The noise produced by a helicopter introduces different typologies of sources of noise that can be reassumed in: The rotor that generates different types of noise: Thickness noise is caused by the blade periodically displacing air during each revolution. This sound propagates in the plane of the rotor. Moreover, a rotating blade at a non-zero angle of attack imposes rotating forces onto the surrounding air, causing blade loading noise. This sound generally propagates in a direction perpendicular to the plane of the rotor. These two types of noise always occur, even in a hover condition.