Helicopters are able to hover, and perform special vertical lift missions that most other aircraft cannot. Furthermore, helicopters generally fly at lower altitudes. Helicopters can be used in locations and missions for which fixed wing aircraft may not be appropriate, such as emergency medical evacuations, power line maintenance, law enforcement, electronic news gathering and air tours. The unique characteristics that make them a valuable tool may lead, at times, to a sensitivity to helicopter noise by some people in certain areas.
Helicopter aerodynamics are complex and vary by model. The noise signature may also be affected by weather, wind direction, mode of flight, altitude and other factors. The complex aerodynamics is due to both the required speed and characteristics of the main rotor and the speed and characteristics of the tail rotor; there is also noise from both the engine and airframe. In addition, noise is created by both the blade vortex interaction and thickness noise. The blade vortex interaction occurs when a rotor blade passes within a close proximity of the vortex created from a previous blade. Thickness noise is dependent only on the shape and motion of the blade, and can be thought of as being caused by the displacement of the air by the rotor blades. As helicopter noise is periodic and impulsive and is located in the low frequency bands, the noise travels farther and lasts longer.