Only once or twice (i did some crazy stuff. Found out about it going trough the older stuff at my blog
)
A guy that build a sh*t load of lab horns needed to cancel around 50Hz since his main thing was working with DJ's working on turn tables.
So a little sub was used to create a reduction at the 50Hz region which was just enough to get what the guy wanted......
Turns out there was 2 of those little guys (it's been some time ago so i forgot about the twins instead of 1)
The challenge on this kind of array is not the reduction at the back near field nor far field. It's finding out which of the 2 you need to not get in to trouble with local permits regarding SPL levels.
I've had a close encounter with the D&B cardioids. On that production my spot was 3 to 4mt away at the back from a whole bunch of them at a hardcore dance event a couple of years ago. After the 2nd DJ i had to get away from there because the pressure at the back of them was literally sickening......