Determination of aircraft noise variability using an acoustic camera

Nowadays, aircraft noise is one of the major problems to be dealt with by the aerospace industry and especially suffered by the residents living in the vicinities of airports. The enforcement of noise control environmental laws around airports is hindered due to the large variability observed in the noise levels for flyovers of the same aircraft type. These variations are not properly considered by the current assessment tools, such as the Noise-Power-Distance tables. In this paper, a measurement campaign was performed in Amsterdam Schiphol Airport using a 32 microphone array, where 115 flyovers of landing aircraft were recorded in order to determine these variations for various aircraft types and investigate the causes. It was assumed that the main cause of this variability are the differences in the emitted aircraft noise, since previous experience confirmed that the effect of the variable atmosphere (for the distances considered) is negligible. A strong correlation was found between the noise levels and the fan rotational speed (determined from the spectrograms), with almost 40% of the observed total noise variation explained by the engine settings variation. The use of a microphone array allows for acoustic imaging, thus discerning the noise of the aircraft elements from those of other sound sources. It was shown that for many aircraft types the turbofan engines were the main noise sources. After applying functional beamforming to the acoustic data focusing on the engines location, this correlation is even higher, explaining over 62% of the noise level variability.