I'm with Captain swing and wee little drummer on this one, far too many precious people around these days who are fucking useless and demand people to be quiet while they perform something boring, crap, or just plain uninteresting. Why shouldn't you yawn if someone is boring? As long as you do it quietly it's OK by me. I've played in plenty of pubs and bars where I've not been listened to and frankly, not been expected to be listened to! you put up with it! If you are any good you transcend it!