I find it interesting how this sign names the Portland Ordinance number. It is at once authoritative and completely rhetorical - an argument both from and with authority.
It is, when you think about it, a logical fallacy: Amplified music is prohibited because it is prohibited (argument in a circle).
Imagine a city in which signs attempt to convince you of the rightness and justness of their commands. Signs that make brilliant inductive leaps, reasonable inferences, and sound deductive arguments. How would this look? How would we behave?