Forty years ago in the UK some places were installing these db monitors - still see them about. They indicate like any db meter, a series of green lights then edging into yellow, then finally into red at which point power to the stage is automatically cut. Nice, eh?

We played a dump one night where just one table was occupied by a loud crowd of about ten people. They were sat under the meter's mic and were triggering well into the yellows. We asked them to move to another part of the hall. They did, and all was fine after that.