Fire Alarm Audibility: What About Closets?

NFPA 72 states that audibility from a fire alarm system is required to be provided throughout a building. Most jurisdictions are less concerned with attaining audibility in small, non-normally-occupied rooms such as janitor closets, small electrical rooms, or small mechanical rooms. However, several military reviews have pointed to exactly these rooms and asked how audibility is going to be achieved. 

I typically provide speakers just outside of these groups of rooms such that there should be an achieved audible level, but how could I prove that during the design phase?

Is there any calculation for audible loss through a door or software that could model this loss?

I feel like there's a good handful of variables that play into whether audibility is achieved in these small rooms that may be difficult to predict.

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