At a very crowded P.F. Chang’s, people find themselves speaking loudly to be heard by the people next to them.  This starts a self-sustaining “noisiness” where everyone has no choice but to almost shout to be heard.  At odd moments, the conversation levels drop, and you can hear an awkward conversation:

“…and then I dropped the soap!  I was OUT OF THERE!” followed by laughter at his table…

Followed by a good 30 seconds of COMPLETE SILENCE from the room.