Thanks a lot for the Japanese link!
I haven't figured out how to translate it yet.
8-10 dB reduction is significant indeed ... it means a Honda EU2000i could be left on Eco mode idling all night for certain emergencies and hardly bother anyone (it wouldn't bother me camped next to it, anyway). It would also be very easy on nearby camper's ears even when running full speed during the day powering a small enough air conditioner.
I wonder why it's so hard for the various experimenters in these forums to build an enclosure that small that will do the same without damaging the generator on hot days?? That Honda enclosure even looks too small to have any additional fans in it to move air. I guess maybe that's the difference between solid engineering and do-it-your-selfers ... of course ~$1000 for the enclosure is a big difference, too.