My mum was a teacher and I so recognise the approach of the other teachers on here (yes, MB, I am talking about you).
My DCs get the "taken to one side, down to their level, look in their eye, this is a final warning in a 100% 'I mean business' low and direct voice". Sometimes I even scare myself.
I have only once done something more loudly when DS was arsing around in a coffee shop and non accidentally slopped a huge amount of his hot chocolate everywhere by doing something I had specifically told him for the last time not to do about 3 seconds earlier. I was very exasperated, particularly as he cried as a kind of way of preempting a telling off . I wasn't shouting per se but was very exasperated and cross. I could feel the eyes of the people in the next table and felt like a mean fishwife, but was absolutely buggered if I was going to say "there, there my little angel poppet - let mummy mop it up and buy you another one".