And crucially, the context of the shared space makes a large difference. You simply cannot compare 6ft. outdoors with lots of ventilation vs. indoors and forced air vs. indoors and no ventilation.

The choir practice should not be an example of how easy it is to spread; rather, it should be an example of the unreasonable intuitions people have about 'distance'.

Remaining in close contact, indoors, while singing, for a long period of time is not a good idea. It is quite reasonable to expect that to lead to transmission.

Similarly, two people passing by each other on a sidewalk, with nose breathing and mouths closed, even if it's closer than 6 ft., is a very small risk. Still good to maximize space, but nothing to get worked up about.