Aircraft Propeller/Piston-Engine Propulsion: A Moving Source Pair Susceptible to Active Noise Control

Abstract

For many years work has been under way to investigate the possibility to reduce the noise of piston-engine driven propeller/aeroplanes by appropriately superposing the very propeller/noise with exhaust noise from the piston engine to pro- duce a destructive interference. In this context, it should be understood that certain propeller driven aeroplanes are equipped with piston engines whose fundamental firing frequency coincides with the rotational (blade passing) 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 technique can be applied. Indeed, the signature of both the propeller and the engine exhaust noise is characterized by periodic sound pressure fluctations. In establishing a suitable phase shift between these two periodic sound signals a destructive interference can be attained, in principle resulting in a lesser noise level on the ground.