I think, we should not call a conversation we hear "noise", it disturbs our focus but it's not noise. Noise has no pattern and no meaning. But the most disturbing sound is the sound that repeats in fixed intervals. I believe repeating sounds in high volume is even used as a form of torture. So airplane noise is torture because it's loud and it repeats.
I find a conversation more distracting if it's relevant to me.
If we have to cram developers into open plan offices then each feature team should be dispersed throughout the building, so that the discussions each person overhears are less relevant to the person overhearing them.
