Return Ratio
“Listen up, main! Stick together. Here’s our mission: Go deep then gradually come closer for 20 degrees and then instantly go deep again and repeat.”
If you’re not convinced yet that this is mission impossible, then add range ratio until you surrender. A wider balcony front gives us more angle to work with, but with friends like this, who needs enemies?
Let’s make a single modification to the previous shape and do the exercise again. Slide the upper floor backwards so its front aligns with the lower floor’s rear.
What’s different? VBOT1 and VTOP2 are still both at 0 degrees, but they now have a 1:1 range ratio. There’s no longer a zig-zag in the middle. We would surely cover this with a single main.
It’s also not a balcony any more but it reveals the mechanism, the return ratio, the primary indicator for splitting the array.
Every inch we slide the upper floor forward increases the discontinuity between VBOT1 and VTOP2. Such sharp turns in coverage require angular isolation and we don’t have it. Return ratio (in dB) quantifies the level difference the balcony forces us to overcome.
We can saw a line of best fit through a shallow balcony with a small return ratio and keep the array together. Return ratios of 6 dB or more cannot be smoothed over (Figure 3).
Secondary Options For Upper/Lower Mains
There are still options short of breaking up. We can outsource coverage to others, specifically the underbalcony area, which can be covered by delays. The area covered by the delay is taken off the custody requirements of the main. VTOP2 moves closer, reducing the range ratio and opening up angle, a double bonus. We may be able to tough it out as a single main if the delays can bring the return ratio in bounds.
The height of the mains also plays a role. We’ve looked at them in the middle. Going upward reduces the angular spread between VBOT1 and VTOP2 (as if it wasn’t small enough already). Going higher leads to occultation (the blocking of the sight line to the loudspeaker) underneath, which reduces return ratio by coverage reduction.
Delays, however, have moved from optional to mandatory. Occultation seriously downgrades the underbalcony area and should not be considered fair trade for return ratio gains. Moving the mains under the balcony line ensure sightlines to the back and opens up the angular spread (the mains can see the underbalcony ceiling now).
Return ratio shows no improvement and balcony coverage will become more challenging. The upper level slope is flattening (from the mains POV). The upper level needs more severe coverage shaping due to reduced angle and rising range ratio. Such severe asymmetry is difficult with a single slope. Asking the mains to do that upstairs and on the floor is a very tall order, especially if it’s a tall balcony.