the standard sound level meter (SLM) will be weighted - A-weighting is typical, C-weighting is also common. You won't find unweighted SLM's in most cop cars.
each noise ordincance is specific to that municipality. Usually, they are responding to noise complaints, either residential or commercial property. Considering it takes time to get it out, turn it on, and you can't be in a moving car because wind noise will generate a false reading - i don't see them being necessary for mobile sound system citations. some cops will have fairly sophisticated SLM's for community noise, L90, L50, etc.
if they can hear you, they can usually cite you.
i've been driving around with a remote in hand, finger on pause, for years now - ever since they decided they can impound your car if the system is audible from 75ft. when i see a cop, i pause it. no sudden movements, just a press of the button. I haven't had a ticket since. basically, don't give them a reason.
I have no problem with enforcement of sound systems. it's annoying. showing off at a stop light is juvinile. no one thinks you're cool because your trunk can rattle more than someone else's. The only people who are impressed are other juviniles. I know, i used to be one of them. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
Now, there is no point in showing off. no one outside the vehicle can appreciate an accurate sound stage.