If law enforcement were to take a sound level reading for the sake of enforcing an ordinance, at what distance from the source would they take a measurement?
At whatever distance it is from the source of the sound to the complainant's dwelling. From the St Tammany Ordinances:
"(b)To determine the sound level, three readings will be taken at the complainant's dwelling or structure and the mean of these readings will determine the actual decibel count."
The ordinance has nothing to do with the noise level coming from their speaker. It has to do with the noise level entering your ears.
Municode is your friend.