This is pretty amazing to watch.
When the metronomes cannot connect to each other, they get out of synch. As soon as they can connect, or 'hear' each other they get in synch pretty much immediately. It's like a lesson in microcosm showing how musicians will always play together if they listen to each other, but can't if they're in their own world. I've noticed this phenomena sometimes when working with classical musicians. They'll be great players and can read anything, but when playing a piece together they sometimes just read their own part and don't try to hear where they are in relation to the ground pulse and in relation to where everybody else is. As a result they get out of sync pretty easily. Not all the time of course, but it's a phenomenon I've noticed before - reading the part in isolation and not relating it to everyone else's part. In symphony orchestras of course this is endemic - but that's a whole other story!
Duke Ellington: Denmark, 1970
15 hours ago