Last year about 35 windmills were built near the city where I live. They're spread over a very large area, it was hard to get more then 5 in the frame at the same time. The towers are 80m (262 feet) high and each blade is 45m (148 feet) long. When I was there, they would do one rotation ever 3.6 seconds so that means the tips were moving at 283 km/h (176 mph). The blades moving through the air sounds like a jet plane, but not as loud.