The distinctive whup-whup sound of the UH-1 Iroquois, or "Huey" as it is better known, can be heard for miles. However, its larger brother, the UH-60 Black Hawk, is much quieter. This is partly due to the design of the Black Hawk's rotor blades; all four blades have tips which are swept back twenty degrees, decreasing the size of the rotor tip vortices and reducing the noise of the aircraft. (Purists can expound upon this for hours, I know).
Engineers at Eurocopter have gone a step further, designing rotor blades which are almost silent, thanks to technologies such as Blue Edge and Blue Pulse--innovations in blade design which minimize the interaction between the rotor blade and the rotor tip vortices.
Listen for yourself: