The Grateful Dead part of that post reminded me about a yacht club dining room we played in about once a month for 20 years. The dining room was huge with high ceilings and big plate glass windows on 3 sides to take in the view of the water and the docks. We played opposite the middle pane, I'd guess about 50 feet away and up about 4 feet on a stage. You could clap your hands and hear a second clap bounce off the windows. I estimate about 1/10 of a second. It's at sea level, in air conditioning, but I don't have the exact measurements.
The first time we set up it sounded like two drummers playing with that old slap-back echo they used on 1950s recordings.
The best we could do was aim the speakers in an X pattern aiming them at the opposite corners of the room. That created a hot spot in the middle right behind the dance floor, so we had the club refrain from seating anyone who didn't like a little volume there. We played a dinner set for about an hour and then cranked it up to about 100dba on the dance floor which resolved to 85dba in the hot spot.
After a hurricane, the club got remodeled, and now we play in the lounge, which has much better acoustics.