The subject of the quiet helicopter is receiving increasing attention throughout the world because it is now established that a helicopter can be made quiet. Quietening is also needed as more helicopters are being used over cities and residential areas. To date, little attention has been paid to helicopter quietening because the emphasis has been on achieving maximum payload and other performance. Now that the basic helicopter design has been established, refinements for quietening are becoming practical. The comparison is much like the evolution of the automobile and the truck in the city. Initially these vehicles were noisy. As their number increased and their design evolved, mufflers and sound-deadening materials were skilfully applied to reduce their noise to more acceptable levels for each type vehicle.