Helicopter noise reduction

Helicopter noise reduction is a topic of research into designing helicopters which can be operated more quietly, reducing the public-relations problems with night-flying or expanding an airport. In addition, it is useful for military applications in which stealth is required: long-range propagation of helicopter noise can alert an enemy to an incoming helicopter in time to re-orient defenses (see acoustic signature).

Source: Wikipedia — Helicopter noise reduction (CC BY-SA 4.0)