This is a problem that was addressed with the IMPBA (yeah, boat guy) as the measurement taken at one site could be louder or quieter at another depending on if the noise is reflected or absorbed.
We use 92 dB at 50', 4 feet over the water using the Radio Shack digital dB meter set on "A" fast. I've compared mine to the high-dollar ones and have gotten the exact same readings.
Vibrations from fuselage's or in our case, hulls, can greatly affect the noise readings. Some have even found that the air intaking the carb alone can make a lot of noise itself regardless of how quiet of a pipe that you place on the exhaust.