Okay, here's some straight dope.
The current Honda generator brochure has a typo, incorrectly listing the distance for noise measurements as 3 meters. Actual measurements are taken at 7 meters (23 ft). This error will be corrected in the next version of the generator brochure.
FYI, published measurement data is unchanged, e.g., noise measurements were indeed taken at 7 meters.
No firm word yet from Honda R&D on an exact method or protocol used for noise measurements.
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I work for Honda, but the preceding is my opinion alone.
Robert@Honda