Flyover Noise Reduction of Piston-Engine Propeller Aeroplanes Using an Active Noise Control Technique 931218
Certain propeller driven aeroplanes are equipped with piston engines whose fundamental “firing”-frequency coincides with the rotational frequency of the propeller. Taking the two sources as a system consisting of a primary source (propeller) and a secondary anti-noise source (engine exhaust), an active noise control (ANC) technique can be applied. The far field noise of both the propeller and the engine exhaust is characterised by periodic sound pressure fluctuations; by generating a suitable phase shift between these two periodic sound-fields, a destructive interference may be attained, resulting in a reduced overall noise level immission on the ground. Technically, this is achieved by (1) adjusting the spatial distance between the exhaust orifice and the propeller and (2) an appropriate azimuthal positioning of the propeller on the crankshaft of the engine.