Thanks, I see what you mean.However i am not sure if the issue was in the lack of air volume, or if the edges were creating internal reflections.
But I couldn't help but notice that new slots have quite jarred edges. Wouldn't it introduce micro-reflections/diffractions?
I guess the speed of the air is too low to consider effects of turbulent flow (diminishing effective opening).
I'm not savvy enough to know at which point "bad" edge starts to matter acoustically.