The noise of the generator is measured in Decibels at a distance of 21 feet (or 7 meters). Decibels are logarithmic, so the difference between 50 and 60 is much more to the human ear than the difference
between 100 and 110.
Generally, the air cooled generators fall between 62 and 69 db at 21 feet. The liquid cooled are quieter, generally between 58 and 62db. The speed of the engine also determins how noisy they are. The 1800 RPM generators are very quiet.