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Balcony Battles: When To Split Upper & Lower Mains?

A look into main loudspeaker system trade-offs involved in dealing with balconies, accompanied by design examples...

Previously, I addressed the impact of breaking and tapering a line of loudspeakers (here). Now let’s turn our attention to balconies.

Everybody seems to love breaking main loudspeakers horizontally into left and right, but breaking them vertically into upper and lower? Not so much.

We can be repeat offenders when it comes to multi-main horizontal breaks, such as a parade route or racetracks.

Multi-mains can go vertical as well, although it’s unlikely that we’ll expand beyond two elements.

Vertical expansion is also driven by room shape, in this case a very specific room shape: the balcony. Let’s look into the trade-offs involved in battling the balcony.

We’re talking about left/right (L/R) mains. There’s no such thing as a lower center (that’s where the band is). Center mains solve balcony coverage with L/R side fills near the deck and lots of delays. The question for the left main is when to surrender to uncoupling. We all want the band to stay together, but sometimes the members need to go their separate ways (Figure 1).

Figure 1: Example applications that question whether or not we need to split the mains into upper and lower sections. The same main loudspeaker height and balcony shape appears in all three examples.

Single & Double Slopes
The typical listener plane is a slope rising with distance. We solve this shape with the asymmetric coupled point source. The simplest listener plane is a constant slope, a consistent rise over distance. We commonly encounter more complexity, with a steeper slope in the rear than front. The coupled point source can adapt to this shape by complementary asymmetry, even with very substantial differences in rate of rise.

Balconies add a second listener plane, which is where the trouble starts. We are now double-sloped, a shape that calls out for uncoupling. There are two primary strategies: treat the shape as a single complex slope (and stay coupled) or treat them as distinct slopes and solve them separately (uncoupled).

We can keep the main array coupled by plowing a line of best fit through the balcony details. The downsides are level variance (balcony front will be louder) and ripple variance (balcony front reflection). The upside is the extended frequency range of the coupling zone, the result of keeping all sources close. A deep balcony increases the level variance. A tall, reflective balcony front increases the ripple. That’s what we’re up against.

How deep is too deep? How do we know when the front will give us trouble? We’ll be able to wrap our heads around balcony acoustics once we see how acoustics wraps itself around balconies.

The double slope has four coverage target milestones: VTOP1 (vertical top), VBOT1 (vertical bottom), VTOP2 and VBOT2. Each has a unique angle and range relative to the mains, and each pair has a unique angular spread and range ratio relative to each other. It’s the inner pair’s relationship (VBOT1-VTOP2) that has the twist. Let’s plug in some numbers and see the results (Figure 2).

Figure 2: Example applications with double sloped vertical shapes. Multiple shallow balconies present a nearly symmetric (1:1) range ratio while the floor is highly asymmetric (>2:1). Coverage can be divided between the two slopes.

We start with a matched pair, two identical slopes, stacked directly on top of each other. Each has a 20-degree spread and 2:1 range ratio. Our loudspeaker is in the middle so it covers from VTOP1 (+20 degrees) to VBOT2 (-20 degrees), with a 2:1 range ratio (6 dB). In between are VBOT1 (+0 degrees) to VTOP2 (-0 degrees), which also have a 2:1 range ratio.

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We can solve a 6 dB range ratio spread over a 40-degree angular spread by aiming an 80-degree loudspeaker at the uppermost seat. How do we solve a 6 dB change that happens in a 0-degree spread? OK, it can’t be 0 degrees because the balcony has to be thick enough to hold people, but it can be very, very small.

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