Hoss, you got it right. AMA rules wouldn't count for beans in controlling community noise emissions. Local ordinances apply. In most ordinances the limits on sound emissions are specified as an averaged dBA limit (typically averaged over a one-hour integration period) at a receptor's location (e.g., his property line), often modified with time-of-day and zoning offsets. When specified (in the US), the measurement method is usually an ANSI standard that is traceable to ISO 3744:1994 Acoustics — Determination of sound power levels of noise sources using sound pressure — Engineering method in an essentially free-field condition over a reflecting plane
The title of that standard is a giveaway to OP's query - the procedural standards for environmental sound level measurement generally specify 'over a reflecting plane,' that is, a hard surface.

Abel