professional JBL-Control 416C/T 6.5” 2-Way, Coaxial, 50 Watt, in-Ceiling Speaker with Backcan, 8 Ohm or 70V / 100V, Magnetic Grill disappear visually and are easy to run on a 70 volt line, which is worked through in how to wire a 70 volt system. Where the ceiling is open structure, a small surface box such as JBL Professional Control 25-1 Compact Indoor/Outdoor Background/Foreground Speaker, Black, Sold as Pair on a rated bracket is better because you can aim it. Spacing for either is worked out in the ceiling speaker spacing calculator.
Driving the fills: amplifier, processor and wiring
Delay has to live somewhere. A loudspeaker processor such as dbx DriveRack PA2 2x6 PA Complete Professional Loudspeaker Management System, Rack Mountable with Android and iOS Control. Black or Behringer Ultradrive Pro Dcx2496 Ultra High-Precision Digital 24-Bit/96 Khz Loudspeaker Management System gives you delay, high-pass and level on a dedicated output, which is the clean way to do it. A digital console can also delay an aux or matrix output, which is free if you already own one, but it ties the fill to a console setting that a volunteer can move by accident. On balance, put the delay in the processor and lock it.
Four to eight small fills on a 70 volt line tapped at 4 or 8 watts each is a total of 32 to 64 watts, which the 80 percent rule turns into a 100 watt amplifier channel from something like Crown CDi 1000 Two-Channel, 500-Watt @ 4Ω, 70V/140V Power Amplifier. Low impedance wiring works too for two or three boxes, but cable runs under a balcony are long: at 150 feet into an 8 ohm load you need 10 AWG to stay inside the 5 percent rule, which the speaker cable gauge calculator will confirm.
What goes wrong with delay systems
The most common failure is a fill with no delay at all, installed by somebody who wired it in parallel with the mains. It is louder than doing nothing and worse than doing nothing, because the image collapses to the ceiling and speech gets a hard doubling artefact. The second most common is a delay set to the exact path difference with no Haas offset, which puts both arrivals on top of each other and produces the same image collapse in a subtler form.
Third is level. Fills set by ear in an empty room are always too loud, because an empty room is more reverberant and a full room absorbs. Set the level low, then check it during a service with people in the seats. Fourth is bandwidth: a full-range fill under a balcony fights the low frequency already arriving from the mains and turns clarity into mud. High-pass it.
Once the fills are timed, tune the system as a whole rather than as parts, which is the subject of how to tune a church sound system.
Sources
- Speed of sound at 68 F and the Haas fusion window as implemented in src/lib/avmath.mjs
- Loudspeaker processor delay and high-pass ranges from manufacturer published specifications
Frequently asked questions
Why add extra delay instead of matching the path difference exactly?
Because matching exactly makes both arrivals simultaneous, and the ear then localises the sound at whichever source is closer, which is the fill above the listener. Adding 10 to 15 ms puts the main system first, so the ear places the voice at the platform and hears the fill only as added clarity. That is the Haas effect, and it is why a well set fill is invisible.
Does temperature change the delay time?
Slightly. The speed of sound rises with temperature, so a room at 80 F is about 1 percent faster than the same room at 68 F. On a 75 foot path difference that is under a millisecond, which is inaudible. Outdoor systems with long throws are a different case, because a 200 foot path difference across a 30 degree temperature swing does shift audibly.
Can I set the delay by ear?
You can get close. Play speech, stand at the design seat, and increase the delay until the voice snaps forward to the platform and the fill stops calling attention to itself. That point is usually within a few milliseconds of correct. Measuring both distances and calculating is faster and repeatable, which matters when the next volunteer has to understand what was done.
Should under-balcony fills carry low frequency?
No. High-pass them at 120 to 160 Hz. Long wavelengths already bend under the balcony from the main system, so low end from the fill arrives late, adds comb filtering and muddies the speech the fill was installed to clarify. Restricting the fill to the range above about 150 Hz is what makes it disappear as a source.
How loud should the fill be relative to the mains?
About 6 to 10 dB below the level the mains deliver to that seat, measured with a level meter during speech. Louder than that and listeners localise the ceiling. Quieter and it is not doing the job. Set it low in an empty room and check it again with a congregation present, because a full room absorbs noticeably more than an empty one.
Do I need a separate amplifier channel for the fills?
Yes, because the fills need their own delay, level and high-pass, and none of those can be applied if they share a channel with the mains. One amplifier channel serves all the fills on that ring, since they share the same delay time. A second ring, further back or in a different area, needs its own channel and its own delay figure.
Researched, not professional advice. This page is compiled from published manufacturer specifications, operator manuals, FCC rules, published standards and owner-review consensus, not hands-on testing. Sound system design, rigging loudspeakers overhead, and any electrical work are jobs for a qualified professional: have flown loudspeakers and their attachment points signed off by a structural engineer or a certified rigger, and have all wiring done by a licensed electrician to your local code. Wireless microphone rules change, so confirm the current FCC position before buying. As an Amazon Associate we earn from qualifying purchases.