The thing that's special about the Honda EU series is that they will produce the correct AC voltage and frequency with the engine running at a wide range of RPM's. For example, if one needs only half the capacity of the generator, the engine runs at the idle speed. That's what contributes to its being super-quiet. When using our EU1000i to charge our camper's battery, we power a conventional charger with the generator and the generator runs at its slowest RPM and is so quiet that you almost cannot hear it from 50 feet away.
The system is actually a DC generator which supplies energy to a rather sophisticated inverter to provide the AC output. The output is not a truly "pure" sine wave (as seen on my oscilloscope), but is a very smooth waveform with a tiny ripple riding on it.