Have you ever wondered what the sound waves of your sub-alignment look like?
Coupled, Equidistant, Aligned
If your main and sub are right next to each other and phase aligned when equidistant then you can send them the exact same signal and expect the audience to be in the coupling zone.
Here you can see a ripple tank example of two sources encountering an audience represented by a white box. No reflective boundaries have been enabled.
Uncoupled, Unequal, Aligned
If your main and sub get separated, as is often the case with flown mains and ground stacked subs, then they are still phase aligned somewhere, but maybe not where your audience lives.
In this example you can see a power alley (coupling) where arrival times are match along the isosceles triangle and power valleys (cancellation) where the triangle becomes obtuse.
Uncoupled, Equidistant, Aligned
You can equalize the displacement problem with a physical offset or digital delay. Notice how the isosceles triangle rotates down towards the audience. Edit this example here.
Uncoupled, Equidistant, Misaligned 120º
What if you’re not sure if your main and sub are aligned when they are equidistant? You might fix the displacement problem with delay, but unknowingly leave an alignment problem. Now part of the audience is in the cancellation zone.
In this example I have introduce a 120º phase offset into the bottom source.
Uncoupled, Equidistant, Misaligned 180º
In this worst case scenario, you have equalized the distance offset, but accidentally put your audience fully into the cancellation zone. This could happen with an accidental polarity inversion or simply with two speakers that are not designed to be phase compatible when they are equidistant.
The moral of the story is to know your gear. Don’t assume a successful alignment has been created by only equalizing the distance offset.