alt Hacker NewsThey really aren't particularly quiet imo. Yes, there's no motor, but it turns out that the whirring sound of blades rotating and cutting grass is quite loud even without a motor. I would say mine is perhaps 3/4 as loud as a gas mower, which isn't a very impressive reduction in noise.
With logarithmic perception, it's about a 50% reduction in sound energy.
My battery mower is quiet enough that I don't feel terribly rude mowing at twilight.