Helicopters make a lot of noise because of a physical phenomenon called blade-vortex interaction. Eurocopter engineers have developed a new kind of rotor blade that attenuates this problem. It's called Blue Edge, and—as you can hear—it works beautifully:

The new blade shape is combined with another technology called Blue Pulse, which adds three flaps to the edge of the rotor blades. These flaps move up and down at 15 to 40 times per second, using piezoelectric motors that also help to reduce the blade-vortex interaction. [Autopia]