I can't find any info on those formula you posted earlier, Tomas. And nobody I talk to seems to know anything usefull about them either. Maybe I just hang out with the wrong people.
Re-reading the first formula, it occured to me that (assuming a broadband source) the only variable is mass. So that must be what people refer to as the 'mass law', right? Doubling the mass means doubling the isolation of a boundary. That seems reasonable for a single leaf wall, from what I read, up to its primary resonance frequency.
Second question: in those formulas, what does 'D' stand for?